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• Published: 22 June 2021

Evaluating the climate impact of aviation emission scenarios towards the Paris agreement including COVID-19 effects

• Volker Grewe   ORCID: orcid.org/0000-0002-8012-6783 1 , 2 , 3 ,
• Arvind Gangoli Rao   ORCID: orcid.org/0000-0002-9558-8171 2 , 3 ,
• Tomas Grönstedt 3 , 4 ,
• Carlos Xisto   ORCID: orcid.org/0000-0002-7106-391X 3 , 4 ,
• Florian Linke   ORCID: orcid.org/0000-0003-1403-3471 3 , 5 ,
• Joris Melkert 2 , 3 ,
• Jan Middel 3 , 6 ,
• Barbara Ohlenforst   ORCID: orcid.org/0000-0002-5793-6059 3 , 6 ,
• Simon Blakey   ORCID: orcid.org/0000-0001-6478-7170 3 , 7 , 8 ,
• Simon Christie   ORCID: orcid.org/0000-0003-2631-5425 3 , 9 ,
• Sigrun Matthes   ORCID: orcid.org/0000-0002-5114-2418 1 , 3 &
• Katrin Dahlmann 1 , 3  

Nature Communications volume  12 , Article number:  3841 ( 2021 ) Cite this article

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• Climate change
• Climate-change mitigation
• Environmental impact
• Projection and prediction

Aviation is an important contributor to the global economy, satisfying society’s mobility needs. It contributes to climate change through CO 2 and non-CO 2 effects, including contrail-cirrus and ozone formation. There is currently significant interest in policies, regulations and research aiming to reduce aviation’s climate impact. Here we model the effect of these measures on global warming and perform a bottom-up analysis of potential technical improvements, challenging the assumptions of the targets for the sector with a number of scenarios up to 2100. We show that although the emissions targets for aviation are in line with the overall goals of the Paris Agreement, there is a high likelihood that the climate impact of aviation will not meet these goals. Our assessment includes feasible technological advancements and the availability of sustainable aviation fuels. This conclusion is robust for several COVID-19 recovery scenarios, including changes in travel behaviour.

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Introduction

Fuel efficiency of jet aircraft has been increasing right from the dawn of jet aviation in the late ’50 s and early ’60 s. This improvement cannot be attributed to one single source but has been achieved by a combination of factors such as improvements of the airframe aerodynamics, weight reductions due to better engineering, materials and manufacturing techniques, larger engines with a lower specific thrust, higher overall pressure ratios and component efficiencies, lighter structures and lighter on-board systems. Kharina and Rutherford 1 report an average reduction in fuel consumption per passenger-km at the global fleet level of 1.3% per year over the years 1960–2014. Without any further specific measures this reduction is expected to continue at a similar rate until 2037 2 in a business as usual scenario.

Air transport as a sector has been growing rapidly in most regions of the world. The total number of passengers transported annually passed 4 billion in 2017. The number of flights in all regions of the world has increased (Supplementary Fig. 1 ) and aircraft have on average greater seating capacity and are operated with a higher load factor (Supplementary Fig. 2 ). It is expected that air transport will continue to grow in the coming decades. Airbus 3 predicts in its Global Market Forecast continued annual growth of 4.4% in revenue passenger kilometre (RPK) for the next two decades. Boeing 4 expects in its Commercial Market Outlook an annual growth of 4.6%. The effects of the COVID-19 pandemic are expected to only have a temporary effect on this growth.

Without any measure the climate impact of aviation will continue to grow. Several measures, both political and technical, are in place or will be introduced in the near future. Via a number of scenarios, we analyse their effect on global warming and assess the effectiveness of these measures. Since many of these measures are set top-down we also want to assess the technical feasibility. Therefore, we have performed a bottom-up expert assessment on the feasibility of technical advances and their effect on climate change. We confront the two approaches with each other.

The profitability for the airlines is small. Their average net profit per passenger is <10 USD (Supplementary Fig. 3 ). Competition amongst airlines is fierce and therefore sensitive to airline costs differences. Fuel costs play an important role, which is of particular concern for the uptake of sustainable alternative fuels (SAF) that currently have a significantly higher cost than conventional fossil fuels. The COVID-19 pandemic has led to a large decrease in the number of flights and passenger load factors in 2020. In May 2020, the International Civil Aviation Organisation (ICAO) estimated a decrease of global total available seat kilometres of 94% in April 2020 compared to the 2019 baseline. However, they expect a recovery leading to an annual decrease in available seat kilometres of 45% to 63% for 2020 5 , but assume growth will resume beyond 2020.

Approximately 5% of the current anthropogenic climate change is attributed to global aviation 6 , 7 and this number is expected to increase since aviation passenger transport is projected to grow by ~4% per year whilst other sectors continue to decarbonise. Aviation emits carbon dioxide (CO 2 ), water vapour (H 2 O), nitrogen oxides (NO x ), sulphate aerosols, compounds from incomplete combustion (unburnt hydrocarbons, UHC) and particulates (soot). The emitted species are transported in the atmosphere and alter a wide range of atmospheric processes including the formation of contrail-cirrus and ozone and the depletion of methane 7 , 8 , 9 .

The formation of persistent contrails-cirrus depends on aircraft and fuel parameters as well as atmospheric conditions, as the propensity of contrail formation is higher in the cold and saturated atmosphere 10 , 11 , 12 . Contrail-cirrus influence the incoming solar radiation and the outgoing infrared radiation emitted by the Earth and its atmosphere. The net change, the radiative forcing (RF), is on average positive and hence contrail-cirrus act to warm the climate 13 . The emitted nitrogen oxides (NO x ) react with hydroxyl radicals (HO x ), which eventually form ozone and contribute to the depletion of methane in the atmosphere. Therefore, emissions of nitrogen oxides increase the ozone concentration and decrease the methane concentration (which itself leads to a reduction in ozone production and is called primary mode ozone, PMO). Ozone and methane are greenhouse gases and changes in their concentrations cause changes in the RF, which are in total positive, i.e. leading to warming 7 , 14 , 15 . The net direct impact of aerosol emissions on RF (soot: warming and sulphate: cooling) is small 7 and are not further regarded in this study, whereas the impact of soot emissions on contrail-cirrus properties are important 16 and considered in our calculations (see ‘Methods’). An open question, which is currently under investigation is whether aerosol emissions significantly alter or even induce natural clouds, both low-level and cirrus clouds 17 .

The Advisory Council for Aviation Research and Innovation in Europe (ACARE) has set targets for the reduction of emissions in its Flightpath 2050 document 18 . Among these targets is a reduction of 75% of CO 2 and 90% of NO x emission per passenger-km by 2050. The datum for these reductions is a typical new aircraft in the year 2000. These targets are set for the research, with intended outcomes to be realised at a technological readiness level (TRL , The European definition of TRLs range from 1 to 9, i.e. from ‘basic principles observed’ to ‘actual system proven in operational environment’) of 6.

The ICAO of the United Nations has agreed on a global market-based measure scheme to abate the growth of CO 2 emissions from international aviation. This scheme is the Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA). According to this scheme, the post-2020 growth in the sector must be offset such that the net carbon emissions do no longer grow. They must either be reduced via more efficient aircraft and/or the use of SAF or must be compensated via offsets. CORSIA starts as a voluntary pilot scheme in 2021 and becomes mandatory, with some exceptions, in 2027 for all member states 19 . Aviation is a growing sector that has committed to reduce net CO 2 emissions and thus contributes to the international goals of limiting climate warming ‘to well below 2.0° C above preindustrial levels and pursuing efforts to limit the temperature increase to 1.5 °C above preindustrial levels’, as stated in the Paris Agreement 20 . The Paris Agreement does not set emission targets for specific sectors. Furthermore, international aviation and shipping are not included in the national contributions that countries have to make to comply with the agreement. However, we assume that the international aviation community will contribute to the goal of the Paris Agreement. We will investigate the effect of measures and policies on global warming and also assess their feasibility. Thereby we will not distinguish between domestic and international aviation but treat the sector as a whole. There are two bridges to cross between the emission goals set by ACARE and ICAO and the climate targets set by the Paris Agreement: First, how do the emission goals translate into near-surface temperature changes, i.e. climate change. Second, how large are the non-CO 2 effects?

Here we close these gaps and show that the emissions goals set by Flightpath 2050 very likely will stabilise aviation’s climate impact, though the sector’s contribution to global warming remains considerable. Contrarily, we find that ICAO’s offsetting scheme, CORSIA, will surpass the climate target set to support the 1.5 °C goal between 2025 and 2064 with a 90% likelihood. In both cases non-CO 2 effects will have a considerable contribution to aviation’s climate impact, however, they are currently not included in ICAO’s goal of climate neutral growth and only partly addressed in Flightpath 2050. We assess the feasibility of achieving the Flightpath 2050 goals by technological improvements and the availability of sustainable alternative fuels as an ECATS (Environmentally Compatible Air Transportation System) expert group and reveal the risk of a large discrepancy, leading to an increasing aviation induced global warming effect rather than stabilisation.

Results and discussion

Top-down scenarios for future aviation.

Figure  1a presents the global growth of revenue passenger kilometres, showing an exponential increase between 5.2 and 6% per year (dotted lines). From this basis, we developed eight top-down scenarios, which consider a further increase in aviation, though with a decreasing rate of growth (down to 1.2%/year). These rates are based on simulations of the aviation sector, relying on the Randers scenario 21 , which is independent from aircraft manufactures. This scenario was employed within the WeCare project 22 and considers worldwide saturation effects of economic growth. This Randers scenario leads to a growth rate of 1.2%/year in 2050 which we extrapolate to 0.8%/year in 2100 (see also Supplementary Material). Our industry-independent scenario shows lower estimates of the transportation volume for the coming two decades compared to the Airbus and Boeing forecasts (see above), though still slightly higher than other estimates for 2050 23 , 24 . Advances in airline operating efficiency, including changing the type of aircraft, the number of seats and load factor lead to a reduced increase of flown kilometres (Fig.  1b ; green line) compared to the transport volume measured in RPKs (violet line). More fuel-efficient technologies even lead to a smaller increase in fuel use compared to flown distances (blue and orange lines). Taking the targets of Flightpath 2050 into account, a more aggressive reduction in emissions can be achieved up to 2050. In the scenario FP2050, we consider a development of these technologies until 2050 followed by an introduction into the market. In the scenario FP2050-cont we apply a continuous introduction of these innovative technologies into the market (Fig.  1b , early and continuous/late introduction light/dark brown, respectively). Using these assumptions, the modelled results show that after 2050 the increase in RPK is balanced by technology enhancements leading to almost constant fuel consumption until 2100.

They include the future use of current technology, i.e. without technology improvements (CurTec), with a business-as-usual future technological improvement (BAU), the offsetting scheme of the international civil aviation organisation (CORSIA), and 2 Flightpath 2050 scenarios which differ in the speed of technology improvements (FP2050 and FP2050-cont). a Revenue passenger kilometres as provided by ICAO; dotted lines provide exponential growth rates. b Future changes relative to their respective values in the year 2000 for revenue passenger kilometres (violet), flown distances (green), the fuel consumption of the scenarios BAU and CORSIA (blue), and the FP2050 scenarios (brown). c Future CO 2 emissions for the scenarios CurTec (red), BAU (blue), CORSIA (light blue), FP2050 with late technology advancements (dark brown) and continuous technology advancements (brown). Note that for CORSIA the effective CO 2 emission is considered, including reductions due to the use of sustainable alternative fuels (SAF) and capping net emissions. d as bottom-left, but for NO x emissions; note that the NO x emissions for BAU and CORSIA are identical. The order in the legend is the same as the lines appear in the graph.

We take into account five different scenarios (Table  1 ): (1) Current Technology (CurTec), which describes the emission pathways with current (2012) technology, (2) Business-as-usual (BAU), which, in addition, takes into account some of the future improvements in technology, (3) CORSIA, which is identical to BAU, but yearly CO 2 emissions are reduced by offsetting CO 2 emissions beyond 2020 values, (4) and (5) Flightpath 2050 (FP2050 and FP2050-cont), which utilise the targets of FP2050 (Fig.  1c, d ). Note that for the CORSIA scenario, we assume an optimistic future availability and a price premium of SAF based on an analysis of feedstocks and the evolution of SAF production. As a result, approximately half (53%) of the CO 2 reduction that is required to achieve CORSIA’s CO 2 -neutral growth stems from the use of SAF and the other part results from carbon caps. This leads to a larger reduction in climate impact compared to a scenario where the total amount of CO 2 is capped. The explanation is that SAF do not only reduce the climate impact via CO 2 but also the reduction in contrail-cirrus climate impacts since a change in their chemical composition changes the contrail-cirrus properties (see ‘Methods’).

Aviation climate impact

We use these five scenarios to calculate their climate impact with the non-linear climate-chemistry response model AirClim 25 , 26 in terms of near-surface temperature change by taking into account effects from CO 2 as well as NO x and H 2 O emissions and contrail-cirrus (Fig.  2 ). The three scenarios CurTec (red line), BAU (dark blue line), and CORSIA (light blue line) show an increase in temperature until the end of the simulation (2100), though the rate of increase slows down. For CurTec, since the technology is frozen in this scenario, the rate of increase arises from the assumed development of the transport volume (Fig.  1 ). The increased efficiency in scenario BAU in comparison to the scenario CurTec clearly shows a substantial temperature reduction of roughly 25% in 2100. The temperature reduction is even larger for CORSIA (35–40%), due to a reduction in the effective CO 2 emissions from the CORSIA scheme and changes in contrail-cirrus properties from the extensive use of SAF. Terrenoire et al. 27 calculated a temperature increase in 2050 for a CORSIA scenario of 32 mK, which is consistent with our calculated value of 30.4 mK. The two implementations of the Flightpath 2050 scenarios (FP2050 and FP2050-cont) show a clear stabilisation of their climate impact, though with an overshoot around 2050. Allowing 5% of the anthropogenic temperature increase to be contributed by the aviation sector, as motivated by the current estimate of aviation to global warming, both scenarios show compliance with a 2 °C target and the scenario FP2050-cont even with the 1.5 °C target. The inertia of the climate system delays the impact of both FP2050 scenarios, which overshoot these targets around the year 2050. However, after 2050, the FP2050 measures are sufficient to cause significant temperature decreases beyond these targets from this point on.

The horizontal lines indicate 5% of a 2 °C and 1.5 °C climate target. The scenarios describe a future use of current technology, i.e. without technology improvements (CurTec, red), a business-as-usual future technological improvement (BAU, blue), the offsetting scheme of the international civil aviation organisation (CORSIA, light blue), and 2 Flightpath 2050 scenarios which differ in the speed of technology improvements (FP2050 and FP2050-cont, brown and orange, respectively).

Temperature change is a complex response to the individual measures through the various climate agents. The reductions of the CO 2 emissions (Fig.  1 ) for all scenarios compared to the CurTec scenario lead to a significant reduction of aviation’s absolute contribution to climate change (Fig.  2 ). However, the relative contribution to climate change, i.e. the share of CO 2 to the aviation’s climate impact, increases from 25% in 2005 to between 33% and 56% in 2100. The reason is that the reduction in NO x emissions reduces the temperature increase via ozone faster than the reductions in CO 2 emissions. The short lifetime of both NO x and ozone in the atmosphere compared to CO 2 enables this faster response. On the other hand, the contrail-cirrus climate impact is largely driven by the distances flown. Here two factors play a role, the increase in the efficiency of the transportation system and the use of sustainable alternative fuels. These two effects lead to a reduction in the contrail-cirrus climate impact by roughly 20% in the scenarios BAU and CORSIA compared to CurTec (Fig.  3 ). The relative contribution of contrail-cirrus to the climate impact (Table  2 ) shows a reduction from 33% in 2005 to around 20% and 24% in 2100 for BAU and CORSIA scenarios, respectively, and is only slightly reduced for the FP2050 scenarios (27% and 30%). Recently, the non-CO 2 effects of aviation were revised concerning NO x emissions 15 and contrail-cirrus 13 . While Grewe et al. 15 stressed methodological improvements, like how to correctly attribute ozone concentrations to aviation NO x emissions, Bock and Burkhardt 13 focussed on improved contrail-cirrus microphysics. Our results include most aspects of these new developments and hence show, e.g. a larger ozone-RF as well as NO x -RF compared to earlier studies, such as Lee et al. 28 (Fig.  3 , left bars). The current results are in accordance with those new findings.

The individual bars are grouped into four categories. (1) The two bars on the left describe the radiative forcing of aviation in the year 2005 (RF 2005). Results from Lee et al. (2009) are expanded by contrail-cirrus estimates based on Bock and Burkhardt (2019), denoted by L09 + BB19, respectively; (2) temperature change in the year 2005 (dT 2005); (3) temperature change in the year 2050 for the 5 scenarios (dT 2050); (4) as (3), but for the year 2100 (dT 2100). For 2100, i.e. the right-hand columns, the scenarios are presented in the same order as for 2050. The order in the legend is the same as the colours appear in the individual boxes. The scenarios describe a future use of current technology, i.e. without technology improvements (CurTec), a business-as-usual future technological improvement (BAU), the offsetting scheme of the international civil aviation organisation (CORSIA), and 2 Flightpath 2050 scenarios which differ in the speed of technology improvements (FP2050 and FP2050-cont).

Hence to summarise, the increase in transport volume leads to an increase in the overall climate impact from aviation, which also increases the relative importance of CO 2 (25% in 2005 Base to 39% in 2100 CurTec, Table  2 ), even if aviation net CO 2 emissions are regulated and capped to 2020 values. The increase in fuel efficiency of aviation technologies at a current rate decreases the overall climate impact, especially for CO 2 and NO x . By this, it mainly reduces the relative contribution of NO x (41–16%). The introduction of the CORSIA scheme further reduces the climate impact of CO 2 emissions and that increases the relative importance of contrail-cirrus and NO x . The technological measures from FP2050 have a similar reduction efficiency for CO 2 as CORSIA, however, the strong measures for NO x largely reduce the overall climate impact so that the remaining climate impact from aviation is due to CO 2 (50–60%) and contrail-cirrus (around 30%).

Contrasting aviation climate impact with 1.5 °C and 2 °C climate targets

The climate impact of aviation emissions has a considerable uncertainty range, especially, for the non-CO 2 effects 22 , 28 , which influences not only the absolute change of near-surface temperatures but also the importance of the individual climate agents. In this work, we take into account uncertainties in the atmospheric lifetime of aviation-related species, uncertainties in the RF of individual species and the climate sensitivity parameter, the last of which relates the RF to temperature changes. These parameters are varied in a Monte–Carlo analysis with 10,000 simulations to obtain a range of possible atmospheric responses. The results of the Monte–Carlo analysis provide a basis for estimating a range when the temperature thresholds, 5% of 1.5 °C and 5% of 2 °C, are surpassed. For example, Fig.  4a shows the first 20 simulations of the CORSIA scenario. Figure  4b shows the probability density function (blue) and cumulative probability density function (green) of these times of surpassing the 5% of 1.5 °C for the CORSIA scenario. The mid 90% range (between the 5% and 95% percentile) indicates that this threshold is surpassed between 2025 and 2064 in the CORSIA scenario. The 5% of 2 °C is surpassed roughly 10 years later (Fig.  4c ). Both, the CurTec and BAU scenario, show that both thresholds are surpassed very likely well before 2050 (Fig.  4c ).

a Potential pathways (first 20 realisations of the Monte–Carlo simulation) for the CORSIA Scenario (grey). 5% of the 1.5 °C ( = 75 mK) is indicated as a black line. Crossings of the brown line with the grey line indicate the year when the threshold is surpassed. b Probability density function (PDF, blue) and cumulative probability density function (CPDF, green) for the year in which the climate target of 5% of 1.5 °C is surpassed (and stays above). The horizontal bar indicates the 95%, 50% and 5% percentiles. c 95%, 50% and 5% percentiles of the year in which the climate target is surpassed for 5% of 2 °C (thin top lines with crosses) and 5% of 1.5 °C (thick bottom lines). For both FP2050 scenarios, the 5% of 2 °C target is not surpassed in >95% of the cases, hence there is no thin line, and for FP2050 scenario with continuous improvements (FP2050-cont) the 50% percentile is beyond 2100. The scenarios describe a future use of current technology, i.e. without technology improvements (CurTec), a business-as-usual future technological improvement (BAU), the offsetting scheme of the international civil aviation organisation (CORSIA), and 2 Flightpath 2050 scenarios which differ in the speed of technology improvements (FP2050 and FP2050-cont) and bottom-up estimates based on a group of experts from ECATS (Environmentally Compatible Air Transportation System).

