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climate change definition research paper

The Science of Climate Change Explained: Facts, Evidence and Proof

Definitive answers to the big questions.

Credit... Photo Illustration by Andrea D'Aquino

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By Julia Rosen

Ms. Rosen is a journalist with a Ph.D. in geology. Her research involved studying ice cores from Greenland and Antarctica to understand past climate changes.

  • Published April 19, 2021 Updated Nov. 6, 2021

The science of climate change is more solid and widely agreed upon than you might think. But the scope of the topic, as well as rampant disinformation, can make it hard to separate fact from fiction. Here, we’ve done our best to present you with not only the most accurate scientific information, but also an explanation of how we know it.

How do we know climate change is really happening?

How much agreement is there among scientists about climate change, do we really only have 150 years of climate data how is that enough to tell us about centuries of change, how do we know climate change is caused by humans, since greenhouse gases occur naturally, how do we know they’re causing earth’s temperature to rise, why should we be worried that the planet has warmed 2°f since the 1800s, is climate change a part of the planet’s natural warming and cooling cycles, how do we know global warming is not because of the sun or volcanoes, how can winters and certain places be getting colder if the planet is warming, wildfires and bad weather have always happened. how do we know there’s a connection to climate change, how bad are the effects of climate change going to be, what will it cost to do something about climate change, versus doing nothing.

Climate change is often cast as a prediction made by complicated computer models. But the scientific basis for climate change is much broader, and models are actually only one part of it (and, for what it’s worth, they’re surprisingly accurate ).

For more than a century , scientists have understood the basic physics behind why greenhouse gases like carbon dioxide cause warming. These gases make up just a small fraction of the atmosphere but exert outsized control on Earth’s climate by trapping some of the planet’s heat before it escapes into space. This greenhouse effect is important: It’s why a planet so far from the sun has liquid water and life!

However, during the Industrial Revolution, people started burning coal and other fossil fuels to power factories, smelters and steam engines, which added more greenhouse gases to the atmosphere. Ever since, human activities have been heating the planet.

We know this is true thanks to an overwhelming body of evidence that begins with temperature measurements taken at weather stations and on ships starting in the mid-1800s. Later, scientists began tracking surface temperatures with satellites and looking for clues about climate change in geologic records. Together, these data all tell the same story: Earth is getting hotter.

Average global temperatures have increased by 2.2 degrees Fahrenheit, or 1.2 degrees Celsius, since 1880, with the greatest changes happening in the late 20th century. Land areas have warmed more than the sea surface and the Arctic has warmed the most — by more than 4 degrees Fahrenheit just since the 1960s. Temperature extremes have also shifted. In the United States, daily record highs now outnumber record lows two-to-one.

climate change definition research paper

Where it was cooler or warmer in 2020 compared with the middle of the 20th century

climate change definition research paper

This warming is unprecedented in recent geologic history. A famous illustration, first published in 1998 and often called the hockey-stick graph, shows how temperatures remained fairly flat for centuries (the shaft of the stick) before turning sharply upward (the blade). It’s based on data from tree rings, ice cores and other natural indicators. And the basic picture , which has withstood decades of scrutiny from climate scientists and contrarians alike, shows that Earth is hotter today than it’s been in at least 1,000 years, and probably much longer.

In fact, surface temperatures actually mask the true scale of climate change, because the ocean has absorbed 90 percent of the heat trapped by greenhouse gases . Measurements collected over the last six decades by oceanographic expeditions and networks of floating instruments show that every layer of the ocean is warming up. According to one study , the ocean has absorbed as much heat between 1997 and 2015 as it did in the previous 130 years.

We also know that climate change is happening because we see the effects everywhere. Ice sheets and glaciers are shrinking while sea levels are rising. Arctic sea ice is disappearing. In the spring, snow melts sooner and plants flower earlier. Animals are moving to higher elevations and latitudes to find cooler conditions. And droughts, floods and wildfires have all gotten more extreme. Models predicted many of these changes, but observations show they are now coming to pass.

Back to top .

There’s no denying that scientists love a good, old-fashioned argument. But when it comes to climate change, there is virtually no debate: Numerous studies have found that more than 90 percent of scientists who study Earth’s climate agree that the planet is warming and that humans are the primary cause. Most major scientific bodies, from NASA to the World Meteorological Organization , endorse this view. That’s an astounding level of consensus given the contrarian, competitive nature of the scientific enterprise, where questions like what killed the dinosaurs remain bitterly contested .

Scientific agreement about climate change started to emerge in the late 1980s, when the influence of human-caused warming began to rise above natural climate variability. By 1991, two-thirds of earth and atmospheric scientists surveyed for an early consensus study said that they accepted the idea of anthropogenic global warming. And by 1995, the Intergovernmental Panel on Climate Change, a famously conservative body that periodically takes stock of the state of scientific knowledge, concluded that “the balance of evidence suggests that there is a discernible human influence on global climate.” Currently, more