ECATS technology scenarios and their climate impact

The emission reductions formulated in the Flightpath 2050 are aspirational goals, which the aviation community is aiming to achieve. Here we now contrast this with technologies which are currently discussed in the research, such as boundary layer ingestion, distributed propulsion, laminar flow control, lightweight structures, advanced geared turbofan engines, etc., and assess their potential to reduce fuel use and NO x emissions (Table  3 and Supplementary Material for more details). The majority of technology enhancements for a 2050 aircraft should, at least as an idea, be available today since the time from the development of basic research ideas (TRL 1) to having this aircraft operational in service (TRL 9) takes decades. We take into account developments for different aircraft segments, such as single-aisle and twin-aisle aircraft for entry into service between 2035 and 2050. General aviation, regional aircraft and business jet have been left out from this study, as their current contribution to total aviation CO 2 emission is around 5–6%, only. We take into account a large range of technologies and engine airframe integrations (see Supplementary Figs. 7, 12–16 ) and find a 18–22% improvement in fuel efficiency, which is similar to the analysis presented by Cumpsty et al. 2 , which indicates an 18% reduction. For the far future (2050), we consider one variant for a single-aisle aircraft, while three variants are considered for a future long-range twin-aisle aircraft. These include (1) a conventional tube-and-wing wide-body aircraft (TW), (2) the so-called Flying-V (FV) or multi-fuel blended wing body (MF-BWB). Both have similar aerodynamic characteristics and were developed by TU Delft 29 , 30 , 31 , 32 and (3) NASA’s N3-X (N3) blended wing body 33 , 34 . We find that the fuel consumption of a 2035 aircraft might be reduced between 18% and 22% compared to new 2015 aircraft and between 34% and 44% in 2050. Note that though far future technologies, i.e. in 2075 or later, are in principle of interest they do not significantly impact our results, since their diffusion into the fleet delays their impact and more importantly, the impact on global temperatures will mainly occur beyond 2100 due to the inertia of the atmosphere-ocean system in the order of decades. These findings result in 9 ECATS emission scenarios with 3 variants (TW, FV, N3) including a pessimistic base and an optimistic implementation, which differ by ±10%. The scenarios are developed consistently with the top-down scenarios following the same transport volume development and SAF usage as in the scenario CORSIA. Figure  5a presents the fuel use and NO x emissions relative to the year 2000, resulting in a roughly fivefold increase in fuel consumption by 2100 and a fourfold increase in NO x emission. The new technologies introduced from 2035 onwards lead to a reduction in fuel use and NO x emission around 2050, which is then offset by the further increase in transport volume, resulting in a slight increase in fuel use and NO x emission until 2100. This analysis shows that an emission pathway better than BAU might be feasible, but that the goals set by Flightpath 2050 are unlikely to be achieved. The fuel use and NO x emission from the FP2050 scenario (Fig.  1 ) are drastically lower than the range of our ECATS scenarios (Fig.  5 ).

a as in Fig.  1 , changes in fuel use (red) and NO x emissions (blue) taking into account a bottom-up analysis of aviation technologies; three far future technology pathways are taken into account, with a ± 10% uncertainty range, each leading to a scenario range; b Transport volume for the scenario taking into account a reduction of flight due to COVID-19 with three assumptions: a short recovery of 3 years (red); a longer recovery of 15 years (brown) and in addition to a long recovery a behavioural change after COVID-19 (yellow). c Resulting temperature changes as in Fig.  2 for the range of ECATS scenarios (green) and the BAU scenario for comparison (blue). d Resulting temperature changes from the 3 COVID-19 scenarios (red, brown, and yellow) in comparison to the BAU scenario (blue). The scenario BAU describes a business-as-usual future technological improvement and ECATS bottom-up estimates based on a group of experts from ECATS (Environmentally Compatible Air Transportation System).

The climate impact of the ECATS aviation scenario (Fig.  5c ) shows clearly a reduction compared to the BAU scenario. However, the stabilisation of the temperature, as it was found for the Flightpath 2050 scenarios, is not achieved. The ECATS scenarios fall in between the BAU and FP2050 scenarios. The absolute change in temperature and the contribution from individual climate drivers (Table  2 ) contribute to climate warming in 2100 from CO 2 of 33–37% and the effects from non-CO 2 emissions roughly equally shared between contrail-cirrus and NO x emissions.

Sensitivities to growth, global targets, sustainable fuels and technologies

The future evolution of the aviation system and the resulting impact on climate relies on too many variables to be predicted with one outcome. To tackle this problem, we present a range of scenarios. Those are based on either an analysis of climate impacts based on set emission targets, the five scenarios mentioned in Table  1 , which we call top-down scenarios, or an analysis of the climate impact of technological changes that can be expected in future aircraft, which we call the bottom-up scenarios (see ‘Method’). Both approaches define possible future pathways. Even though this approach includes a large range of uncertainties, we feel that such analysis should be an important part of the debate around the impact of aviation and the potential for change within the sector. A major uncertainty is the future demand for air travel. Here we present a scenario, which lies between the estimates from Boeing and Airbus (see above) other estimates from academia 23 , 24 which levels off in the future. In this sense, we present a more conservative estimate of the future climate impact of aviation as compared to industry forecasts. A variation of the future growth rates by ±50% on top of the general declining growth rate leads to a change of fuel usage in the scenario BAU of roughly ±20% in 2100 and a shift in the median surpass year of 3 years (Table  4 ). Demand-suppressing effects from the use of more expensive SAF might end up at about 10–15% reduction of demand by 2050 for an elasticity of −1 35 and a SAF price, at best, two times that of conventional kerosene 36 . Hence, our ‘−50% growth rate’ sensitivity simulation can be taken as an indicator for the impacts of such demand-suppressing effects, implying that the median year at which the temperature rise of 5% of 1.5 °C is surpassed will be delayed by a few years only. Most other scenarios lead to a similar shift in the median surpass year. A change in future efficiency improvements has in principle similar effects. The overall setting of the climate target and a shift from 5% to either 3.5% or 6.5% leads to a shift of the median surpass year in the order of one to two decades (Table  4 ). Sustainable aviation fuels are an important means in reducing the climate impact of aviation. However, according to CORSIA, whether a cap in net CO 2 is achieved by offsetting or the use of SAF has only a limited impact on the temperature evolution. And hence a reduction of the SAF availability by 50% leads to negligible changes in the distribution of the surpass years.

COVID-19 effects on aviation climate impact

The recent COVID-19 pandemic might question the discussed future aviation pathways we analysed so far. To better understand the possible implications of this pandemic on the climate impact of aviation, we altered the BAU scenario in a parametric way to assess three different pathways for the international recovery from the lock-down of nation states and the associated dramatic reduction in air travel, based on reported transport volumes and scenario projections 5 . We take into account a fast recovery of 3 years, a slow recovery of 15 years (C19-03, C19-15) and a change in habits due to experiences during the lock-down, for example, a shift towards web conferences instead of face-to-face meetings. Figure  5b shows a drop in RPK due to COVID-19 and the three recovery pathways. The respective, resultant temperature change (Fig.  5d ), however, is only significant if a sustained reduction in RPKs follows the crisis (yellow curve). Otherwise, the changes in 2020 due to COVID-19, as dramatic as they are for individuals and the global economy, only have a minor effect on the overall climate impact of aviation as long as a recovery follows. From the experience of other crises (e.g. SARS, 9-11, etc. see Fig.  1 ) we might expect a fast recovery. However, the consideration of which COVID-19 scenario is more likely is outside the scope of this study.

Top-down-scenario building

In the top-down scenario building, we combine top-level assumptions on the evolution of aviation (transport volume, technologies, SAF availability) with a detailed description of the air transport system for specific years. Details are given in the Supplementary Material as textual description and EXCEL sheet. Five scenarios are assessed, which all have some common characteristics (Table  1 ). They have identical evolution in transport volume, defined by the revenue passenger kilometres, which resemble ICAO data for the past (1971–2017) and are extrapolated to future with the assumption of a slow decrease in traffic growth rates in future. The observed increase rate in transport volume of roughly 6% per year in the decade 2008–2017 are reduced by to roughly 1% per year in 2050 following the results from the WeCare analysis and the Randers scenario. We employed the Randers scenario named 2052 that includes the temporal development of socio-economic factors, such as population and Gross-Domestic Product, for different world regions and is complemented by reasonable narratives and scientific evaluations. Within the WeCare project, it was combined with an air passenger demand model that calculates the demand between settlements. The resulting air traffic scenario shows lower estimates of the transportation volume for the coming two decades compared to the Airbus and Boeing forecasts. The resulting air traffic scenario is not based on an extrapolation of historical trends and manufacturer expectations but considers realistic assumptions for the socio-economic growth and an associated expected saturation around 2040. Details on the forecasting methodology developed and applied in WeCare can be found in Terekhov 37 and Ghosh 38 . Future fuel efficiency improvements are based on the ICAO’s environmental report 39 , with 1%/year in 2018 decreasing to 0.25% in 2100. These two assumptions lead to a fuel consumption of 823 Tg in 2050, which agrees well with the mean of the ICAO scenarios 39 . The geographical distribution follows the emission inventories developed within the WeCare project 22 . Two time horizons are taken, one for the recent past ( = 2012) and one representative for the future (2050), describing the geographical and vertical distribution of the emissions. All scenarios are identical between 1940 and 2018, and deviate afterwards, according to scenario assumptions, derived from the basic storylines. Thereby, we obtain 5 scenarios CurTec, BAU, CORSIA, FP2050, FP2050-cont (see main text and Table  1 ). The carbon-neutral growth from 2020 onwards in the CORSIA scenario is achieved by using a combination of sustainable aviation fuels (SAF) and emission offsets. Based on the EU-Renewable Energy Directive (RED-II), we assume an effective 65% net CO 2 reduction in SAF production and use compared to conventional kerosene in the year 2020. We assume a mix of different feedstocks, such as agricultural residues, algae, dedicated energy crops and also e-fuels (power-to-liquid), which enables an improvement of the overall CO 2 reduction potential to 80% in 2100. An analysis of the current growth rates and forecasts of the availability of SAF are used to optimistically estimate future availability of SAF and to allow a conservative estimate of the climate impact of the CORSIA scenario. Note that we have not explicitly considered any closed loop demand-supressing effects of increased costs 35 , such as SAF costs, since EUROCONTROL has indicated that these effects might be marginal 40 and there is a high degree of uncertainty in the prediction of these costs. Instead, we have addressed this sensitivity by changing the growth rates (see below) by ±50% as open loop scenarios, which would cover a number of changes in transport volumes including those arising from demand suppressing costs increases. These assumptions lead to a scenario where 1/3 of the fuel used in 2100 is assumed to be SAF. We consider two different pathways of achieving the Flightpath 2050 objectives, late and continuous (FP2050 and FP2050-cont). Both scenarios have the same transport volume as BAU and consider technological improvements by 2050, which are formulated as ‘CO 2 emissions per passenger kilometre have been reduced by 75%, NO x emissions by 90% and perceived noise by 65%, all relative to the year 2000.’ 41 .

In addition to these five main scenarios, we introduce three possible development pathways related to the COVID-19 pandemic by varying the timing and degree of recovery (see main text).

Bottom-up-scenario building

In the Bottom-up scenario building, we present possible different development pathways and analyse how those scenarios influence the contribution of future aviation to climate change. Evolutionary technology scenarios are developed by expert judgement (TU-Delft, Chalmers, DLR, TU-Hamburg) with comprehensive knowledge on the possible availability of advanced technologies in future aircraft programmes along with in-house tools and models for engine performance, aircraft design and aircraft performance (explained in detail within the Supplementary Material). We assess a broad spectrum of possible aircraft configurations, technologies, systems and procedures currently under research and development and evaluate their viability and provide best estimates on fuel consumption and NO x emission reduction potentials (Table  3 ). Comparing with the work by Schäfer et al. 42 , the improvement rates are quite similar when matching our 2035 single-aisle aircraft with the evolutionary year 2035 configuration presented by Schäfer et al. The reference used in our paper is more recent and is comparable to Schäfer’s ‘intermediate’ aircraft. They predict an 18% fuel burn reduction of the evolutionary aircraft over the intermediate aircraft, which is similar to that obtained in our analysis. In a similar approach, Hileman et al. 43 investigated at the US domestic market considering single-aisle aircraft, only. According to them, a double bubble fuselage design 44 with lower cruise speed would have 42% lower fuel consumption when compared to B737-800, which is an older generation of aircraft than the A320neo. However, it is less likely that the next generation of single-aisle aircraft will deviate from a tube and wing geometry.

In this work, the fuel efficiency and emission analysis are done for both single-aisle and twin-aisle aircraft market segments, as those two segments will account for about 95% of globally available seat kilometres. Single-aisle aircraft serving short and short-to-medium distance routes are responsible for 47% of the worldwide aviation fuel consumption. Single-and twin-aisle aircraft serving the medium and long-range routes are responsible for another 47% of the fuel consumption. Hence, differently to the top-down FP2050 scenarios, we analyse possible future technology developments and derive the expected fuel efficiencies and NO x emission evolutions in a bottom-up approach and combine that with the same overall scenario definition as for the top-down scenarios, e.g. with respect to transport volume.

We compute emission inventories based on global fleet forecast data developed in the WeCare project 22 for the years 2015–2070, in 5-year steps, for single-aisle and twin-aisle market segments. As a simplification, we assume that for each segment there is one representative aircraft type which can be used to model the entire market segment appropriately, while multiple aircraft generations are considered. The aircraft Airbus A320neo and A350 are selected as best of class for the current generation and serve as reference aircraft types for the single-aisle and twin-aisle markets, respectively. Entry into service year of the current generation is assumed to be around 2015. The next generations of single-aisle aircraft are assumed to be conventional tube-and-wing configurations entering into service in 2035 and 2050 with the fuel consumption and NO x emission improvement factors as shown in Table  3 relative to the reference aircraft. For the twin-aisle market, we estimate the next generation aircraft entering into service in 2035 being a tube-and-wing configuration. In 2050, three different options, viz. a conventional tube-and-wing widebody aircraft, an aerodynamically improved aircraft, the so-called Flying-V or multi-fuel blended wing body (MF-BWB) with an advanced turbofan engine, both developed by TU Delft, and NASA’s N3-X blended wing body with a turbo-electric propulsion system, are considered and used as possible twin-aisle aircraft configurations. For each of the years considered, the actual fleet composition is calculated considering a fleet diffusion of the new aircraft generations, i.e. introducing and partly replacing old aircraft. The market penetration of an aircraft generation is modelled as an S-curve applying the Bass diffusion model that has been calibrated to reach >95% market penetration within roughly 15 years, which is a typical diffusion time for new aircraft 45 , 46 , starting from their respective entry into service (EIS) [2015, 2035, 2050].

For the calculation of the reference emission inventories (those based on the reference aircraft types), we apply the GRIDLAB methodology developed in DLR 47 . In a next step, those inventories are multiplied with the improvement factors (CO 2 and H 2 O inventories scaled according to fuel improvement, NO x inventory scaled according to NO x improvement) to determine the emissions for the respective aircraft generations. Finally, for all years, the corresponding emission inventory is obtained by combining the inventories of the individual aircraft types and generations according to their market share.

Climate modelling

We use the non-linear climate-chemistry response model AirClim 25 , 26 to analyse the climate impact of the various scenarios. AirClim is a surrogate model, which relies on a multitude of pre-calculated responses to emissions with a global climate-chemistry model and has been verified against reference models to correctly simulate scenarios, such as flying lower or higher 26 . AirClim considers changes in concentration of CO 2 , water vapour, ozone, methane and the formation of contrail-cirrus, and takes their lifetimes, effects on the Earth radiation budget and eventually the changes in the near-surface temperature into account. The spatial resolution of the relation between emission location and response depends on the kind of effect and related atmospheric lifetimes. For CO 2 , with a very long atmospheric perturbation, the emission location is unimportant and hence CO 2 concentration changes are simulated in a box model. The relation between emission location and chemical concentration changes largely depends on the altitude and geographical location of the emission. The lifetime of aviation NO x and aviation ozone is in the order of several weeks and months, respectively 48 . Accordingly, chemical responses are dependent on emission altitude and latitude, whereas for short-term contrail-cirrus effects, the longitude is also taken into account. As a background atmosphere, we take the RCP2.6 scenario into account, assuming a world which tries to achieve the Paris Agreement. The effect of sustainable aviation fuel on contrail-cirrus properties is taken into account by utilising the results from Moore et al. 49 and Burkhardt et al. 16 : A linear scaling between SAF use and reduction of soot number particle emissions is assumed, taking into account the results from measurements, which indicate that a 50–50 blend reduces the number of emitted soot particulates by 50% 49 and the change in contrail-cirrus properties and lifetime changes the contrail-cirrus RF following the results of Burkhardt et al. 16 by parameterising their results in their Fig.  1f :

where \(\triangle {{RF}}^{{contr}}\) is the relative change in contrail-cirrus radiative forcing (dimensionless value between 0 and 1) and Δ pn the relative change in particle number emissions (dimensionless value between 0 and 1). Note that the formula is only valid for Δpn \(\ge 0.1\) .

The effect of SAF use on contrail-cirrus properties and lifetime changes are qualitatively in agreement with Caiazzo et al. 50 . The increase in RF when using SAF in comparison to a kerosene baseline as calculated by Caiazzo et al. 50 stems from the increase in the calculated potential contrail-cirrus coverage, which is caused in their calculations by the change in the Schmidt-Appleman criterion.

Monte–Carlo analysis

Uncertainties in climate impact estimates are quantified by using a Monte–Carlo Simulation. As indicated in Lee et al. 7 , 28 the climate impact of aviation emissions upon the atmosphere is associated with large uncertainties. The approach has been tested in Dahlmann et al. 26 and successfully applied to obtain a robust climate impact for the mitigation option Flying slower and lower 51 . Here we categorise the uncertainties into three groups following Dahlmann et al. 26 : (1) uncertainty in atmospheric residence time ( ± 20%), (2) strength of RF ( ± 5% for CO 2 , ± 10% for CH 4 , and ± 50% for H 2 O, O 3 (incl. PMO), and contrail-cirrus), (3) relation between RF and near-surface temperature change (climate sensitivity parameter; ±5% for CO 2 , ± 10% for CH 4 and contrail-cirrus, ±30% for H 2 O and O 3 (incl. PMO)). Hence, we consider 11 uncertainty parameters, which are drawn individually for each simulation. A total of 10,000 simulations are performed to assess the uncertainty ranges, which are displayed in Fig.  4 for the top-down scenarios. A total of 3400 simulations combined with nine different ECATS scenarios resulting in 30,600 simulations are utilised for the Monte–Carlo analysis employed in the ECATS scenarios.

Data availability

The scenario data and result data are available on Zenodo 10.5281/zenodo.4627860.

Code availability

The code for deriving the scenarios is given in an excel spreadsheet and available on Zenodo 10.5281/zenodo.4627860. The software code AirClim is confidential proprietary information of DLR. Therefore, the code cannot be made available to the public or the readers without any restrictions. Licensing of the code to third parties is conditioned upon the prior conclusion of a licensing agreement with DLR as licensor. The codes used for analysing the data and plotting the analysed data are available from the corresponding author upon reasonable request.

Kharina, A., Rutherford, D., Fuel efficiency trends for new commercial jet aircraft: 1960 to 2014 . White paper of the International Council on Clean Transportation (2015).

Cumpsty, N., Mavris, D., Kirby, M. Aviation and the environment: outlook, Ch. 1 in ICAO 2019 Environmental Report: Aviation and Environment, Destination Green – The Next Chapter, https://www.icao.int/environmental-protection/Documents/EnvironmentalReports/2019/ENVReport2019_pg24-38.pdf (2019).

Airbus, Global Market Forecast 2018-2037, Global Networks, Global Citizens , ISBN: 978-2-9554382-3-6 (2018).

Boeing, Commercial Market Outlook 2019–2038 , (Boeing, 2019).

ICAO (International Civil Aviation Organisation), Effects of novel coronavirus (COVID‐19) on civil aviation: economic impact analysis (ICAO, Air Transportation bureau, Montréal Canada, 2020).

Skeie, R. B. et al. Global temperature change from the transport sectors. Atmos. Environ. 43 , 6260–6270 (2009).

Article   ADS   CAS   Google Scholar  

Lee, D. S. et al. Transport impacts on atmosphere and climate: Aviation. Atmos. Environ. 44 , 4678–4734 (2010).

IPCC, Intergovernmental Panel on Climate Change: Special report on aviation and the global atmosphere (eds Penner, J. E., et al.) (Cambridge University Press, New York, NY, USA, 1999).

Brasseur, G.P. et al. Impact of Aviation on Climate: FAA’s Aviation Climate Change Research Initiative (ACCRI) Phase II . Bull. Amer. Meteor. Soc., 97, 561–583, https://doi.org/10.1175/BAMS-D-13-00089.1 (2016).

Schmidt, E. Die Entstehung von Eisnebel aus den Auspuffgasen von Flugmotoren. in: Schriften der deutschen Akademie der Luftfahrtforschung , Heft 44, pp 1–15 (Verlag R. Oldenbourg, München, 1941).

Appleman, H. The formation of exhaust condensation trails by jet engines. Bull. Am. Meteorol. Soc. 34 , 14–20 (1953).

Article   ADS   Google Scholar  

Schumann, U. On conditions for contrail formation from aircraft exhausts. Meterol. Z. 5 , 4–23 (1996).

Article   Google Scholar  

Bock, L. & Burkhardt, U. Contrail cirrus radiative forcing for future air traffic. Atmos. Chem. Phys. 19 , 8163–8174, https://doi.org/10.5194/acp-19-8163-2019 (2019).

Holmes, C. D., Tang, Q. & Prather, M. J. Uncertainties in climate assessment for the case of aviation NO. Proc. Natl Acad. Sci. 108 (27), 10997–11002, https://doi.org/10.1073/pnas.1101458108 (2011).

Article   ADS   PubMed   Google Scholar  

Grewe, V., Matthes, S., Dahlmann, K. The contribution of aviation NO x emissions to climate change: are we ignoring methodological flaws? Env. Res. Lett. , https://doi.org/10.1088/1748-9326/ab5dd7 (2019).

Burkhardt, U., Bock, L. & Bier, A. Mitigating the contrail cirrus climate impact by reducing aircraft soot number emissions. npj Clim. Atmos. Sci. 1 , 37, https://doi.org/10.1038/s41612-018-0046-4 (2018).

Article   CAS   Google Scholar  

Sausen, R., Matthes, S., Gierens, K. et al. The EU project ACACIA (Advancing Science for Aviation and Climate), in: Making Aviation Environmentally Sustainable, Book of Abstracts of the 3rd ECATS conference (eds Matthes, S: and Blum, A.) Vol. 1 ISBN: 978-1-910029-58-9, page 98 (2020).

European Commission, Flightpath 2050, Europe’s vision for aviation, report of the high level group on aviation research , Luxembourg: Publications Office of the European Union, ISBN 978-92-79-19724-6, https://doi.org/10.2777/50266 (2011).

ICAO, Annex 16 to the Convention on International Civil Aviation, Environmental Protection , Vol. IV Carbon Offsetting Scheme for International Aviation (CORSIA), First Edition, October 2018, ISBN 978-92-9258-611-9 (2018).

United Nations, UN Framework Convention on Climate Change, Adoption of the Paris Agreement, Conference of Parties, Twenty-first session Paris, FCCC/CP/2015/L.9/Rev.1, 30 November to 11 December 2015 (2015).

Randers, J. 2052: A Global Forecast for the Next Forty Years (Chelsea Green Publishing, White River Junction, VT, USA) ISBN-10: 1603584218, ISBN-13: 978-1603584210 (2012).

Grewe, V. et al. Mitigating the climate impact from aviation: achievements and results of the DLR WeCare Project, Aerospace 4 (34), 1–50, https://doi.org/10.3390/aerospace4030034 (2017).

Owen, B., and Lee, D. S. Allocation of international aviation emissions from scheduled air traffic – Future cases, 2005 to 2050 (report 3 of 3) (No. CATE-2006-3(C)-3). Manchester: Centre for Air Transport and the Environment (CATE) (2006).

Peeters, P. Tourism’s impact on climate change and its mitigation challenges: how can tourism become ‘climatically sustainable’? Delft University of Technology. https://doi.org/10.4233/uuid:615ac06e-d389-4c6c-810e-7a4ab5818e8d (2017).

Grewe, V. & Stenke, A. AirClim: an efficient climate impact assessment tool. Atmos. Chem. Phys. 8 , 4621–4639 (2008).

Dahlmann, K., Grewe, V., Frömming, C. & Burkhardt, U. Can we reliably assess climate mitigation options for air traffic scenarios despite large uncertainties in atmospheric processes. Trans. Res. Part D. 46 , 40–55, https://doi.org/10.1016/j.trd.2016.03.006 (2016).

Terrenoire, E., Hauglustaine, D. A., Gasser, T. & Penanhoat, O. The contribution of carbon dioxide emissions from the aviation sector to future climate change. Environ. Res. Lett. 14 , 084019 (2019).

Lee, D.S. et al. Aviation and global climate change in the 21st century. Atmos. Environ. 43 , 3520–537 (2009).

Faggiano, F., Vos, R., Baan, M. & van Dijk, R. Aerodynamic Design of a Flying V Aircraft, 5-9 June 2017, 17th AIAA Aviation Technology, Integration, and Operations Conference, https://doi.org/10.2514/6.2017-3589 (2017).

Yin, F., Gangoli Rao, A., Bhat, A. & Chen, M. Performance assessment of a multi-fuel hybrid engine for future aircraft. Aerosp. Sci. Technol. 77 , 217–227 (2018).

Gangoli Rao, A., Yin, F. & van Buijtenen, J. P. A hybrid engine concept for multi-fuel blended wing body. Aircr. Eng. Aerosp. Technol.: Int. J. 86 (6), 483–493 (2014).

Grewe, V. et al. Assessing the climate impact of the AHEAD multi-fuel blended wing body. Met. Z. 26 , 711–725, https://doi.org/10.1127/metz/2016/0758 (2017).

Felder, J.L., Hyun, D.K., Brown, G. V. Turboelectric Distributed Propulsion Engine Cycle Analysis for Hybrid-WingBody Aircraft, 47th AIAA Aerospace Sciences Meeting, 5th – 8th January, 2009, Orlando, Florida, AIAA 2009-1132, https://arc.aiaa.org/doi/abs/10.2514/6.2009-1132 , (2009).

Felder, J.L. NASA N3-X with Turboelectric Distributed Propulsion, GRC-E-DAA-TN19290, Disruptive Green Propulsion Technologies; November 17, 2014 - November 18, 2014; London; United Kingdom, https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150002081.pdf (2014).

Morlotti, C., Cattaneo, M., Malighetti, P. & Redondi, R. Multi-dimensional price elasticity for leisure and business destinations in the low-cost air transport market: evidence from easyJet. Tour. Manag. 61 , 23–34, https://doi.org/10.1016/j.tourman.2017.01.009 (2017).

World Economic Forum (WEF). Clean Skies for Tomorrow Sustainable Aviation Fuels as a Pathway to Net-Zero Aviation , Insight Report November 2020. http://www3.weforum.org/docs/WEF_Clean_Skies_Tomorrow_SAF_Analytics_2020.pdf (2020).

Terekhov, Ivan. Forecasting Air Passenger Demand between Settlements Worldwide Based on Socio Economic Scenarios , doctoral dissertation, Hamburg University of Technology (2017).

Ghosh, Robin, Ein Ansatz der quantitativen Zukunftsforschung als Grundlage der Systemanalyse für das globale Lufttransportsystem , doctoral dissertation, Hamburg University of Technology, published in German (2019).

ICAO, Onboard a sustainable future , Environmental report 2016, https://www.icao.int/environmental-protection/Pages/env2016.aspx (2016).

EUROCONTROL, Does taxing aviation really reduce emissions? https://www.eurocontrol.int/publication/does-taxing-aviation-reduce-emissions (2020).

ACARE, Strategic Research & Innovation Agenda . Vol. 1 (ACARE, 2012).

Schäfer, A., Evans, A. & Reynolds, T. et al. Costs of mitigating CO 2 emissions from passenger aircraft. Nat. Clim. Change 6 , 412–417, https://doi.org/10.1038/nclimate2865 (2016).

Hileman, J. I., Rosa Blanco, E. D. l., Bonnefoy, P. A., Carter N. A. The carbon dioxide challenge facing aviation. Prog. Aerospace Sci. 63 , 84-95 (2013).

Drela, M. Design Drivers of energy-efficient transport aircraft. SAE Int. J. Aerospace 4 (2) 602–618, https://doi.org/10.4271/2011-01-2495 (2011).

IATA, Technology Roadmap, 4th edition July 2013, (2013).

Kar, R., Bonnefoy, Philippe A., Hansman, R. J. Dynamics of Implementation of Mitigating Measures to Reduce CO 2 Emissions from Commercial Aviation. Report No. ICAT-2010-01, Massachusetts Institute of Technology (2010).

Linke, F., Grewe, V., Gollnick, V. The implications of intermediate stop operations on aviation emissions and climate. Meteorologische Zeitschrift , 26 (6), 697–709. https://doi.org/10.1127/metz/2017/0763 (2017).

Grewe, V. et al. Aircraft routing with minimal climate impact: The REACT4C climate cost function modelling approach (V1.0). Geosci. Model Dev. 7 , 175–201, https://doi.org/10.5194/gmd-7-175-2014 (2014).

Moore, R., Thornhill, K. & Weinzierl, B. et al. Biofuel blending reduces particle emissions from aircraft engines at cruise conditions. Nature 543 , 411–415, https://doi.org/10.1038/nature21420 (2017).

Article   ADS   CAS   PubMed   PubMed Central   Google Scholar  

Caiazzo, F., Agarwal, A., Speth, R. L. & Barrett, S. R. H. Impact of biofuels on contrail warming. Environ. Res. Lett. 12 , 114013 (2017).

Dahlmann, K. et al. Climate-Compatible Air Transport System - Climate Impact Mitigation Potential for Actual and Future Aircraft. Aerospace 3 , 38, https://doi.org/10.3390/aerospace3040038 (2016).

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Acknowledgements

The authors like to thank Dr. Christoph Kiemle for providing an internal review. The non-profit ECATS-Association IASBL (Environmentally Compatible Air Transportation System, http://www.ecats-network.eu/ ) promotes and supports its Members’ joint activities and interests in the field of aviation and environmental impact. Its higher-level aim is to help making aviation sustainable. This study was launched and performed by ECATS members.

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Volker Grewe, Arvind Gangoli Rao, Tomas Grönstedt, Carlos Xisto, Florian Linke, Joris Melkert, Jan Middel, Barbara Ohlenforst, Simon Blakey, Simon Christie, Sigrun Matthes & Katrin Dahlmann

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V.G. developed the paper idea, prepared the emission data excel sheet, and performed the AirClim simulations. A.G.R., T.G., C.X., F.L. and Jo.M. analysed the top-level objectives, gave advice on how to use them in the top-down emission calculation and developed the bottom-up scenario for technical improvements. Ja.M. and B.O. analysed the legislative objectives and advised on how to use them in the top-down emission calculation. S.B., S.C. and A.G.R. analysed the effects of SAF, their potential for future use, gave advice on how to use them in the top-down emission calculation and developed the SAF part of the bottom-up scenario. K.D. and S.M. supported the AirClim simulations and interpretation.

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Grewe, V., Gangoli Rao, A., Grönstedt, T. et al. Evaluating the climate impact of aviation emission scenarios towards the Paris agreement including COVID-19 effects. Nat Commun 12 , 3841 (2021). https://doi.org/10.1038/s41467-021-24091-y

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• 5 Volpe National Transportation Systems Center, Cambridge, MA, United States

The aviation sector seeks to reduce greenhouse gas (GHG) emissions, with manufacturers and airlines announcing “zero-emission” goals and plans. Reduced carbon aviation fuels are central to meeting these goals. However, current and near-term aircraft, which will remain flying for decades, are designed around the combustion of petroleum-based aviation kerosene (e.g., Jet A/A-1). Therefore, the industry has focused on the qualification and approval of synthesized (e.g., non-petroleum-based) aviation fuel components with maximum blend limit percentages to avoid the blended fuel having properties outside the accepted ranges for Jet A/A-1. The synthesized components approved for blending are not necessarily interchangeable with Jet A/A-1. They may lack certain required chemical components, such as aromatics, or may have other characteristics outside the allowable ranges. To ensure safety, these synthesized aviation fuel components are only qualified to be used in commercial aviation when blended up to approved limits. The sector seeks to move toward the capability of using 100% synthesized aviation fuels that also meet sustainability criteria, known as sustainable aviation fuels, or SAF. However, these fuels must be developed, assessed, and deployed appropriately. This paper explores key questions relating to the introduction of 100% SAF, concluding that:

• Near-term unblended synthesized aviation fuels must be “drop-in,” meaning they are compatible with existing aircraft and infrastructure.

• Stand-alone complete fuels could be qualified within 1–2 years, with blends of blending components to reach 100% synthesized fuels to follow.

• Sustainability criteria, while critical to sector acceptance, will continue to be assessed separately from technical performance.

Introduction

The aviation industry seeks cost-competitive synthesized aviation fuels with a carbon benefit and sustainability performance to counter the effects of price spikes, competition for finite oil supplies, and aviation’s high profile as a greenhouse gas (GHG) and particulate emitter. This decarbonization is additionally needed to meet long-term net-zero emissions goals ( ATAG, 2021 ). Current and near-term (10–20 years) aircraft will remain in operation for decades and are designed around aviation kerosene (e.g., Jet A/A-1). Technologies to increase the efficiency of new aircraft by a fleet average of 1–2% each year are offset by a 4–5% compound average annual travel growth rate ( Fleming and de Lepinay, 2019 ) leading to projected emissions increases ( IATA, 2019 ). Proposed “zero emissions” options, such as batteries ( Hepperle, 2012 ; Schäfer, et al., 2019 ) or cryogenic fuels, are of low technology readiness, have restricted range, and require new energy supply networks ( McKinsey and Co., 2020 ). However, reducing the carbon footprint of jet fuel reduces aviation’s impact on the environment now and in the long-term.

Since 2006, the Commercial Aviation Alternative Fuels Initiative (CAAFI ® ), a public-private partnership including United States government, aviation sector stakeholders, and aviation fuel supply chain participants, has worked to enhance energy security and environmental sustainability for aviation with alternative jet fuels by facilitating their deployment in the marketplace ( CAAFI, 2021 ). Initially, due to safety and compatibility concerns, CAAFI and the industry focused on qualification of synthesized fuel blending components that come from sources other than petroleum to be added to conventional aviation fuel sourced from petroleum, which are qualified by ASTM D4054 for use in ASTM D7566 ( ASTM International, 2021 ) (see Figure 1 ). These blending components are limited to a maximum blend percentage to ensure that all blended fuels properties are within accepted ranges for Jet A/A-1 (particularly aromatic content) ( Zschocke et al., 2012 ). Thus, the resultant blended fuels are interchangeable with unblended Jet A/A-1 regarding handling, operability, and safety and referred to as “drop-in” aviation fuels ( Colket., et al., 2016 ). The blended fuel is re-identified as Jet A/A-1 for transport, storage, purchase, and use under the ASTM D1655 petroleum-based jet fuel specification ( ASTM International, 2020 ).

FIGURE 1 . Types of synthesized fuel blendstocks and their ability to be used as drop-in fuels as blendstocks or as stand-alone fuel. The red apples indicate fuels that can be drop-in, green apples are similar to drop-ins and may be usable in some existing aircraft but would require modification of the existing specifications and possibly infrastructure, and bananas indicate fuels that are completely different from petroleum-based kerosene and could not be used as stand-alone jet fuel in current infrastructure or equipment.

The existing synthesized blending components are not necessarily sustainable, as environmental, social, and economic performance requirements are not part of ASTM qualification. To address sustainability goals, aviation stakeholders and International Civil Aviation Organization (ICAO) member States put a process in place to evaluate production, feedstock, land-use, social impact, and life-cycle carbon footprint of various possible paths, and consider relevant environmental, social, and economic risks, formalized via the ICAO Carbon Off-setting and Reduction Scheme for International Aviation (CORSIA) ( ICAO, 2021a ). Fuels certified as sustainable under this, and similar approaches, are called sustainable aviation fuels (SAF). The synthesized blending components complying with ASTM D7566 can be made according to sustainability criteria to become SAF.

The current production of SAF globally is much less than 1% ( Csonka, 2020 ). However, the United States has set a target of producing 3 billion gallons of SAF per year by 2030, totaling about 10% of anticipated annual jet fuel consumption, and targets complete replacement of petroleum-based jet fuel by 2050 ( U.S. White House, 2021 ), and SAF mandates are in place or are under consideration globally ( Malicier, 2021 ). While overall SAF availability is currently low, 100% SAF may be available at particular locations very soon. Airfields could provide limited amounts of 100% SAF to those willing to pay for that distinction, such as private jet owners. Or a particular airport or nation could set goals to fuel a certain number of flights with 100% SAF, possibly within the decade. Aircraft manufacturers have made commitments to compatibility with 100% SAF by 2030 even in the absence of an agreed upon definition for 100% SAF ( Boeing, 2021 ). Furthermore, the qualification of 100% SAF would eliminate the need for controlled blending of that SAF into the fuel pool, reducing supply chain complexity. Thus, the 100% synthesized fuel definition and qualification process should happen now to prepare for these future needs.

The ability to use neat SAF (without blending with conventional fuel), or “100% SAF” could further reduce aviation’s global GHG generation and human health impacts. Additionally, 100% SAFs containing reduced or no aromatics reduce non-volatile particulate matter (nvPM) emissions, which are linked to contrail formation ( Voigt, et al., 2021 ), and contrails are suggested to contribute more to aviation radiative forcing than CO 2 emissions ( Lee, et al., 2021 ). Finally, 100% SAF has very low levels of sulfur, which leads to low levels of sulfur oxide (SOx) emissions ( Moore, et al., 2015 ). Thus 100% SAF would reduce aviation’s GHG production, contrails, and SOx emissions, all significant environmental benefits.

However, the synthesized blending components approved to date are not by themselves necessarily drop-in or interchangeable with Jet A/A-1, and so cannot be used as 100% synthesized fuels alone ( Figure 1 ). They may lack aromatics required in legacy aircraft and aircraft engines for seal compatibility ( Anuar et al., 2021 ). Properties such as freeze point or mass density may be near the limits for the accepted Jet A/A-1 range ( Edwards, 2017 ; Colket and Heyne, 2021 ). They may contain a restricted number of chemical species or have limited carbon number range and not meet the Jet A/A-1 distillation curve requirements ( Bell et al., 2018 ; Won et al., 2019 ). These differences may impact the performance, operability, and/or safety of some aircraft models and engines currently flying ( Bell et al., 2018 ; Won et al., 2019 ).

The challenge is to achieve 100% SAF that meets all the safety and operability requirements of the ASTM qualification process, as well as the affordability and sustainability goals of the industry. In this paper, we explain the importance of the “drop-in” requirement, highlight potential approaches to achieve fully synthesized aviation fuels and provide perspectives on the viability of those approaches, and discuss how sustainability criteria can be layered onto fully synthesized jet fuels to create complete sustainable aviation fuels.

Importance of “Drop-In” as a Requirement

Jet A/A-1s are unique mixtures of hydrocarbons that cannot be simply defined by a certain chemical composition. The characteristics of Jet A/A-1 are derived from petroleum going through modern refinery processes, e.g., distillation, hydrotreatment, catalytic reforming, etc. Specifications and tests have been developed to measure performance properties such as net heat of combustion, thermal stability, viscosity, distillation curve, freezing point, flash point, smoke point, density, lubricity, aromatic content, sulfur content, etc. ( Hemighaus, et al., 2007 ). Over time, the property specifications have been reviewed and updated, testing improved, and new specifications added. The intent has been to make Jet A/A-1 the safest and most suitable possible fuel for aviation.

Aviation gas turbines are designed to operate on and utilize the properties of Jet A/A-1 ( Heyne et al., 2021 ). Jet A/A-1 properties are tightly linked to the reliability and safety of aviation. For example, the flash point of Jet A/A-1 is such that a match will not ignite the fuel at room temperature ( ASTM International, 2020 ). Yet gas turbines can ignite the fuel at conditions as cold as Fairbanks, Alaska, or as hot as Saudi Arabia, and can be re-lit in mid-flight at 30,000 feet.

The ASTM qualification process (ASTM D4054) has been adapted to enable synthesized fuel approvals ( Rumizen, 2021 ). Thus far, all synthesized blended fuel has been required to be drop-in and meet every specification for petroleum-based Jet A/A-1 ( ICAO, 2018 ), because it was not known what specific properties of the fuel were critical for operability and safety and which properties could be relaxed. An 8% minimum aromatics content was set to ensure compatibility for nitrile seals. Combustor performance includes factors such as cold weather ignition, altitude relight, lean blow-out characteristics, interactions with combustor acoustics and dynamics, flame stability, flame luminosity, heat release patterns, and so on. Safety and reliability in external components must consider such factors as cold fuel viscosity system performance, vapor pressure characteristics and impact on pump performance, cavitation potential, low lubricity, seal compatibility, thermal stability and tendency to varnish, icing characteristics, entrained water, biocide compatibility, flammability, and other criteria ( Colket et al., 2016 ; Colket and Heyne, 2021 ).

Although jet fuel combustion has been studied for decades, unknowns remain. For example, critical factors for altitude relight are not well characterized: atomization is a complex interplay of fuel surface tension, viscosity, density, and air temperature and pressure; fuel vapor pressure, and molecular composition are also important ( Peiffer et al., 2019 ; Boehm et al., 2021 ). Research, such as the National Jet Fuels Combustion Program ( Colket et al., 2016 ; CAAFI R&D Team, 2019 ), has added to the understanding of the interaction of fuel chemical composition and physical properties with combustion. However, that understanding is not complete, and any uncertainty may impact safety.

Jet fuel is not only used for combustion in the aircraft. Fuel is used to exchange heat with the oil, to power fueldraulic actuators, and to lubricate (or at least not excessively wear) pumps ( Heyne et al., 2021 ). Additionally, in legacy aircraft, the nitrile seals are sensitive to fuel composition and their performance might be impacted ( Graham, et al., 2013 ). To be drop-in, the fuel must satisfy these functionalities as well.

The industry position is that safety for all past, present and future aircraft must be addressed in the specification of any fuel. For 100% synthesized fuels, new requirements may need to be added to the specification. For example, the NJFCP suggested several characteristics as potentially critical to the safety and operability of jet fuels, such as derived cetane number ( Colket et al., 2016 ; Stachler et al., 2020 ; Boehm et al., 2021 ). These new requirements need to be considered, researched and verified. Similarly, to better enable synthesized fuel cost-effectiveness, it may be desired to redefine or add other specifications and properties, which would require additional research and verification to insure 100% drop-in compatibility.

Without knowing the impact of a fuel not meeting all Jet A/A-1 characteristics, a proposed non-drop-in fuel must be limited to validated applications. A non-drop-in fuel requires separate handling, storage, and logistics, and must be compatible with that separate infrastructure (e.g., fueling trucks, hydrant system, tanks, etc.). It requires safety measures to eliminate any possible mistakes in fuel identity. It may require separate fittings, separate fuel tanks, unique identification procedures, and testing similar to procedures used for gasoline and diesel fuels at gas stations, with the potential for much more severe safety consequences if mistakes are made.

Thus, 100% synthesized fuels should be drop-in for all aviation applications, at least in the short- to mid-term. However, the specifications for Jet A/A-1 may be refined and expanded as additional learning is acquired.

Approaches to Achieve 100% Synthesized Jet Fuel

Here are approaches to achieve 100% synthesized jet fuel that should be considered.

1. Replicate All Jet A/A-1 Properties in a Single Fuel. In 1999, Sasol developed Fischer-Tropsch (FT) fuels from coal, first as no more than a 50% blending component, then, with a process change to include aromatics to replicate all Jet A/A-1 properties, as a 100% fully synthesized jet fuel. Extensive testing, including long duration engine tests, ensured that all the required Jet A/A-1 performance characteristics were met. Similarly, there are current biomass-based pathways, some of which are FT processes, that replicate all Jet A/A-1 performance properties ( MODUK, 2020 ).

2. Replicate All Jet A/A-1 Properties in a Blended Fuel. Current synthesized blending components must be combined with conventional Jet A/A-1 to meet all necessary performance properties including aromatic content. In the future, a synthesized blendstock with no aromatics could be combined with a synthesized blendstock with aromatic content to achieve a 100% synthesized that meets all required Jet A/A-1 properties. Conceivably, as many blending components as needed could be used to replicate the properties of Jet A/A-1.

3. Substitute for Aromatics or Reduce Aromatics Requirement. The requirement for aromatics is linked to the performance of nitrile seals in older engines: without the aromatics, the seals shrink and fuel leaks occur. Other molecules, such as cycloparaffins, can act like aromatics from seal performance perspective ( Graham, et al., 2013 ). This potential is currently under evaluation. Additionally, the 8% lower limit for aromatics is known to be safe with margin ( Heminghaus, et al., 2006 ). If 100% SAF with low or no aromatics is sought to reduce nvPM, the specification for aromatics could be reduced or removed, while retaining all other Jet A/A-1 performance properties. Research could identify substitute molecules and the true lower limit of aromatic content.

4. Remove the Requirement for Seal-Swelling Components (non-drop-in). Modern engines have replaced nitrile seals with better performing fluorocarbons and fluorosilicone seals. Engine and flight tests have been performed on “neat” SAFs ( Applied Research Associates, 2016 ; Airbus, 2021 ; Palmer, 2021 ; Rolls-Royce, 2021 ). Thus, a fuel without aromatics, e.g. 100% paraffinic, that matches all other specifications for Jet A/A-1 could be considered for use in compatible aircraft. However, this fuel would not be “drop-in” for legacy aircraft and would face the reliability and safety concerns outlined previously.

5. Redefine Jet Fuel Requirements. It is possible that not all the current specifications for Jet A/A-1 are necessary for engine and aircraft performance. Changing, removing, or adding alternative requirements may make it easier to produce improved synthesized fuels from biological sources. Bacteria and yeast tend to produce very specific chemicals rather than a broad range of chemical components like petroleum-based Jet A/A-1. For example, the “Hydroprocessed Fermented Sugars to Synthetic Isoparaffins” process (ASTM D7566 A3, HFS-SIP) produces solely farnesene, a 15-carbon molecule ( ASTM International, 2021 ), Extensive research and testing are needed to define specification modifications and assure the reliability and safety of fuels produced to the redefined specifications.

Currently, the first two options (100% synthesized fuels from a single or blended fuels) could be near term paths, while Options 3 and 5 (redefining jet fuel requirements based on further research) have future potential with sufficient research and learning. Option 4 (non-drop-in fuels) is not desirable since it would require significant, expensive changes to aircraft equipment and infrastructure.

Thus far, the ASTM D4054 specification process has been viewed from the perspective of comparison to a conventional fuel ( Rumizen, 2021 ). A key question is whether and how this process could be changed to better enable 100% synthesized fuels. Does the ASTM specification process need to become more stringent to capture unknowns that have been ignored because they have been unknown for petroleum-based fuels? Or can it be simplified due to more physics and chemistry-based understanding? Recently an optional prescreening approach has been formalized that uses only a small quantity of fuel to perform analyses that help identify the suitability and potential gaps for a particular proposed fuel ( CAAFI R&D Team, 2019 ). It may be possible to make other changes to make the process more effective and efficient while continuing to ensure the safety of aviation fuels.

The ASTM qualification process should be continuously reviewed and improved as additional learning with respect to 100% synthesized fuels and their properties is acquired, which in the longer term would enable modifications to jet fuel specifications needed for Options 3 and 5 above.

Sustainability of 100% Synthesized Jet Fuel

To further refine the definition of a 100% synthesized aviation fuel to 100% Sustainable Aviation Fuel, any SAF needs to be produced in a way that demonstrably meets sustainability criteria to ensure environmental, social, and economic performance. Sustainability requirements are applied to synthesized aviation fuels separately from the technical, safety, and performance characteristics that qualify a fuel to be used in aviation under the ASTM specifications; therefore, a 100% synthesized fuel is not necessarily a 100% sustainable aviation fuel, even if it comes from a renewable feedstock. Nevertheless, the sustainability performance is critical to the value proposition of these fuels and must be ensured.

There are existing approaches to evaluate the environmental performance of SAF, including regulatory scheme compliance, such as qualification for the United States Renewable Fuel Standard ( US Environmental Protection Agency, 2010 ) or California’s Low Carbon Fuel Standard ( State of California, 2020 ). The full sustainability (environmental, social, and economic) performance of SAF can be evaluated and assured through the use of voluntary sustainability certification schemes, such as those used for CORSIA qualification ( ICAO, 2021b ) or the European Union’s Renewable Energy Directive ( European Union, 2021 ). The certification approach assures sustainable production of SAF to the extent possible.

Currently, there is no consistent definition for the use of the term SAF. The aviation sector must continue to decide how to evaluate the sustainability of SAF, which sustainability factors to address, and whether the existing approaches for regulatory compliance and voluntary certification are sufficient to qualify fuels as sustainable. At a minimum, it is reasonable to expect that the aviation sector will call fuels SAF that meet the sustainability criteria agreed upon by ICAO for CORSIA ( ICAO, 2021c ), as these are clearly defined and can be used to meet existing emissions obligations. Some nations and some airlines or aviation groups may commit to greater sustainability requirements or specific requirements in isolation (e.g., the United States Grand Challenge defines SAF as having a 50% reduction in carbon intensity for SAF, whereas CORSIA requires a 10% reduction). A minimum standard for labeling fuels as SAF will reduce confusion and ensure that SAF achieve the sustainability performance on which their value proposition depends.

It should be noted that sustainability certification as currently implemented does not fully address important societal choices and tradeoffs that are beyond the scope of aviation, such as interactions among economies, competition/balance with other renewable energy approaches, how wastes should be credited as feedstocks, and how these considerations should be valued both locally and internationally. Previous studies have concluded that biofuels in particular may have issues of overall scalability and environmental impact if deployed at a global level as a primary fossil energy replacement solution ( de Castro et al., 2014 ; Gomiero, 2015 ). These factors are not addressed by certification at the fuel/feedstock producer level. The choice of which sectors of the economy use bio-based fuels and the scale of their use are societal decisions to which the aviation sector can contribute.

While the ongoing ICAO Long Term Aspirational Goals (LTAG) exercise is considering technology horizons in aviation of 2050 and 2070 ( ICAO, 2021d ), action must be taken now, as the actions with the greatest impacts will take time to penetrate the global aviation market.

Considering the options presented herein, the definition and qualification process for Option 1—Replicate All Jet A/A-1 Properties in a Single Fuel—could be achievable within 2 years. This approach has already been pioneered by SASOL, and other fuels are following that pathway (DefStan 91-091). Option 2—Replicate All Jet A/A-1 Properties in a Blended Fuel—could follow closely, a year or two behind, since in essence, it is the pathway being followed for current blended SAFs. The ASTM Task Force AC598 (Standardization of Jet Fuel Fully Comprised of Synthesized Hydrocarbons) has begun work to consider the definition and qualification process for Options 1 and 2 ( Polek, 2021 ); that process will need to take into account the challenges laid out in this paper.

Options 3 and 5 that would modify Jet A/A-1 properties require significant research and testing before the safety of either replacing or lowering the aromatics is assured, and any redefinition of, or addition to, current specifications or standards is made. Future research must include investigation of how fuel compositions interact with the operability, reliability and safety of aircraft and flight. Finally, since Option 4—Remove the Requirement for Seal-Swelling Components—leads to a non-drop-in fuel, it is not likely to be supported by industry.

Author Contributions

The individual authors listed here were all engaged in the drafting, revision, and referencing of this mini-review based on individual and organizational experience and knowledge.

KL and PH participation was funded by the United States Federal Aviation Administration Office of Environment and Energy through agreement number 693KA9-20-N-00013 under the supervision of Nathan Brown. JH work was funded by United States Federal Aviation Administration Office of Environment and Energy through ASCENT, the FAA Center of Excellence for Alternative Jet Fuels and the Environment, Project 34 through FAA Award Number 13-C-AJFE-UD-024 under the supervision of Anna Oldani. Any opinions, findings, conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the FAA. Open access publication fees are provided by the FAA through the Volpe National Transportation Systems Center.

Conflict of Interest

Author SK was employed by the company Pratt & Whitney. Author GA was employed by the Company General Electric. Author JE was employed by the company Boeing.

The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Airbus (2021). An A350 Fuelled by 100% SAF Just Took off. Available at: https://www.airbus.com/newsroom/stories/A350-fuelled-by-100-percent-SAF-just-took-off.html (Accessed September 9, 2021).

Google Scholar

Anuar, A., Undavalli, V. K., Khandelwal, B., and Blakey, S. (2021). Effect of Fuels, Aromatics and Preparation Methods on Seal Swell. Aeronaut. J. 125, 1542–1565. doi:10.1017/aer.2021.25

CrossRef Full Text | Google Scholar

Applied Research Associates (2016). ARA's History-Making Sustainable Aviation Fuel (SAF) Technology. Available at: https://www.ara.com/products/readijet/ (Accessed September 9, 2021).

ASTM International (2020). D1655-20d: Standard Specification for Aviation Turbine Fuels . West Conshohocken, PA, USA: ASTM International .

ASTM International (2021). D7566-20c: Standard Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbons . West Conshohocken, PA, USA . doi:10.1520/D7566-20C

ATAG (2021). Waypoint 2050. Air Transport Action Group. Available at: https://aviationbenefits.org/environmental-efficiency/climate-action/waypoint-2050/ .

Bell, D. C., Heyne, J. S., Won, S. H., and Dryer, F. L. (2018). “The Impact of Preferential Vaporization on Lean Blowout in a Referee Combustor at Figure of Merit Conditions,” in Proceedings of the ASME 2018 Power Conference collocated with the ASME 2018 12th International Conference on Energy Sustainability and the ASME 2018 Nuclear Forum , Lake Buena Vista, FL, USA , June 24–28, 2018 ( ASME ). doi:10.1115/POWER2018-7432

Boehm, R. C., Colborn, J. G., and Heyne, J. S. (2021). Comparing Alternative Jet Fuel Dependencies between Combustors of Different Size and Mixing Approaches. Front. Energ. Res. 9, 701901. doi:10.3389/fenrg.2021.701901

Boeing (2021). Boeing Commits to Deliver Commercial Airplanes Ready to Fly on 100% Sustainable Fuels. Available at: https://boeing.mediaroom.com/2021-01-22-Boeing-Commits-to-Deliver-Commercial-Airplanes-Ready-to-Fly-on-100-Sustainable-Fuels (Accessed December 8, 2021).

CAAFI R&D Team (2019). Prescreening of Synthesized Hydrocarbons Intended for Candidates as Blending Components for Aviation Turbine Fuels (Aka Alternative Jet Fuels or AJF). Available at: https://www.caafi.org/tools/docs/CAAFI_RD_Prescreening_Guidance_Document_v1.0.pdf .

CAAFI (2021). About CAAFI. Available at: https://caafi.org/about/caafi.html .

Colket, M. B., Heyne, J., Rumizen, M., Edwards, J. T., Gupta, M., Roquemore, W. M., et al. (2016). An Overview of the National Jet Fuels Combustion Program. AIAA J. , 1087–1104. doi:10.2514/1.J055361

Colket, M., and Heyne, J. (2021). Fuel Effects on Operability of Aircraft Gas Turbine Combustors. Prog. Astronautics Aeronautics , 1–534. doi:10.2514/4.106040

Csonka, S. (2020). Aviation’s Market Pull for SAF (Sustainable Aviation Fuel). Available at: https://www.caafi.org/focus_areas/docs/CAAFI_SAF_Market_Pull_from_Aviation.pdf (Accessed September 3, 2021).

de Castro, C., Carpintero, Ó., Frechoso, F., Mediavilla, M., and de Miguel, L. J. (2014). A Top-Down Approach to Assess Physical and Ecological Limits of Biofuels. Energy 64 (1), 506–512. doi:10.1016/j.energy.2013.10.049

Edwards, J. (2017). “Reference Jet Fuels for Combustion Testing,” in 55th AIAA Aerospace Sciences Meeting , Grapevine, TX , January 9-13, 2017 ( American Institute of Aeronautics and Astronautics ). Available at: https://www.caafi.org/news/pdf/Edwards_AIAA-2017-0146_Reference_Jet_Fuels.pdf .

European Union (2021). Voluntary Schemes. Available at: https://ec.europa.eu/energy/topics/renewable-energy/biofuels/voluntary-schemes_en .

Fleming, G., and de Lepinay, I. (2019). Environmental Trends in Aviation to 2050. International Civil Aviation Organization (ICAO). Available at: https://www.icao.int/environmental-protection/Documents/EnvironmentalReports/2019/ENVReport2019_pg17-23.pdf .

Gomiero, T. (2015). Are Biofuels an Effective and Viable Energy Strategy for Industrialized Societies? A Reasoned Overview of Potentials and Limits. Sustainability 7 (7), 8491–8521. doi:10.3390/su7078491

Graham, J. L., Rahmes, T. F., Kay, M. C., Belières, J.-P., Kinder, J. D., Millett, S. A., et al. (2013). Impact of Alternative Jet Fuel and Fuel Blends on Non- Metallic Materials Used in Commercial Aircraft Fuel Systems. Available at: https://www.faa.gov/about/office_org/headquarters_offices/apl/research/aircraft_technology/cleen/reports/media/Boeing_Alt_Fuels_Final.pdf .

Hemighaus, G., Boval, T., Bacha, J., Barnes, F., Franklin, M., Gibbs, L., et al. (2007). Aviation Fuels Technical Review . San Ramon, CA: Chevron Products Company .

Heminghaus, G., Boval, T., Bosley, C., Organ, R., Lind, J., Brouette, R., et al. (2006). Alternative Jet Fuels, Addendum 1 to Aviation Fuels Technical Review . San Ramon, CA: Chevron Corporation .

Hepperle, M. (2012). “Electric Flight – Potential and Limitations,” in AVT-209 Workshop on Energy Efficient Technologies and Concepts Operation , Lisbon, Portugal . Available at: https://www.sto.nato.int/publications/STO%20Meeting%20Proceedings/Forms/Meeting%20Proceedings%20Document%20Set/docsethomepage.aspx?ID=30995&FolderCTID=0x0120D5200078F9E87043356C409A0D30823AFA16F602008CF184CAB7588E468F5E9FA364E05BA5&List=7e2cc123-6186-4c30-8082-1ba072228ca7&RootFolder=%2Fpublications%2FSTO%20Meeting%20Proceedings%2FSTO%2DMP%2DAVT%2D209 (Accessed August 10, 2021).

Heyne, J., Rauch, B., Le Clercq, P., and Colket, M. (2021). Sustainable Aviation Fuel Prescreening Tools and Procedures. Fuel 290, 120004. doi:10.1016/j.fuel.2020.120004

IATA (2019). Aircraft Technology Roadmap to 2050. International AIr Transport Association (IATA). Available at: https://www.iata.org/contentassets/8d19e716636a47c184e7221c77563c93/technology20roadmap20to20205020no20foreword.pdf .

ICAO (2021a). Carbon Offsetting and Reduction Scheme for International Aviation (CORSIA). Montreal, Quebec, Canada. Available at: https://www.icao.int/environmental-protection/CORSIA/Pages/default.aspx .

ICAO (2021b). CORSIA Approved Sustainability Certification Schemes. Available at: https://www.icao.int/environmental-protection/CORSIA/Documents/ICAO%20document%2004%20-%20Approved%20SCSs.pdf .

ICAO (2021c). CORSIA Eligible Fuels. Available at: https://www.icao.int/environmental-protection/CORSIA/Pages/CORSIA-Eligible-Fuels.aspx .

ICAO (2021d). Feasibility of a Long Term Global Aspirational Goal for International Aviation. Available at: https://www.icao.int/environmental-protection/Pages/LTAG.aspx .

ICAO (2018). Sustainable Aviation Fuels Guide Version 2. Available at: https://www.icao.int/environmental-protection/Documents/Sustainable%20Aviation%20Fuels%20Guide_100519.pdf .

Lee, D. S., Fahey, D. W., Skowron, A., Allen, M. R., Burkhardt, U., Chen, Q., et al. (2021). The Contribution of Global Aviation to Anthropogenic Climate Forcing for 2000 to 2018. Atmos. Environ. 244, 117834. doi:10.1016/j.atmosenv.2020.117834

PubMed Abstract | CrossRef Full Text | Google Scholar

Malicier, V. (2021). Fuel Suppliers Urge Governments to Impose SAF Blending Mandates . Editor J. Fox (London, UK: S&P Global Platts ).

McKinsey and Co (2020). Hydrogen-powered Aviation: A Fact-Based Study of Hydrogen Technology, Economics, and Climate Impact by 2050. Brussels: European Commission. Available at: https://www.fch.europa.eu/sites/default/files/FCH%20Docs/20200507_Hydrogen%20Powered%20Aviation%20report_FINAL%20web%20%28ID%208706035%29.pdf .

MODUK (2020). DEF STAN 91-091, Revision I12. MODUK - British Defense Standards (MODUK). Available at: https://www.academia.edu/28340693/Ministry_of_Defence_Defence_Standard_91_91 .

Moore, R. H., Shook, M., Beyersdorf, A., Corr, C., Herndon, S., Knighton, W. B., et al. (2015). Influence of Jet Fuel Composition on Aircraft Engine Emissions: A Synthesis of Aerosol Emissions Data from the NASA APEX, AAFEX, and ACCESS Missions. Energy Fuels 29 (4), 2591–2600. doi:10.1021/ef502618w

Palmer, W. (2021). United Flies World’s First Passenger Flight on 100% Sustainable Aviation Fuel Supplying One of its Engines. GE Future of Flight. Available at: https://www.ge.com/news/reports/united-flies-worlds-first-passenger-flight-on-100-sustainable-aviation-fuel-supplying-one .

Peiffer, E. E., Heyne, J. S., and Colket, M. (2019). Sustainable Aviation Fuels Approval Streamlining: Auxiliary Power Unit Lean Blowout Testing. AIAA J. 57 (11), 4854–4862. doi:10.2514/1.J058348

Polek, G. (2021). OEMs All in on SAF Development. AIN Online. Available at: https://www.ainonline.com/aviation-news/air-transport/2021-06-01/oems-all-saf-development .

Rolls-Royce (2021). Rolls-Royce Conducts First Tests of 100% Sustainable Aviation Fuel for Use in Business Jets. Available at: https://www.rolls-royce.com/https://www.rolls-royce.com/media/press-releases/2021/01-02-2021-business-aviation-rr-conducts-first-tests-of-100-precent-sustainable-aviation-fuel.aspx (Accessed September 9, 2021).

Rumizen, M. A. (2021). Qualification of Alternative Jet Fuels. Front. Energ. Res. 9, 760713. doi:10.3389/fenrg.2021.760713

Schäfer, A. W., Barrett, S. R. H., Doyme, K., Dray, L. M., Gnadt, A. R., Self, R., et al. (2019). Technological, Economic and Environmental Prospects of All-Electric Aircraft. Nat. Energ. 4, 160–166. doi:10.1038/s41560-018-0294-x

Stachler, R., Heyne, J., Stouffer, S., and Miller, J. (2020). Lean Blowoff in a Toroidal Jet-Stirred Reactor: Implications for Alternative Fuel Approval and Potential Mechanisms for Autoignition and Extinction. Energy Fuels 34 (5), 6306–6316. doi:10.1021/acs.energyfuels.9b01644

State of California (2020). Low Carbon Fuel Standard Regulation . Sacramento, CA, USA: State of California .

U.S. White House (2021). FACT SHEET: Biden Administration Advances the Future of Sustainable Fuels in American Aviation. Washington, DC, USA. Available at: https://www.whitehouse.gov/briefing-room/statements-releases/2021/09/09/fact-sheet-biden-administration-advances-the-future-of-sustainable-fuels-in-american-aviation/ (Accessed December 07, 2021).

US Environmental Protection Agency (2010). 40 CFR Part 80, Subpart M. Renewable Fuel Stnadard . USA. Available at: https://www.epa.gov/renewable-fuel-standard-program/regulations-and-volume-standards-renewable-fuel-standards (Accessed August 10, 2021).

Voigt, C., Kleine, J., Sauer, D., Moore, R. H., Bräuer, T., Le Clercq, P., et al. (2021). Cleaner Burning Aviation Fuels Can Reduce Contrail Cloudiness. Commun. Earth Environ. 2 (114). doi:10.1038/s43247-021-00174-y

Won, S. H., Rock, N., Lim, S. J., Nates, S., Carpenter, D., Emerson, B., et al. (2019). Preferential Vaporization Impacts on Lean Blow-Out of Liquid Fueled Combustors. Combustion and Flame 205, 295–304. doi:10.1016/j.combustflame.2019.04.008

Zschocke, A., Scheuermann, S., and Ortner, J. (2012). High Biofuel Blends in Aviation (HBBA): ENER/C2/2012/420-1 Final Report. Available at: https://ec.europa.eu/energy/sites/ener/files/documents/final_report_for_publication.pdf .

Keywords: ASTM fuel qualification, drop-in, fungible, sustainable aviation fuel, synthesized aviation fuel

Citation: Kramer S, Andac G, Heyne J, Ellsworth J, Herzig P and Lewis KC (2022) Perspectives on Fully Synthesized Sustainable Aviation Fuels: Direction and Opportunities. Front. Energy Res. 9:782823. doi: 10.3389/fenrg.2021.782823

Received: 24 September 2021; Accepted: 31 December 2021; Published: 24 January 2022.

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Copyright © 2022 Kramer, Andac, Heyne, Ellsworth, Herzig and Lewis. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Kristin C. Lewis, [email protected]

This article is part of the Research Topic

Sustainable Aviation Fuels

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Airports and environmental sustainability: a comprehensive review

Fiona Greer 1,2 , Jasenka Rakas 1 and Arpad Horvath 1

Published 8 October 2020 • © 2020 The Author(s). Published by IOP Publishing Ltd Environmental Research Letters , Volume 15 , Number 10 Citation Fiona Greer et al 2020 Environ. Res. Lett. 15 103007 DOI 10.1088/1748-9326/abb42a

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1 Department of Civil and Environmental Engineering, University of California, Berkeley, CA 94720, United States of America

2 Author to whom any correspondence should be addressed.

Fiona Greer https://orcid.org/0000-0001-9453-0640

Jasenka Rakas https://orcid.org/0000-0001-9694-3588

• Received 23 June 2020
• Accepted 1 September 2020
• Published 8 October 2020

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Over 2500 airports worldwide provide critical infrastructure that supports 4 billion annual passengers. To meet changes in capacity and post-COVID-19 passenger processing, airport infrastructure such as terminal buildings, airfields, and ground service equipment require substantial upgrades. Aviation accounts for 2.5% of global greenhouse gas (GHG) emissions, but that estimate excludes airport construction and operation. Metrics that assess an airport's sustainability, in addition to environmental impacts that are sometimes unaccounted for (e.g. water consumption), are necessary for a more complete environmental accounting of the entire aviation sector. This review synthesizes the current state of environmental sustainability metrics and methods (e.g. life-cycle assessment, Scope GHG emissions) for airports as identified in 108 peer-reviewed journal articles and technical reports. Articles are grouped according to six categories (Energy and Atmosphere, Comfort and Health, Water and Wastewater, Site and Habitat, Material and Resources, Multidimensional) of an existing airport sustainability assessment framework. A case study application of the framework is evaluated for its efficacy in yielding performance objectives. Research interest in airport environmental sustainability is steadily increasing, but there is ample need for more systematic assessment that accounts for a variety of emissions and regional variation. Prominent research themes include analyzing the GHG emissions from airfield pavements and energy management strategies for airport buildings. Research on water conservation, climate change resilience, and waste management is more limited, indicating that airport environmental accounting requires more analysis. A disconnect exists between research efforts and practices implemented by airports. Effective practices such as sourcing low-emission electricity and electrifying ground transportation and gate equipment can in the short term aid airports in moving towards sustainability goals. Future research must emphasize stakeholder involvement, life-cycle assessment, linking environmental impacts with operational outcomes, and global challenges (e.g. resilience, climate change adaptation, mitigation of infectious diseases).

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List of acronyms

1. introduction.

Airport infrastructure is a vital component of society's transportation network. There are more than 40 000 airports worldwide (CIA 2016 ). Around 2500 airports processed over 4 billion passengers in 2018 (IATA 2018 ). The onset of COVID-19 has drastically decreased air traffic levels (IATA 2020 ). It is likely that air travel will recover over the next couple of years and continue to rise. In the United States, massive investment is required (ASCE 2017 , ACC 2020 ) to modernize and retrofit aged, inadequate airport infrastructure (e.g. terminals, airfields, service equipment). Similar expansion projects and necessary reconfiguration projects for post COVID-19 processing of passengers are occurring worldwide. Airports are not solely transport nodes. The onset of 'airport cities' make this critical infrastructure a catalyst for economic, logistical, and social development (Appold and Kasarda 2013 ).

The environmental impacts attributed to airport construction and operational activities (e.g. building operation, ground service equipment (GSE)) are significant to consider, especially in light of the fact that as other transport sectors go 'green,' the air transport sector will face more challenges in reducing their environmental impacts. It is estimated that the aviation industry accounts for approximately 2.5% of global greenhouse gas (GHG) emissions in 2018 (IEA 2019 ), but that estimate excludes the impacts from airport construction and operation. An analysis of 2019 data for San Francisco International Airport (SFO 2018 , 2020 ) reveals an approximate annual breakdown of 85% for aviation GHG emissions and 15% for airport GHG emissions. Although not accounting for life-cycle impacts and not representative of every airport, this breakdown offers a sense of scope of how GHG impacts are divided between aviation (i.e. flights) and airport activities. The environmental impact of airport infrastructure/operations is not just limited to their GHG emissions. Airport construction and operation also results in emissions of air pollutants such as carbon monoxide (CO), nitrogen oxides (NO x ), and particulate matter (PM), displacement of and damage to natural ecosystems, generation of waste, and consumption of resources such as water.

In the public policy sphere, airport sustainability is an emerging area of interest. The aviation and airport communities recognize the important role that airport infrastructure plays in promoting beneficial environmental and human health outcomes. However, how the public sector addresses airport sustainability is fragmented and lacks rigorous appraisal of suggested best practices. Oftentimes, airport operators rely on other airports' existing sustainability guidelines for selecting 'green' practices that are not explicitly defined and quantified (Setiawan and Sadewa 2018 ). This review offers the public aviation sector, in particular, a much-needed overview of relevant sustainability indicators and methods for airport infrastructure and guidance in pursuing future research and implementation of sustainable practices and projects.

The expected increase in demand for air travel and the necessary upgrades for airport infrastructure compound the environmental impacts of airport construction and operation. In designing and operating the next generation of airport infrastructure (e.g. terminal buildings) there must be a systematic way for evaluating the resulting environmental impacts. Measures that assess the sustainability of the design, construction, and operation of airport infrastructure offer a potential solution for airport operators to consider.

1.1. History and background

Sustainability, as defined in the United Nations' Brundtland Report, states that present society must manage and consume resources so as not to compromise future society's needs (Brundtland et al 1987 ). While the Brundtland definition acknowledges human activity's environmental impact, it does not offer concrete guidance for achieving sustainability. A less abstract framework is the 'triple bottom line' approach, which aims to identify solutions that balance environmental, social, and economic interests (Elkington 1994 ).

Sustainability indicators, or metrics, can be used to measure the 'sustainability performance' of an airport. Metrics are critical because they allow for:

• Comparing the sustainability of one airport (or one type of airport) against another;
• Identifying the weak points or opportunities for improvement in airport infrastructure;
• Measuring progress towards meeting targeted goals.

A standardized, empirical metric is also crucial for making decisions about sustainable design and operation of airport infrastructure (Longhurst et al 1996 ). Stakeholder involvement in developing these indicators is necessary (Upham and Mills 2005 ). Sustainability metrics are a component of a larger-scale sustainability plan. Ideally, formalized sustainability plans developed by airports should incorporate metrics for tracking progress towards goals.

Airport sustainability, as defined by the aviation industry, incorporates the 'triple bottom line' concept with a fourth pillar focused on operational efficiency. Airport Council International (ACI) refers to this approach to sustainability as EONS (Martin-Nagle and Klauber 2015 , Prather 2016 ). Common subcategories of EONS are shown in table 1 . An important research dimension of the airport industry is the U.S. National Academies of Sciences' Airport Cooperative Research Program (ACRP), which researches and publishes synthesis reports and guidance for current sustainability practices at airports. ACRP reports are largely compiled through literature reviews of airports' published sustainability reports and through interviews, surveys, and questionnaires with airport operators. Recent topics of ACRP reports include:

• overall sustainability (Brown 2012 , Delaney and Thomson 2013 , Lurie et al 2014 , Prather 2016 , Malik 2017 );
• feasibility of on-site energy provision (Lau et al 2010 , Barrett et al 2014 ) and microgrids (Heard and Mannarino 2018 );
• GHG emission reduction strategies (ACRP, FAA, Camp, Dresser, & McKee et al 2011 , Barrett 2019 );
• air quality impacts (ACRP, FAA, CDM Federal Programs Corporation et al 2012a , ACRP 2012b , Lobo et al 2013 , Kim et al 2014 , 2015 )
• water efficiency (Krop et al 2016 ) and stormwater management (Jolley et al 2017 );
• habitat management (Belant and Ayers 2014 );
• sustainable ground transport (Kolpakov et al 2018 );
• sustainable construction practices (ACRP, FAA, Ricondo & Associates et al 2011 );
• waste management (Turner 2018 );
• climate change adaptation of airports (Marchi 2015 ).

Table 1.  Airport industry concept of sustainability or EONS, as defined by Prather, 2016.

The definition of environmental airport sustainability in the academic literature varies with some defining it according to multiple categories of environmental impacts (Chao et al 2017 , Ferrulli 2016 , Gomez Comendador et al 2019 ; Kilkis and Kilkis 2016 ) and others limiting that definition to traditional environmental aviation impacts such as emissions and noise (Lu et al 2018 ). Environmental sustainability is assessed using both quantitative and qualitative metrics/measures, and using both generalized, average airports (Chester and Horvath 2009 ) and data from operating airports (Chao et al 2017 ; Kilkis and Kilkis 2016 , Li and Loo 2016 ).

In both industry and academic research, environmental impacts are often disaggregated according to the airside and landside components of the airport system boundary. Figure 1 shows a plan view schematic of the typical features included in the airport system boundary. It should be noted that energy generation, water/wastewater (WW) treatment, and waste management infrastructure can be located within airport-owned property (i.e. decentralized) or within the surrounding community of the airport (i.e. centralized). Table 2 identifies the purpose and primary stakeholders for each airport component. Understanding the scope of airport infrastructure aids in identifying the most relevant environmental impacts and the stakeholders best equipped to mitigate those impacts.

Figure 1.  Plan view of airport system boundary. Key infrastructure features are identified.

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Table 2.  Purpose and primary stakeholders of key airport infrastructure.

1.2. Research objectives and goals

While previous studies have examined sustainability practices of individual airports (Berry et al 2008 , Prather 2016 ), this work represents the first comprehensive systematic review of academic and industry literature on airport environmental sustainability. The five objectives of this research are: (1) synthesize the existing literature on environmental sustainability indicators and metrics for airports; (2) review the application of sustainability indicators developed for the construction of terminals and other airport facilities at a case study airport (San Francisco International Airport also known as SFO); (3) identify gaps in the literature; (4) recommend what sustainability indicators/metrics should be employed at airports based upon the results of the literature review; (5) provide recommendations for future directions of research. Sustainability indicators are grouped according to the SFO framework: Energy and Atmosphere, Comfort and Health, Water and Wastewater, Site and Habitat, Materials and Resources. These five categories provide a framework for stakeholders to begin exploring the scope of relevant environmental impacts. The breadth of the five categories also highlights that sustainability encompasses more than one type of impact (e.g. GHG emissions) and underscores that airports have multiple priorities in addressing their environmental impacts. The expected outcome from this review is the identification of gaps in the existing literature and practice as it pertains to evaluating the sustainability of airport infrastructure. Recommendations for future research directions will provide those in the academic realm, as well as in the public aviation sector, a robust assessment of what metrics, practices, and methods should be applied to achieve optimal performance outcomes.

1.3. Overview of article

Section 2 presents the methodology for conducting the systematic review. Section 3 follows with a characterization, trend analysis, and synthesis of the reviewed literature, along with a review of the sustainability indicators used at a current SFO infrastructure project. Section 4 discusses the limitations and gaps of the existing literature, analyzes the efficacy of SFO's sustainability assessment framework, and provides guidance for future research directions. Section 5 concludes with a summary of the overall work and a recommendation for practices that airports should implement in the short term.

2.1. Systematic literature review

2.1.1. criteria for selecting research papers.

The foremost criterion in selecting peer-reviewed research articles and technical reports is that they pertain to indicators (i.e. metrics or measurements) for environmental sustainability. Although the concept of sustainability also includes economic and social factors, they are outside the scope of this review. We excluded corporate sustainability reports published by individual airports as data from these reports often appear in non-standard formats. However, individual airport sustainability practices were explored as part of the review of academic and ACRP literature. We iteratively searched for peer-reviewed research articles and technical reports in Web of Science, Google Scholar, and the National Academies of Science' ACRP database that were relevant to 'airport sustainability,' using the key terms of 'airport' and variations of 'sustainability' including 'environmental sustainability,' 'sustainable development,' and 'environmental impact.'

Searches were conducted with key terms related to the five categories of the SFO framework (i.e. Energy and Atmosphere, Comfort and Health, Water and Wastewater, Site and Habitat, Materials and Resources). Additional searches also included articles that incorporated life-cycle assessment (LCA), a method for assessing the 'cradle-to-grave' environmental impacts of a product, process, or project. We elected to also include search terms for Scope 1, Scope 2, and Scope 3 GHG emissions. Table 3 summarizes the definitions and examples of Scope GHG emissions.

Table 3.  Summary of GHG scope emissions for airports.

Characterizing GHG emissions according to the three Scopes aligns with airport industry practice of allocating responsibility for GHG emissions among airport stakeholders (ACA 2020 ). Exact search terms for all criteria are provided in table A1 in appendix A (available online at https://stacks.iop.org/ERL/15/103007/mmedia ). Articles that were relevant to at least more than one of the five sustainability categories were considered as part of a Multidimensional category.

Articles that focused on sustainability indicators for the construction and operation of physical airport infrastructure were prioritized. Articles were excluded if they concentrated on aircraft, aircraft fuel, or on aircraft operations within the airport boundary such as taxiing, queuing, and the landing and take-off (LTO) cycle. The rational for this exclusion is that aircraft-related sustainability is an already extensively reviewed subject (Agarwal 2010 , Blakey et al 2011 , Sarlioglu and Morris 2015 ). However, articles pertaining to aircraft servicing operations at airports (e.g. ground service equipment or GSE, de-icing) were included. All screening criteria are listed in table A2 in appendix A. Note that the time period of 2009 to 2019 is selected to provide a meaningful analysis of the academic literature, as interest in airport environmental sustainability as a research field began in earnest at the end of the 2000s.

The searches yielded a total of 108 articles grouped according to Energy and Atmosphere ( n = 22), Comfort and Health ( n = 25), Water and Wastewater ( n = 14), Site and Habitat ( n = 16), Materials and Resources ( n = 18), Multidimensional ( n = 13). Common themes of sustainability indicators for each category are depicted in figure 2 . A bibliography for all articles included in this systematic review is provided in appendix A (table A3). Section 3 provides a trend analysis of the articles included in the systematic review.

Figure 2.  Themes for each of the five sustainability categories.

3.1. Characterization of systematic literature review

A trend analysis of the reviewed articles indicates that interest in airport environmental sustainability has steadily increased over the period of 2009 to 2019 (figure 3 ). Article counts in each category theme (figure 4 ) reveal that research among the various categories is relatively balanced, with some prominent exceptions. Article counts for 'Ambient Air Quality,' 'Airfield Materials,' and 'Multidimensional' research themes are the highest. The high article counts for 'Ambient Air Quality' and 'Airfield Materials' suggests that research in the field of airport environmental sustainability largely focuses on the characteristics of an airport that are most prominent and apparent (i.e. the runway, taxiway, and apron). The high article count for the 'Multidimensional' category indicates that the research community is beginning to recognize that airport sustainability is comprised of multiple environmental impacts across multiple airport functions. In categories such as 'Waste Management' and 'Building Materials,' the small article counts imply that these specific subjects are still emerging as relevant research areas.

Figure 3.  Cumulative articles by year (dotted line = moving average).

Figure 4.  Cumulative articles by theme.

3.1.1. Synthesis of research by category

3.1.1.1. energy and atmosphere.

Common themes among the articles featured in the Energy and Atmosphere category include energy management of airport infrastructure, use of renewable energy on-site, and energy-related air emissions.

3.1.1.1.1. Energy management

Energy management refers to a process by which airports can characterize and monitor their energy consumption and enact measures to reduce it. Airports use fossil fuels (natural gas, petroleum) and electricity to perform various operational requirements such as controlling the thermal environment of buildings, lighting runways and buildings, and fueling airport ground equipment and vehicles. Using Seve Ballesteros-Santander Airport in Spain as a case study, it is estimated that most of the energy consumption at an airport is attributable to the terminal building with heating, ventilation, air conditioning (HVAC) and lighting being the most energy-intensive practices (Ortega Alba and Manana 2017 ). A best practice for energy management is implementation of an energy monitoring system (Lau et al 2010 ). Although not analyzed from an environmental perspective, airports represent an opportunity for exploring the implementation of microgrids, which allow for on-site energy generation and storage (Heard and Mannarino 2018 ).

Some literature indicates that if an airport has implemented specific energy management practices, then those practices are a marker of sustainability. A sample of practices that are considered sustainable and have been implemented at two case study airports (Baxter et al 2018a , 2018c ) is provided in table 4 . An airport that implements a standardized energy management system is considered to be sustainable (Uysal and Sogut 2017 ). Implementation of specific practices depends upon site characteristics including climate, occupancy level, and operating hours (Malik 2017 ). An analysis of energy related to the lighting of a Turkish airport terminal indicates that indoor lighting is a critical energy consumer (Kiyak and Bayraktar 2015 ).

Table 4.  Example energy conservation practices at airports as reported in Baxter et al ( 2018a , 2018c ).

3.1.1.1.2. Renewable energy

Implementation of on-site renewable energy is another typical indicator of sustainability as discussed in the literature. There are safety concerns (e.g. glare, radar interference) with some forms of renewable energy such as solar and wind (Barrett et al 2014 ), but airports are ideal candidates for employing on-site renewables because of their expansive land areas (Lau et al 2010 ). Metrics for evaluating the efficacy of on-site renewable energy such as solar photovoltaic (PV) systems include percentage of energy demand met by on-site renewables (Dehkordi et al 2019 ) and exergy (Kilkis and Kilkis 2017 , Sukumaran and Sudhakar 2018 ). Exergy, as it relates to provision of on-site solar PV, refers to the quality of the energy delivered; solar power tends to have high thermal losses unless cooling intervention is taken. In assessing the emissions impact from different energy sources in a district heating system at Schiphol Airport in the Netherlands, it is argued that GHG emissions should be estimated by accounting for both the first and second laws of thermodynamics (Kilkis and Kilkis 2017 ). Accounting for GHG emissions from both the quantity (first law) and quality (second law) of energy provides a more realistic analysis of the feasibility for achieving practices that are considered sustainable (e.g. net zero-carbon airport terminal buildings). Another metric for assessing environmental impacts from renewable energy at airports is absolute reduction of fossil fuel consumption, which is applied to evaluate a solar PV and battery storage project at Cornwall Airport Newquay in the United Kingdom (Murrant and Radcliffe 2018 ). Modeling of a solar PV farm at a rural U.S. airport indicates that this form of renewable energy can meet both the airport's and local community's electricity needs without compromising pilot or airspace safety (Anurag et al 2017 ). A groundwater source heat pump was found to meet indoor thermal requirements in a more energy-efficient manner (i.e. a higher coefficient of performance) than conventional heat pumps for a Tibetan airport (Zhen et al 2017 ). LCA is used to inventory the GHG emissions from using a biomass-fired combined heat and power plant at London Heathrow Airport to meet terminal building heating needs (Tagliaferri et al 2018 ).

3.1.1.1.3. Energy-related emissions

Recommended GHG emission reduction strategies related to energy use at airports pertain to designing building envelopes to be more energy efficient, using energy efficient equipment and fuels, relying on renewable energy, and managing use of refrigerants (ACRP, FAA, McKee, Dresser Camp, & Synergy Consulting Services 2011 , Barrett 2019 ). GHG emissions from annual airport energy consumption are a typical sustainability evaluation metric (Monsalud et al 2015 , Baxter et al 2018a , 2018c ). In practice, GHG emissions are often inventoried according to a framework developed by ACI, which recognizes that an airport is under direct control of GHG emissions from Scope 1 sources (e.g. on-site power generation) and Scope 2 sources (e.g. purchase from grid electricity), and only able to influence Scope 3 sources (e.g. emissions from an airline's GSE) (ACRP, FAA, Camp, Dresser, & McKee et al 2011 , Ozdemir and Filibeli 2014 ). The ACI framework accounts for the annual amount of electricity and natural gas consumed and the amount of fuel used to power airport ground vehicles. A similar method allocates emissions to each macro unit (e.g. GSE) at an Italian airport (Postorino and Mantecchini 2014 ). A more holistic approach for measuring an airport's energy consumption accounts for the loss of a carbon sink from the deforestation of the site on which Istanbul International Airport was built (Kılkış 2014 ).

3.1.1.2. Comfort and health

The Comfort and Health themes in the literature include building occupant comfort and health impacts related to ambient and indoor air quality.

3.1.1.2.1. Building occupant comfort

Passengers and airport/airline employees spend a considerable amount of time inside airport buildings such as terminals, maintenance facilities, and control towers. Occupant comfort in these buildings is relevant for environmental sustainability because aspects of comfort (i.e. thermal, ventilation, lighting) are directly related to metrics such as energy consumption. Research into novel air conditioning and heating systems in terminals at Chinese airports indicates that thermal and ventilation comfort can be satisfied while saving energy (Meng et al 2009 , Zhang et al 2013 , Zhao et al 2014 ; Liu et al 2019). An investigation of preferences at airports in the U.K. demonstrates that occupants tolerate higher thermal levels and prefer natural lighting, which have energy-saving implications (Kotopouleas and Nikolopoulou 2018 ). Designing airport buildings to emphasize natural lighting should incorporate the functional operational characteristics of air travel (i.e. operational peaks occur in the early morning and early to late evening) (Clevenger and Rogers 2017 ).

3.1.1.2.2. Indoor air quality

Exposure to air pollutants is known to cause negative human health impacts including increased risk of respiratory illness, cardiovascular disease, and death (Apte et al 2012 , Kim et al 2015 ). Indoor air quality (IAQ) research focuses on the pollutants and factors (e.g. ventilation systems, building design) that contribute to occupant exposure while inside facilities such as terminals and control towers. Research on exposure in indoor settings at airports has been limited to the concentrations of nitrogen dioxide (NO 2 ) and volatile organic compounds (VOCs) in a maintenance room at a Lebanon airport (Mokalled et al 2019 ), PM in a terminal building at a Chinese airport (Ren et al 2018 ), VOCs, PM, odorous gases, and carbon dioxide (CO 2 ) at an Italian airport terminal (Zanni et al 2018 ), and CO, VOCs, and PM in a control tower at a Greek airport (Helmis et al 2009 , Tsakas and Siskos 2011 ). One study linked IAQ at eight large Chinese airports with passenger satisfaction, finding that IAQ satisfaction is correlated with CO 2 concentration (Wang et al 2015 ).

3.1.1.2.3. Ambient air quality

Ambient, or outdoor, air quality at airports is a function of both aircraft and non-aircraft operations. Sources of non-aircraft emissions include the equipment used to clean, load, or reposition parked aircraft (i.e. GSE) or used to provide power to parked aircraft (i.e. ground power units or GPUs). Another source of emissions from parked aircraft is the auxiliary power unit (APU), an external rear engine on the aircraft which provides electrical power and thermal conditioning (ACRP , 2012b , Lobo et al 2013 ). Other outdoor sources include emissions from construction (Kim et al 2014 ) and operation of airport ground access vehicles (e.g. maintenance trucks, firetrucks). Much of the exposure to pollutants such as black carbon (a component of PM) occurs on the airfield's apron where aircraft are often positioned for passenger boarding and luggage loading (Targino et al 2017 ). Outdoor exposure to VOCs near a U.S. airport revealed higher-than-expected concentrations of toluene (Jung et al 2011 ). Construction of a terminal building at a major airport in Spain was a critical contributor to ambient levels of PM (Amato et al 2010 ).

A review of airport contributions to ambient air pollution suggests that research on emissions related to GSE, GPU, and APU operations is more limited relative to research on emissions from aircraft (Masiol and Harrison 2014 ). Concentrations of CO 2 , CO, PM, hydrocarbons, NO x , sulfur dioxide, sulfate, and black and organic carbon are estimated for APU and GSE use at 20 U. K. airports (Yim et al 2013 ), emissions of CO, hydrocarbons, and NO x from APUs and GSE are calculated for turnaround operations at major European airports (Padhra 2018 ), and concentrations of NO x and PM for APUs and GSE at Copenhagen Airport are calculated (Winther et al 2015 ). Provision of fixed electrical power and external air conditioning units is considered a sustainable solution for mitigating PM and NO x emissions from APU, GPU, and GSE operation (ACRP, 2012a , Yim et al 2013 , Winther et al 2015 , Padhra 2018 , Preston et al 2019 ). Use of alternative fuel (hydrogen) for powering GSE is considered another sustainable measure to improve ambient air quality on the airport apron (Testa et al 2014 ).

3.1.1.3. Water and wastewater

The major themes related to Water and Wastewater in the reviewed articles include water conservation strategies at airports and water quality concerns related to airport activities.

3.1.1.3.1. Water conservation

Airports consume water for indoor operations such as toilet-flushing, food preparation, and HVAC systems and for outdoor operations including irrigation and aircraft/infrastructure washing and maintenance (Krop et al 2016 ). The amount of water that major airports consume is not insignificant, and is on par with consumption patterns of small and medium-sized cities (de Castro Carvalho et al 2013 ). A typical metric for assessing airport water consumption is volume per day (Baxter et al 2019 ), but this metric fails to offer a broader picture of what sources of water are consumed and what management practices yield the best results (Couto et al 2013 ). The water conservation techniques proposed for airports include monitoring of water consumption, use of water efficient fixtures/fittings, reducing irrigation demand, and use of alternative water sources (e.g. rainwater, greywater, recycled wastewater).

An important point in the literature is that much of airport water consumption is for activities that do not require potable water. There is an opportunity for airports to rely upon alternative sources of water which have been studied for: rainwater harvesting at an Australian airport (Somerville et al 2015 ); wastewater reclamation for a Brazilian airport (Ribeiro et al 2013 ); greywater usage at a Brazilian airport (Couto et al 2013 , 2015 ); seawater and greywater use at an airport in Hong Kong (Leung et al 2012 ). These studies assess the efficacy of alternative sources in terms of demand met.

3.1.1.3.2. Water quality

Water quality concerns related to airport activity can be categorized as persistent, seasonal (e.g. from de-icing operations), and accidental (e.g. fuel spills) (Baxter et al 2019 ). Airports make efforts to prevent hazardous pollutants and fluids from entering groundwater or surface water bodies. Stormwater management strategies include use of bioretention basins, green roofs, harvesting, porous pavement, sand filters, and wetland treatment systems (Jolley et al 2017 ). The academic literature focuses on water quality issues stemming from de-icing activities, a necessary operation for aircraft and runways in cold-weather climates. De-icing fluid runoff can create negative surface water quality effects that impact aquatic flora and fauna by causing higher levels of chemical oxygen demand and lower levels of dissolved oxygen (Fan et al 2011 , Mohiley et al 2015 ). Potential mitigation measures for managing aircraft de-icing include utilization of novel soil filters (Pressl et al 2019 ) and treatment with constructed wetlands (Higgins et al 2011 ). Most studies assess the water quality impact of de-icing fluid, but one article examined the GHG impact from forgoing collection and treatment of de-icing fluid at a wastewater treatment plant and instead using on-site recycling (Johnson 2012 ).

3.1.1.4. Site and habitat

Major themes of the Site and Habitat category in the literature refer to the impact airport construction and operation have on existing natural ecosystems, the effects from on-site and public transportation options, and the implications of airport resilience to climate change.

3.1.1.4.1. Site

Airport development and operation requires suitable land area. In regions where existing land is not suitable, land reclamation is used to create a suitable airport environment. Research into the effects of land reclamation on existing ecosystems focus on impacts to soil, water, air, and animal species (Yan et al 2017 ; Zhao et al 2019 ). Another indicator in the literature refers to efficiency of airport land utilization, or how many aircraft operations occur per given unit area (Janic 2016 ). Airport operation and its impacts on wildlife populations is another area of research, with the goal of finding specific strategies to discourage and accommodate wildlife populations on airfields, airport water resources, terminal buildings, and control towers (Belant and Ayers 2014 ). Work done in the academic literature focuses on identifying the factors that attract avian species to green roofs (Washburn et al 2016 ), on the impacts of solar arrays on avian species (Devault et al 2014 ), and on the effects of airport expansion on bat populations (Divoll and O'Keefe 2018 ).

3.1.1.4.2. Transportation

Sustainable transportation, as it relates to airports, refers to the modes of transportation for shuttling passengers from terminals to parked aircraft and for bringing passengers to airports. Common sustainability practices for on-site transportation include: use of alternative vehicles (e.g. electric vehicles); restriction of vehicle idling; and reducing the number of empty trips (Kolpakov et al 2018 ). One study examined the use of an underground rapid transport system (URTS) for transporting airport passengers the long distances from main terminal buildings to satellite and midfield concourse terminals (Liu and Liao 2018 ). This study did not include specific environmental indicators, but noted that use of URTS is sustainable because it frees up congestion from passenger transport on the airfield concourse. Sustainable public transport options might include using automated vehicles (Wang and Zhang 2019 ), encouraging passengers to use existing public transport options by enhancing their capacity, discouraging private vehicle use, integrating with other transport hubs (Budd et al 2016 ), or installing dedicated electric vehicle charging infrastructure (Silvester et al 2013 ).

3.1.1.4.3. Resilience

The resilience of airports to climate change impacts is a significantly under-researched subject. Relevant risks that airports in coastal locations will face include impacts from sea-level rise and increased frequency of flooding events (Marchi 2015 , Burbidge 2016 , Poo et al 2018 ). Another site implication related to climate change is that increased mean air temperatures will make it harder for aircraft to generate lift, thereby necessitating the construction of longer runways (Coffel et al 2017 ).

3.1.1.5. Materials and resources

Themes from the literature for Materials and Resources center around selection of materials for the construction of airfield (e.g. runway, taxiway, apron) and terminal building infrastructure, as well as management of waste from airport construction and operation.

3.1.1.5.1. Airfield materials

Estimation of environmental effects of airfield pavements is a fairly well-researched subject area, relative to other airport infrastructure. Airfields are either made from asphalt or concrete, which are known major sources of GHGs (Horvath 2004 , Santero et al 2011 , Miller et al 2016 ). The sustainability of airfield pavements is constrained by structural integrity requirements and safety standards (Pittenger 2011 ).

Evaluation metrics for sustainable airport pavement can be general, such as implementing suggested best practices, including: using recycled aggregate in pavement mixes; using locally sourced construction materials; reducing idling times of construction equipment (Hubbard and Hubbard 2019 ). More specific critical factors of a sustainable airport pavement relate to its construction (i.e. the raw materials and equipment used, transportation, waste management) and its operation, which is a function of the pavement's structural characteristics (Babashamsi et al 2016 ). Table A4 in appendix A highlights the specific sustainable practices and assessment methods/metrics found in the literature as they pertain to different parts of the airfield. Example sustainable practices include use of supplementary cementitious materials (SCM) in concrete runways and use of recycled aggregates in taxiway and apron construction. LCA is frequently used in measuring the environmental sustainability of airfield pavements. The scope of most of the LCAs is limited to impacts from the raw material and construction phases of the airfield.

3.1.1.5.2. Building materials

Relative to the airfield, environmental impact analysis of other airport infrastructure (e.g. terminal buildings) is much more limited. LCAs have been performed to determine the optimum level of thermal insulation for terminal buildings at two Turkish airports with a focus on selecting a design that reduces GHG emissions (Akyuez et al 2017 , Kon and Caner 2019 ). An extensive overview of construction methods and building materials that are standard practice (e.g. using locally sourced materials) among the green building community is applied for airports (ACRP, FAA, Ricondo & Associates, R. &, Center for Transportation, C. for, & Ardmore Associates 2011 ). It is common practice, as mentioned in the ACRP literature, for airports to aim for green building certification from groups such as the U.S. Green Building Council's Leadership and Energy in Environmental Design (LEED) like LEED provides a checklist framework where building owners (municipalities in the case of airports) earn points for choosing 'green' building materials and design attributes, among other criteria. There are over 200 LEED certified airport buildings worldwide (USGBC 2020 ), with SFO's Terminal 2 the first LEED Gold airport terminal in the U.S. (SFO 2011 ).

3.1.1.5.3. Waste management

Analysis of waste management at airports is another emerging research area. Waste sources at airports include food waste from retailers/concessionaires, construction waste, and aircraft-related waste (Turner 2018 ). Metrics applied for analyzing waste at a major international airport include quantity of waste, waste source fraction, and waste amount per operation (Baxter et al 2018b ). One article assessed the life-cycle impact, in terms of air emissions, of six waste management scenarios at Hong Kong International Airport determining that on-site incineration with heat recovery yielded optimal results (Lam et al 2018 ).

3.1.1.6. Multidimensional studies

Sustainability, as expressed in ACRP reports (Brown 2012 , Delaney and Thomson 2013 , Lurie et al 2014 , Prather 2016 , Malik 2017 ), encompasses many categories including energy and climate, water, waste, natural resources, human well-being, transportation, and building design and materials. Many of the metrics that the ACRP literature use to assess the specific categories of sustainability mirror those described in the academic literature. A theme among the ACRP work is the evaluation of sustainability practices from an economic and practical perspective, recognizing that implementation can yield economic benefit but takes concerted, coordinated effort.

Table 5 identifies metrics used for quantifying impacts and strategies used to reduce impacts. These metrics and strategies are extracted from the multidimensional journal articles included in the systematic review. Each metric or strategy is prioritized to the one of the five categories of interest. While the focus of this review paper pertains to metrics/strategies that evaluate the sustainability of physical airport infrastructure, and not does focus on environmental impacts related to the aircraft LTO cycle, some of the multidimensional papers include indicators for evaluating those specific environmental impacts (e.g. noise from near-airport aircraft operations). The indicators in table 5 range from explicit, quantifiable metrics (e.g. tonnes CO 2 per passenger) to more vague best practices (e.g. conserve energy in airport buildings). The metrics and strategies that are explicit and quantifiable are more informative for enacting policy measures than are vague strategies such as 'conserve energy' or 'reduce emissions.' It is also more effective for metrics and strategies that connect environmental impacts to operational outcomes and level of service (e.g. number of passenger-miles traveled). Connecting impacts to level of service allows for airports to track how efficiently they are managing their impacts as numbers of operations increase.

Table 5.  Sustainability indicators from multidimensional papers.

a ISO 50 001 Certification = International Standard Organization's Energy Management System. b Airport Carbon Accreditation = ACI certification that recognizes an airport's efforts to manage CO 2 emissions. c ISO 40 001 Certification = International Standard Organization's Environmental Management System. d WLU = Work Load Unit, a standardized metric for airport operations in terms of number of passengers processed or mass of freight handled.

Indicators from each multidimensional paper do not always span all five categories of environmental sustainability, suggesting that consensus building on the definition of environmental sustainability needs to occur. The Energy and Atmosphere category dominates with metrics often related to reducing airport building and airfield energy consumption and air pollutant emissions. Of the eight journal articles included in table 5 , all include metrics for addressing noise pollution in the Comfort and Health category, but none provide explicit metrics for assessing indoor air quality for airport buildings. The indicators in the remaining three categories vary in level of specificity. As an example, in the Materials and Resources category, four of the articles suggest airports use 'green building materials' but only one article (Ferrulli 2016 ) identifies in some detail what that means.

A theme that emerges from the multidimensional papers are the different methods utilized in determining the overall sustainability of an airport. Utility-based methodologies are utilized in two of the multidimensional articles (Chao et al 2017 , Lu et al 2018 ) in the ranking of the most critical indicators by weights applied from expert opinion. Another method for assessing an airport's environmental sustainability is the application of a checklist-based point system where the most sustainable airport implements the most indicators with the highest level of points (Gomez Comendador et al 2019 ). One method incorporates cost-benefit analysis where each environmental indicator for an airport development project is transformed into a financial amount and the highest benefit-cost ratio yields the most sustainable outcome (Li and Loo 2016 ). A composite ranking indicator is created by normalizing indicators across all categories to compare the environmental sustainability of multiple airports (S. Kilkis and Kilkis 2016 ). Only one method applies life-cycle assessment in inventorying the environmental impact from the LTO cycle, APU and GSE operation, de-icing activities, lighting, and construction of an airport terminal, airfield, and parking lot (Chester and Horvath 2009 ).

The multidimensional articles that include case study airports are listed in table 6 , along with each airport's location. All of the case study airports are considered major international hubs, averaging millions of passengers per year. Their locations span the primary airport markets including Asia, Europe, and the United States, but do not reflect the emerging markets of Latin America and Southeast Asia. By comparing airports of a similar operational capacity, the multidimensional papers offer some insight into how varying regions influence environmental impact. However, more case study airports are necessary to capture local impacts. Insight is lacking on whether the sustainability indicators developed in these multidimensional articles result in distinct environmental outcomes for disparate levels of airport service (e.g. small, regional airports; medium hub airports). Modeling environmental impacts from an average airport (Chester and Horvath 2009 ) allows for generalization of results, which might yield more far-reaching outcomes (i.e. sustainability indicators can be applied to a greater range of airports).

Table 6.  Case study airports/locations from multidimensional papers.

3.1.2. Summary of trends in existing research

Figure 5 shows a word cloud diagram of the article titles included in each of five sustainability categories and the multidimensional category. Frequently used words appear larger relative to less frequently used words. Figure 5 provides a visual representation of the key themes for each category. A summary of key trends in the five sustainability categories and the multidimensional category include:

• Energy and atmosphere: Articles focus on investigating the efficacy of on-site renewable energy at various case study airports. Common sustainability indicators are total energy consumed and mass of GHG emissions from energy consumption. Best practices are considered as: monitoring of energy consumption; utilization of energy efficient HVAC equipment and lighting; installation of on-site renewable energy. There is some effort, particularly in the ACRP literature, to evaluate best practices from a practical perspective (e.g. addressing the safety implications of PV installations). Use of LCA in this category is limited.
• Comfort and health: Most of the research is focused on indoor comfort and health indicators like preferences for thermal and lighting conditions and concentrations of PM, VOCs, CO, and CO 2 . Studies on exposure to ambient air pollutants from non-aircraft sources are limited. Most of the research on ambient air quality aggregates emissions from all sources. There is recent effort to investigate the impact from non-aircraft sources such as APUs, GSE, and GPUs and to identify possible solutions for these equipment (e.g. use of external electrical power and air conditioning units).
• Water and wastewater: Articles focusing on estimating the potential utilization of alternative water sources at airports dominate. Water quality research pertains to impacts from stormwater and de-icing fluids. A typical article in the Water and Wastewater category includes annual water consumption per passenger or flight operation. There is discussion in the literature on whether a disaggregated metric (e.g. indoor water consumption per passenger, outdoor water consumption per passenger) might be a more effective performance indicator.
• Site and habitat: This category is the least explored in the literature. Few articles offer measurable indicators, with most of the quantifiable metrics relating to land use efficiency and destruction of wildlife habitat. There is need for quantifiable indicators for research in on-site, public/private transport and for climate change adaptation practices.
• Materials and resources: Research on the environmental sustainability of airfield pavements dominates this category. LCA is the most frequently used assessment methodology, with life-cycle GHG emissions and energy consumption the most common assessment metrics.
• Multidimensional: Research that investigates airport sustainability from a multidimensional perspective is grouped according to efforts by ACRP and by the academic community. ACRP largely defines environmental sustainability across the five categories (i.e. energy and atmosphere, comfort and health, water and wastewater, site and habitat, materials and resources), but often focuses on economic and practical factors of implementing sustainability best practices. These best practices are often identified through interviewing and surveying U.S. airports. Sustainability indicators in the academic literature predominantly focus on energy consumption and GHG emissions. Sustainability is assessed with a number of methodologies (e.g. utility-based theories, cost-benefit analysis, LCA), suggesting that within the academic community there is a lack of consensus on what attributes and indicators make an airport sustainable.

Figure 5.  Word cloud diagram of article titles included in systematic review. Frequently used terms appear larger relative to less frequently used terms.

3.2. Application of an airport sustainability assessment

This section reviews the application of the SFO environmental sustainability framework on an existing infrastructure project at the airport.

3.2.1. Selection of case study airport

San Francisco International Airport (SFO) is one of the United States' large hub airports and it serves major domestic and international routes. The airport ranked seventh among busiest airports in 2018, with enplanements totaling close to 28 million (FAA 2020b ). The airport was an early adopter in implementing sustainability efforts and in developing metrics to assess the sustainability of construction and operation of airport infrastructure projects (SFO 2020 , FAA 2020a ). A review of the implementation of SFO's sustainability framework answers two critical questions: (1) how sustainability efforts practically get implemented at airports, and (2) how their implementation is or is not effective in yielding measurable benefits. Featuring SFO as a case study offers stakeholders (e.g. regulators, airport operators, the public) insight into what is considered best practices, or acceptable methods, for managing environmental impacts for major international airports. Additionally, it provides some understanding of how sustainability measures at an airport like SFO might not work as well for other airport types (e.g. small hub, regional, general aviation, etc.).

3.2.2. Development of sustainability indicators

SFO is redeveloping their Terminal 1 as part of a capacity-enhancement upgrade for the entire airport; the upgrade will increase the terminal's total number of annual enplanements to 8.8 million. Sustainability indicators were developed in conjunction with SFO's planning, design, and construction guidelines as a measurable index for determining whether the Terminal 1 project will comply with the airport's overarching environmental goals (e.g. achieving GHG emission reductions relative to a baseline year). Each sustainability indicator is grouped according to relevant themes in the five categories of Energy and Atmosphere, Comfort and Health, Water and Wastewater, Site and Habitat, and Materials and Resources. Indicators are either considered 'Mandatory Requirements' or 'Expanded Requirements.' 'Mandatory Requirements' outline metrics and practices that must be achieved according to applicable federal, state, regional building codes and city-wide mandates (e.g. meeting LEED requirements). 'Expanded Requirements' are voluntary metrics and practices that project participants (i.e. contractors) are obligated to implement where feasible. For example, a city-wide 'Mandatory Requirement' in the Energy and Atmosphere category mandates 40% reductions below 1990 GHG emissions by 2025. An example 'Expanded Requirement' calls for reduced GHG emissions from natural gas consumption by using automated HVAC systems.

3.2.3. Implementation of indicators

The indicators are intended to be used for the planning, design, construction, and operation/maintenance phases of airport facilities. An additional level of evaluation is applied to each 'Expanded Requirement.' Requirements are rated as 'Baseline,' 'Baseline Plus,' or 'Exceptional Project Outcome.' Per the previous 'Expanded Requirement' example, 'Baseline,' 'Baseline Plus,' or 'Exceptional Project Outcome' ratings would be given to 10%, 20%, and 30% reductions in GHG emissions, respectively. Such a rating system allows SFO to discern between project outcomes that are more 'sustainable' than others.

The results of an analysis of the projected reduction in annual GHG emissions per square meter from implementing Energy and Atmosphere 'Expanded Requirements' in SFO's Terminal 1 project are shown in figure 6 . The specific 'Expanded Requirements' include practices that rely on reduced natural gas and electricity consumption in terminal buildings (e.g. energy-efficient escalators, dynamic glazing, radiant heating and cooling). It is projected that these 'Expanded Requirements' will reduce Terminal 1's energy use intensity (EUI). The EUI indicates how much natural gas and electricity is consumed by buildings. By converting the EUI to an equivalent amount of GHG emissions per square meter, it can be shown that the GHG intensity of the Terminal 1 project will be less than the average of other SFO buildings. The blue bars in figure 6 show the amount of GHG emissions per square meter, while the dotted outline indicates the amount of annual GHG savings per square meter in the Terminal 1 project. The GHG emissions account for the upstream processes related to natural gas provision and electricity generation. See appendix B for the complete methodology in producing figure 6 . The savings represent an approximate 57% reduction relative to the average GHG intensity for all SFO airport building infrastructure.

Figure 6.  Reductions in GHG Intensity associated with implementing energy reducing 'Expanded Requirements' in Terminal 1 (T1) project relative to the SFO average. Savings are relative to 2018 data.

4. Discussion

4.1. limitations and gaps of existing research.

With few exceptions on airport energy (Kilkis and Kilkis 2017 , Tagliaferri et al 2018 ), overall sustainability (Chester and Horvath 2009 , 2012 , Taptich et al 2016 ), and airfield pavements, much of the research fails to holistically analyze the environmental impacts through supply chains and regional variations. While the ACRP literature provides a sample representation of current best practices at airports, its analysis is sometimes limited by the responses it receives from case-study airports. For both the ACRP and academic literature, analysis of sustainability indicators is often limited by the scope of a case-study airport, so it is difficult to link research results with suggested practice or policy outcomes.

The literature in the Energy and Atmosphere category lacks a broader understanding of how much energy is used at different airports, what it is used for, and where it comes from. Current estimates are limited by the number of existing case-study airports. With an exception (Ozdemir and Filibeli 2014 ), the academic literature limits its characterization of GHG emissions according to Scope 1, Scope 2, and Scope 3. This limitation in the literature indicates that there is a slight disconnect between the academic research community and the airport industry and stakeholders as the Scope characterization is how the industry thinks about and manages GHG emissions. Research that investigates different energy sources (e.g. solar; bioenergy) and energy provision strategies (e.g. grid versus on-site storage) is just beginning, and more effort in this area is needed. Additional gaps in the research include:

• Environmental impacts of energy consumption in terms of other pollutants besides GHG emissions;
• Environmental assessment of airports and supply chains using local and regional models and data (Cicas et al 2007 );
• Characterization and environmental impact assessment of energy consumption patterns for specific airport infrastructure and equipment by region (e.g. U.S. airport terminals are focused on food consumption; European/Asian airports serve as retail/recreational centers);
• Energy consumption impacts from construction of new airport expansion/retrofitting projects.

As with the Energy and Atmosphere category, research in the Comfort and Health category could be broadened to include more research and innovative and exploratory case studies. In light of COVID-19, more research is urgently needed to investigate how terminal building design and ventilation equipment might influence spread of infectious diseases. Ambient air quality research tends to aggregate sources, which makes it difficult to determine if mitigation policies are effective. Additional gaps in the research include:

• More human health-focused exposure studies related to operation of non-aircraft equipment, such as GSE, GPUs, APUs, and ground access vehicles;
• Investigation of air pollutant concentrations related to landside operations, such as passenger pick-up and drop-off;
• Research on human health impacts from airfield and terminal building maintenance, retrofit, and construction;
• Air quality impacts related to selection of different building materials and cleaning/daily maintenance procedures.

As suggested in the Water and Wastewater literature, assessing an airport's water consumption in terms of volume per day provides minimal insight. More research should be conducted to provide a thorough overview of disaggregated water consumption at the airport level so that sustainable practices can be implemented appropriately. A major gap in the literature is the complete lack of research into the linkage between water consumption, water quality, energy needed to convey, treat and heat water, and the resulting GHG and other environmental emissions and impacts. This water-energy nexus is particularly relevant in examining the environmental sustainability of using alternative sources of water at airports, especially with respect to potable versus non-potable demands and options.

Much of the literature in the Site and Habitat category lacks explicit, quantifiable sustainability indicators and there is vast room for investigation into the following gaps:

• Energy and environmental implications of constructing resilience infrastructure, such as sea walls and stormwater systems;
• Environmental impacts of onsite transportation systems, such as underground rapid transit systems;
• Overview of the types of suitable, environmentally efficient transportation modes within and outside of the airport boundary, which is dictated by airport configuration and location;
• Environmental trade-offs between site selection and terminal building orientation and layout of runways.

Research in the Materials and Resources category is predominantly focused on environmental impacts of airfield pavement construction and maintenance, with life-cycle energy consumption and GHG emissions as common metrics. Within the theme of airfield pavements, more research regarding innovative designs and maintenance techniques are warranted. There is a lack of understanding on what sustainable pavement practices can be implemented at airports of different operational capacities. Small and medium-sized airports might be good candidates for testing out innovative practices because their load or volume requirements tend to be smaller than those of larger airports. In terms of sustainable materials and design for airport buildings, research results are limited. In practice, it is more common for airports to strive for LEED certification of airport buildings. LEED, for practical purposes, is a relatively easy standard to implement, but is not sufficient for meeting quantified performance goals throughout the life cycle of airports. Additional gaps in the research include:

• Environmental impact of conventional and alternative construction materials in terminal building infrastructure;
• Sustainability impacts of supply chains and sourcing of airport construction materials;
• Deeper understanding leading to defensible actions on waste generation and waste management techniques at airports, especially in the context of waste-management policies such as 'zero-waste' and bans of single-use plastics.

A review of articles in the Multidimensional category indicates that there is no cohesive, agreed-upon definition of airport environmental sustainability. Gaps in the research include:

• Determining optimal methods for achieving overall environmental sustainability at an airport, also integrated with achieving specified city, regional-level, airline, or civil aviation targets;
• Integration of life-cycle, or holistic, thinking within a specified time horizon into decision making (e.g. should an airport implement an electricity-based strategy if the electricity is generated from fossil fuels?);
• Specifying environmental sustainability indicators in the context of airport operational safety;
• Investigating the overlap between environmental sustainability and airport resilience;
• Rigorous analysis of environmental sustainability and operational parameters;
• Integration of actions in achieving societal sustainable development (economic, environmental, social) with airport, airline, air traffic control, and in general, civil aviation goals.

4.2. Efficacy of case study application

A projected 57% reduction in annual GHG emissions per square meter from consuming natural gas and electricity on-site within the airport terminal buildings suggests that SFO's sustainability assessment indicators have the potential to be effective. A more meaningful expression of results would relate saved GHG emissions to the airport's level of service (e.g. GHG emissions per passenger or per revenue dollar). There are limitations to stating one airport's efforts as 'best practice.' It should be emphasized that applicability from the results of the case study are dependent upon local factors. For SFO, implementing energy-efficient strategies saves more GHG emissions because SFO's electricity is supplied from hydropower, which is less carbon-intensive relative to the state average. Utilizing low carbon-intensive energy is a key sustainability performance indicator. While post-facto analysis would be able to confirm actual GHG reductions from implementing 'Expanded Requirements,' the project is still ongoing. Some important observations can still be made regarding SFO's sustainability indicators.

In discussions with parties involved with the Terminal 1 reconstruction projects, having sustainability criteria at the outset of project development is crucial. All involved parties must be aware of their specific commitments. It is a good practice going forward for project contracts to incorporate strong sustainability performance indicators. SFO plans to integrate language more thoroughly into the Architectural and Engineering standards and guidelines that specifically align with two of SFO's guiding environmental priorities, namely climate change and human and ecological health. Regarding the former, the new contract language will explicitly require that decarbonization be reflected in project design and procurement. For example, instead of a voluntary consideration as part of an 'Expanded Requirement,' low-carbon structural steel would have to be selected as a building material.

The voluntary aspect of the framework (i.e. the 'Expanded Requirements') and the evaluation of 'Expanded Requirements' as baseline, baseline plus, and exceptional project outcome are rather subjective. Such subjectivity does not necessarily result in a completed project with the best environmental performance. Additionally, the SFO framework relies upon building codes that while they are 'state of the art' compared to building codes outside of California, represent a minimum standard. If interested in attaining a facility or project that meets a specified, quantifiable environmental outcome, the subjectivity of a rating system or checklist is not the most effective approach.

SFO's sustainability indicators do not explicitly consider the tradeoffs that potentially occur with prioritizing one criteria over the other; it is a rather static framework that could benefit from incorporating spatial and temporal factors. For example, electing to use a decentralized recycled water source (which is an 'Expanded Requirement' in the Water and Wastewater category) is sometimes an energy-intensive process which can result in increased GHG emissions while enhancing resilience. In this anecdotal example, there is a potential tradeoff between achieving water conservation and reducing GHG emissions. While the SFO framework might work well for an airport that explicitly prioritizes overarching goals (e.g. reducing GHG emissions and climate change impact), it might need to be reevaluated for airports that must equally consider sometimes conflicting environmental priorities.

4.3. Suggestions for direction of future research

The roadmap for future research of airport environmental sustainability emphasizes increased stakeholder involvement, more life cycle-based analysis, linkage of environmental impacts with operational outcomes, and addressing major challenges such as adaptation to climate change and mitigation of infectious diseases like COVID-19.

Airport environmental sustainability is often addressed at project scale. There is a need for investigating the larger role that airports have in impacting the environment, especially in the context of achieving city- and regional-level environmental outcomes that lead most directly to higher environmental quality of people and ecosystems. This ties in with stakeholder involvement because for sustainability indicators including GHG emissions, an airport only claims responsibility for Scope 1 and Scope 2 emissions. Airports often exclude ownership of Scope 3 emissions (e.g. emissions from an airline's GSE, without which there are no airports). The outcome of an airport excluding ownership of Scope 3 emissions is twofold: (1) it is more difficult to manage Scope 3 emissions, and (2) it is difficult to understand an airport's total GHG impact at the city/regional/state/national level, which is important for meeting larger-scale climate performance targets. Therefore, a broader analysis of how different stakeholders should be included in addressing environmental sustainability efforts is necessary.

Society faces important challenges such as adapting to climate change, mitigating the spread of pandemic-causing diseases, and enhancing environmental quality of people and ecosystems. An airport's role in addressing these challenges is largely undefined, but sure to be a significant one. It is imperative that thorough research on an airport's role in managing these challenges gets organized.

5. Conclusion

A comprehensive, systematic review of 108 peer-reviewed articles and technical reports related to assessing and measuring aspects of airports' environmental sustainability has been conducted. Articles have been characterized according to the following categories: Energy and Atmosphere, Comfort and Health, Water and Wastewater, Site and Habitat, Materials and Resources, Multidimensional. Along with a systematic review of academic literature, a review has been undertaken of the application of an existing airport sustainability assessment framework for a case study airport, SFO.

A broad conclusion from the systematic review is that interest in airport environmental sustainability as a research topic is steadily increasing, but that there is ample need for more investigation. Prominent research themes within the scope of airport environmental sustainability include analyzing the environmental impacts (namely GHG emissions) from airfield pavements and energy management strategies for airport buildings, but not from other components of airports and for other environmental emissions and impacts. There is a dearth of research on the impacts of indoor air quality at airports. In the research community, there appears to be a lack of consensus about the scope of environmental impacts that should be included when evaluating the overall sustainability of airports. GHG emissions from energy consumption are one of the most commonly used metrics in research focused on overall airport sustainability.

Methods for evaluating environmental impacts vary. Systems like the World Resource Institute's Scope 1, 2, and 3 designation for GHG emissions and the LEED system for buildings are well-represented in airport-industry practice. The Scope designation primarily divides responsibility for mitigating emissions between airports and airlines, creating a gap whereby airports cannot directly control all emission sources. LEED is a minimum standard that is not sufficient for meeting quantified performance goals throughout the life cycle and supply chains of airports.

Moving forward, the increased use of assessment methodologies such as LCA will be useful in guiding decision-makers and policy outcomes in a more robust, granular direction. In the academic literature, LCA is primarily used for evaluating the environmental impact of airfield pavement construction. However, LCA can and should be applied to evaluate all components of airport construction and operational activities and to guide decision-making as to what practices will yield optimal results. LCA is the only comprehensive, systematic methodology (defined in ISO 14040 and 14044) that estimates the entirety of life-cycle environmental impacts of a product, process, or service. This method is very useful for accounting for regional differences in impacts, for comparing among alternative strategies, and for identifying weak points or activities that result in the greatest environmental burdens. There are also economic and social aspects of LCA that are helpful for decision-makers. One LCA approach, Economic Input-Output LCA, can be used to evaluate the resources, energy, and emissions resulting from economic activity throughout a product's supply chain (Hendrickson et al 1998 ). There are efforts to use a life-cycle approach to focus on the social aspects of a product's impacts (Grubert 2018 ). While addressing the economic and social impacts from airports is beyond the scope of this review, the economic and social implications of airports are likewise very important and demand thorough investigations and actions.

In conjunction with LCA, future research should apply analysis that connects environmental impacts with operational parameters for specific airport occupant groups (e.g. ground handlers), airport infrastructure (e.g. apron), and airport scale (e.g. small, medium, large hubs). Accounting for operational parameters at different scales will provide a better understanding of how environmental sustainability efforts impact different stakeholders and the airport's primary function (i.e. processing passengers and cargo).

A key aspect of addressing the environmental sustainability of airports is the involvement of different stakeholders. As identified in figure 1 , the airport is comprised of airside and landside components. Historically, these components have been managed by distinct stakeholders. Understanding the relationship among the airport components, their respective environmental impacts, and their ways of managing stakeholder groups is critical because it leads to identifying who must act to mitigate environmental impacts. Figure 7 depicts an annotated version of the airport system boundary with suggested best practices for major airport components. Based on the literature review and the application of the SFO case study, effective sustainability practices that airports can implement in the short term are: (1) supply electricity from renewable, low-carbon sources whether on-site or from local utilities; (2) electrify transportation vehicles (e.g. shuttles, maintenance trucks) within the airport system boundary; (3) electrify all gate and ground service equipment; (4) implement water conservation practices like installation of water-efficient faucets and toilets; (5) install energy-efficient fixtures like LED lighting in all airport infrastructure; (6) select durable interior building materials for improved maintainability and reduced waste production.

Figure 7.  Suggested best practices for improving airport environmental sustainability.

These six suggested sustainability practices can result in prompt, substantive environmental benefits without significant tradeoffs. For example, relying on low-carbon electricity reduces GHG as well as other emissions. Electrifying ground service equipment and other airport vehicles results in reductions of air pollutants (NO x , PM) within the airport vicinity, which is a human health benefit. These practices are considered implementable in the 'short term' as opposed to longer-term projects such as changing the material composition of the airfield or installing on-site, decentralized wastewater treatment. These measures cover activities and operations that essentially occur at all airports, but to varying degrees of scale (e.g. all airports consume electricity). In that vein, ease of strategy implementation depends upon airport type, the resources (e.g. cost, accessibility, expertise) available to the airport for successful implementation and the controlling stakeholder. Further analysis of those distinctions is needed in future research.

One common tendency is for airports to adopt a perceived 'best practice' based upon another airport's successful implementation. But progress is needed to ensure that every airport considers all relevant environmental sustainability indicators systematically to account for regional and supply-chain effects rather than simply follow others' actions. This ties in with the further need to connect all relevant environmental impacts with local human health and ecosystem effects as communities living in proximity of airports bare a greater burden of airport operations. Future research should concentrate on the development of quantifiable indicators or performance metrics. Research and practice that increase stakeholder involvement, incorporates life-cycle assessment, and links environmental impacts with operational outcomes will help airports as well as the aviation industry to address their roles in major global challenges (e.g. climate change adaptation, mitigation of infectious diseases).

Acknowledgments

FG and JR acknowledge the financial support of the Sustainability Office at Groupe ADP.

Data availability statement

All data that support the findings of this study are included within the article (and any supplementary information files).

Supplementary material (169 kB, PDF)

Supplementary material (223 kB, PDF)

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Sustainable Aviation Research

NREL's sustainable aviation research aims to not only permanently lower the carbon intensity of flight but also fundamentally improve the carbon footprint, mobility, and resiliency of the entire aviation ecosystem.

A Holistic Approach to Decarbonizing Aviation

New technologies are changing the future of aviation by providing actionable pathways for lowering greenhouse gas emissions in a sector that is among the most difficult to decarbonize.

NREL has instituted a comprehensive, coordinated sustainable aviation strategy that paves the way for research, development, demonstration, and deployment—leading to solutions for decarbonizing aviation.

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NREL develops and helps deploy sustainable aviation solutions that can be scaled globally—from net-zero-carbon energy sources to infrastructure optimization and aircraft propulsion technologies.

Addressing All Energy Aspects of Sustainable Aviation

NREL is uniquely positioned to develop decarbonization solutions that address all energy aspects of the aviation ecosystem—from net-zero-carbon energy sources to infrastructure optimization and aircraft propulsion technologies.

Low- and Net-Zero-Carbon Aviation Fuels and Energy Carriers

NREL offers end-to-end expertise in developing and demonstrating low- and net-zero-carbon aviation fuels—from field to fuel, electron to molecule, and bench to pilot scales.

NREL's broad research portfolio helps develop, scale, and integrate the production of climate-friendly sustainable aviation fuel (SAF), including e-fuels made by upgrading carbon dioxide (CO 2 ) with renewable electricity. This includes research into multiple technology pathways designed to convert diverse fuel feedstocks—from CO 2 to biomass such as lignin, agricultural residues, energy crops, and algae—into finished fuels, including net-zero-emission biofuels . To accelerate the introduction of SAF technologies into the marketplace, NREL closely collaborates with industry partners across the supply chain to scale and mature a range of conversion pathways. Recent projects include:

• Leading a 10-ton-per-day pilot plant project , called SAFFiRE, to cost-effectively produce SAF from corn stover, melding D3MAX's commercial sugar production and ethanol fermentation technology and NREL's patent-pending deacetylation and mechanical refining process
• Integrating electrochemistry with sugar fermentation to produce lipids used to make SAF while avoiding the release of CO 2 into the atmosphere
• Integrating accurate fuel property measurements with advanced aviation turbine simulations to reduce fuel approval time and cost, increase low-carbon fuel blend levels, and improve fuel performance.

NREL's hydrogen and fuel cell research lowers the cost and increases the scale of technologies to safely make, store, move, and use hydrogen across multiple energy sectors, including in aviation. Renewable hydrogen can be used directly as a fuel or combined with bio-based carbon or waste carbon dioxide streams to produce net-zero-carbon liquid fuels. These energy-dense fuels are compatible with today's heavy-duty truck, rail, marine, and aviation engines. NREL researchers are developing advanced technologies to lower the cost of hydrogen production ; novel hydrogen storage materials and carriers; durable, light, efficient fuel cell technologies for long-life, high-use applications; and infrastructure technologies for fast and safe fueling.

Integrated, Decarbonized Ground Aviation Infrastructure

NREL analysis and modeling can resiliently decarbonize airports, military bases, and vertiports to seamlessly integrate them with ground-based transportation systems.

Airport Ecosystem

NREL researchers use advanced methods to optimize airport building design and operations. In the Morpheus project, for example, NREL is developing advanced building controls for the Dallas/Fort Worth International Airport to identify solutions that are replicable across U.S. airports. To accommodate corresponding electrical generation and infrastructure requirements, thermal energy and battery storage—combined with energy efficiency and other forms of load shifting and load shedding—can offer airports cost-effective approaches to achieving their objectives and state and local energy challenges. Also, NREL is leveraging its Advanced Research on Integrated Energy Systems research and demonstration platform to validate and demonstrate integrated solutions that:

• Enable electrification strategies that mitigate integration challenges with high-power charging
• Utilize controllable building and charging loads to minimize or counteract electrification peak loads
• Enhance resilience, reduce operating costs, and de-risk implementation.

Sustainable Aircraft of the Future

Aircraft of the future will transcend the one-size-fits-all approach of today's liquid-fueled aircraft. NREL develops systems and components that enable these new fuel types and propulsion pathways.

To advance the understanding of new low-carbon sustainable aviation fuels and their impact on turbine engine performance, NREL's fuels and combustion researchers use an innovative combination of fuel property measurement, molecular-level chemistry models, and detailed simulations. Fuel properties are measured at the high temperature and high (or very low) pressure of engine operation, and machine learning tools are used to relate properties to performance. NREL also develops combustion kinetic models based on laboratory data. Then, researchers use high-performance computing simulations to evaluate the impact of new fuels and combustion kinetics on turbine engine operation, performance, and emissions.

NREL has multiple specialized energy sciences laboratories to develop, characterize, fabricate, manufacture, and validate hydrogen fuel cell and electrolyzer components and systems, as well as integrate renewable fuels with the grid, transportation, buildings, and other sectors. Researchers use these capabilities to develop advanced hydrogen detection technologies , evaluate the electrochemical properties of novel materials, develop and test advanced materials and cells for fuel cells, and develop methods to scale up renewable energy technology manufacturing . NREL's hydrogen systems and infrastructure research platform integrates hydrogen production, compression, storage, and dispensing into a unified system for developing new infrastructure technologies to enable safe fueling for transportation, stationary, and portable applications. Combined with thermodynamic modeling , these capabilities make it possible to evaluate a range of hydrogen station configurations and associated control strategies at airports, enabling aircraft and equipment manufacturers to improve component design, lower costs, and reduce downtime.

NREL examines the operational requirements and technical challenges of supporting an influx of short-haul electric flights at existing transportation hubs , including major airports such as Denver International Airport. Small electric aircraft could potentially leverage lower operating costs, therefore offering an attractive opportunity for operators to provide direct service between rural communities and large-hub airports, closing the rural–urban transportation gap. Similarly, the exploration of airborne transportation and advanced air mobility is the focus of a partnership between NREL and Supernal —an air mobility company from Hyundai Motor Group developing electronic vertical takeoff and landing vehicles (eVTOL). NREL's research portfolio examines various facets of advanced air mobility, including eVTOL, concentrating on the feasibility, opportunities, and challenges of deploying infrastructure effectively.

Publications

View an overview of NREL's sustainable aviation initiative , all NREL publications about sustainable aviation research , and publications for the Athena airport modeling project .

The Challenge Ahead: A Critical Perspective on Meeting U.S. Growth Targets for Sustainable Aviation Fuel , NREL Technical Report (2024)

Federal Aviation Administration Vertiport Electrical Infrastructure Study , NREL Technical Report (2023)

Impacts of Regional Air Mobility and Electrified Aircraft on Airport Electricity Infrastructure and Demand , NREL Technical Report (2023)

Addressing Electric Aviation Infrastructure Cybersecurity Implementation , NREL Technical Report (2022)

A Roadmap Toward a Sustainable Aviation Ecosystem , NREL Technical Report (2022)

Electrification of Aircraft: Challenges, Barriers, and Potential Impact , NREL Technical Report (2021)

Explore fact sheets on emerging topic areas in energy and aviation, and what they could mean in the push to decarbonize the sector.

Pathways to a Sustainable Aviation Ecosystem: Aviation Energy Research and Operation Simulator (AEROSim) , NREL Fact Sheet (2024)

Learnings From the 2022 NREL Partner Forum: Research Needs for Liquid Fuels , NREL Fact Sheet (2024)

Learnings From the 2022 NREL Partner Forum: Research Needs for Energy Infrastructure , NREL Fact Sheet (2024)

Learnings From the 2022 NREL Partner Forum: Research Needs for Airport Ground Infrastructure and Operations , NREL Fact Sheet (2024)

Learnings From the 2022 NREL Partner Forum: Research Needs for Aircraft , NREL Fact Sheet (2024)

Accelerating Sustainable Aviation Fuel Technology From Laboratory to Deployment: An Overview of NREL’s Sustainable Aviation Fuel Pathways , NREL Brochure (2023)

Sustainable Aviation for Developing Economies , NREL Fact Sheet (2023)

Browse more sustainable aviation fact sheets and brochures .

Electric Aircrafts Will Need Powerful Ports

March 7, 2024

Biorefinery Model Points to Better Method at Producing Co-Products

Feb. 29, 2024

Partner With NREL

NREL works with stakeholders from across the aviation ecosystem to identify critical needs that will achieve deep decarbonization. In this way, cross-sector collaboration helps create targeted solutions for overcoming the biggest barriers to realize low- or net-zero-carbon aviation.

Learn how to partner with NREL .

Brett Oakleaf

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Sustainable Aviation

Sustainable aviation is a multi-disciplinary field that seeks solutions to improve the environmental and societal impacts of air transportation. It aims to reduce aviation’s contribution to climate change through new practices and radical innovation. This specialization covers highly efficient aircraft designs, novel propulsion systems, green aircraft technologies, and energy-optimized flight operations to reduce aircraft energy consumption, noise, and emissions. Research applications include appropriate fidelity and system-level analysis of advanced concept aircraft, electrified and hydrogen-powered aircraft, sustainable aviation fuels, renewable and alternative energy sources, improved air traffic management, emissions, and noise.

At Michigan, research and education in this specialization draw from the expertise of other research areas such as aerodynamics, propulsion, systems design, computation, autonomous systems, controls, structures, and materials. This brings a holistic approach that accounts for the life-cycle impact of greenhouse gas emissions in the design, operation, and sustainment of new aircraft concepts. Michigan’s sustainable aviation research programs involve collaboration with the aerospace industry and government.

SUSTAINABLE AVIATION FACULTY

Carlos E. S. Cesnik

Clarence L. (Kelly) Johnson Professor

Dennis Bernstein

James E. Knott Professor

George F. Halow

Professor of Practice

Gökçin Çınar

Assistant Professor

Ilya Kolmanovsky

Joaquim R. R. A. Martins

Pauline M. Sherman Collegiate Professor

Karthik Duraisamy

Associate Professor

Krzysztof Fidkowski

Mirko Gamba

Venkat Raman

Sustainable aviation research groups, active aeroelasticity and structures research laboratory (a2srl).

Research areas: Multiphysics, multifidelity modeling, analysis, simulation (MS&A) of aero-servo-elastic systems; Multifunctional structural design and integration

Integrated Design of Environmentally-friendly Aerospace Systems (IDEAS) Lab

The Integrated Design of Environmentally-friendly Aerospace Systems (IDEAS) Lab is directed by Professor Gokcin Cinar . Research at IDEAS Lab aims to bring a holistic approach that accounts for the system-level and life-cycle impact of greenhouse gas emissions in the design and operation of unconventional aircraft concepts. Revolutionary technologies, such as electrified propulsion and hydrogen combustion, open up a new and exciting design space with many challenges and uncertainties. 

At IDEAS Lab, we use physics-based modeling, probabilistic and statistical methods, and systems engineering principles to analyze, understand and design the complex system behavior of an aerospace vehicle. We leverage probabilistic design methods to reduce the uncertainty associated with novel concepts in early design phases. We build reduced order models to expedite the analysis and simulation of large-scale systems. These techniques allow us to perform sensitivity analysis, visualize tradeoffs, and explore a vast and uncharted design space under varying constraints at the early design stages.

Multidisciplinary Design Optimization Laboratory

Research areas: Aircraft design, Multidisciplinary design optimization, computational fluid dynamics

Computational Aerosciences Laboratory

Research areas: Analysis & Design of Electrified aircraft (E-VTOL) specifically; Fuel Cell modeling and control

Computational Fluid Dynamics Group

Research areas: Computational fluid dynamics, aerodynamics, reduced models for aircraft analysis and design

Laboratory for Air Transportation, Infrastructure, and Connected Environments (LATTICE)

The Laboratory for Air Transportation, Infrastructure, and Connected Environments (LATTICE) is directed by Assistant Professor Max Li and is focused on identifying and addressing research problems that contribute towards a safer, more efficient, more resilient, and user-oriented air transportation system. Examples of research projects and areas of interest include modeling the disruption and recovery process within air transportation networks, developing advanced air traffic flow management models and mechanisms, control and optimization of networked systems, and systems engineering concepts for UAS airspace and traffic management and Advanced Air Mobility (AAM).

PhD Researcher in Sustainable Aviation Fuel Deployment

Job Information

Offer description.

Four year tax-free stipend of €25,000 per annum + Academic Fees Paid (UK/EU/non-EU).

Commencing September 2024

Job Summary:

Leading to the award of a PhD in four years we are seeking a dedicated PhD candidate to join our pioneering research project focused on the deployment of Sustainable Aviation Fuels (SAF), in the island of Ireland and in an international context. This ambitious project aims to assess the readiness of raw materials, evaluate the compatibility of raw material availability with SAF production technologies, propose viable SAF production instances, and evaluate these instances through comprehensive techno-economic and life cycle assessments. The successful candidate will play a crucial role in driving forward international research to develop feasible, sustainable solutions for aviation fuel, contributing to significant environmental and economic benefits.

Research Environment:

You will join the Low Carbon Technologies Research Centre at Trinity College Dublin led by Profs. Mohammad Reza Ghaani and Stephen Dooley. This collaborative environment is enhanced by the Centre's strong connections with key aviation sector players such as the EU SAF Clearing House, European Union Aviation Safety Agency, The Boeing Company, Siemens Gas Turbines, Ryanair, Aircraft Leasing Ireland, and SMBC Aviation Capital, further emphasizing its pivotal role in driving sustainable energy solutions within the aviation industry. Your research will be collocated at the Trinity College Dublin SAF Research Facility @ SMBC Aviation Capital, a state of the art of research facility dedicated to SAF, sponsored by SMBC Aviation Capital, Ryanair and Science Foundation Ireland as a cross sector collaboration.  

Key Responsibilities:

• Assess the availability, inventory, and economic viability of raw materials for Sustainable Aviation Fuel (SAF) production, including their compatibility with SAF production technologies.
• Develop and evaluate SAF production plans for the Island of Ireland and Internationally, considering factors such as feasibility and scalability.
• Conduct techno-economic analyses (TEA) and life cycle assessments (LCA) to evaluate the sustainability and market potential of SAF projects.
• Engage with stakeholders for supply chain data,  technology foresight and feedback on feedstock on your modelling work.
• Write comprehensive reports summarising research findings and offering recommendations for SAF deployment.
• Publish scientific papers and present findings at international conferences to disseminate the project's results.

Essential Requirements:

• A first or upper second-class honours degree in chemical or mechanical engineering, chemistry, physics, renewable energy, or a related field.
• Expert command of written and spoke English.
• Excellent communication and interpersonal skills, with the ability to engage effectively with a wide range of stakeholders.

B eneficial Skills & Experience

• Experience in supply chain and value analysis is an advantage.
• Self-motivated with the capability to work independently as well as part of a team.
• Prior research experience in areas related to supply chain modelling, biofuels, renewable energy, sustainable aviation fuels is desirable, but not at all essential.
• Proficiency in at least one programming language and familiarity with Python.
• Strong analytical skills, with proficiency in quantitative analysis and familiarity with techno-economic and life cycle assessment methodologies.
• Experience with or understanding of Geographic Information Systems (GIS) software.

Application:

Prospective candidates should send a two-page CV containing names and contact details of two referees and a 1-page cover letter outlining your interest in the position and relevant experience to Dr. Ghaani at [email protected] . Please quote the entire job title in the subject line of your email. The application deadline is 1 st May 2024. Applications will be evaluated as received and candidates of all demographics, educational backgrounds, and genders, that show a good record of academic achievements will be considered.

Requirements

Additional information, work location(s), where to apply.

• Press Releases

Fujitsu signs MoU with Mitsubishi UFJ Financial Group, Inc. to drive nature positive actions

Planning new solutions for conservation of natural capital and biodiversity

Fujitsu limited.

Kawasaki, April, 1, 2024

Fujitsu today announced that on March 29, 2024 it concluded a memorandum of understanding with Mitsubishi UFJ Financial Group, Inc. (MUFG) to deliver nature-positive ( 1 ) outcomes through the conservation of natural capital and biodiversity. Based on the MoU, Fujitsu will leverage its AI, blockchain, and other technologies and its expertise in DX together with MUFG’ s global network, knowledge in financing, and new business creation to work on solutions to achieve nature-positive outcomes. By collaborating with various customers to promote nature-positive activities, Fujitsu will further create business opportunities that lead to the conservation of natural capital and biodiversity for its customers and strengthen its management foundation. Through these activities, Fujitsu will continue to work toward the creation of a future where both people and nature can thrive.

Fujitsu has outlined the solution of global environmental issues, including living in harmony with nature (protection and restoration of biodiversity) as one of its essential contributions within its vision for materiality-based contributions under its corporate social responsibility policy. Based on these goals, Fujitsu is collaborating with customers and partners to contribute to the solution of societal issues and achieve sustainable growth. As part of its commitment to delivering nature-positive outcomes, Fujitsu started collaboration with MUFG to work toward the conservation of natural capital and biodiversity.

Overview of the MoU

Based on the MoU, Fujitsu and MUFG will focus on the following:

• Workshops on nature positive actions (identification of issues, vision design, development of action plan, etc.)
• Planning and developing of digital, finance solutions that contribute to the solution of environmental challenges towards nature positive outcomes
• Trials utilizing newly developed solutions with customers (including solutions in the planning stage)
• Sharing information about activities, and consideration of the establishment of a consortium

Future Plans

Moving forward, Fujitsu will work together with MUFG to create solutions to deliver nature positive outcomes and provide them as offerings under Fujitsu Uvance , Fujitsu’s business model to contribute to the solution of societal issues across industry sectors.

• [1] Nature-positive : Concept that aims to stop and reverse the loss of biodiversity and to positively affect nature through social and economic activities.

Related Links

• Fujitsu Sustainability
• Fujitsu Materiality

Fujitsu’s Commitment to the Sustainable Development Goals (SDGs)

The Sustainable Development Goals (SDGs) adopted by the United Nations in 2015 represent a set of common goals to be achieved worldwide by 2030. Fujitsu’s purpose — “to make the world more sustainable by building trust in society through innovation” — is a promise to contribute to the vision of a better future empowered by the SDGs.

About Fujitsu

Fujitsu’s purpose is to make the world more sustainable by building trust in society through innovation. As the digital transformation partner of choice for customers in over 100 countries, our 124,000 employees work to resolve some of the greatest challenges facing humanity. Our range of services and solutions draw on five key technologies: Computing, Networks, AI, Data & Security, and Converging Technologies, which we bring together to deliver sustainability transformation. Fujitsu Limited (TSE:6702) reported consolidated revenues of 3.7 trillion yen (US$28 billion) for the fiscal year ended March 31, 2023 and remains the top digital services company in Japan by market share. Find out more: www.fujitsu.com .

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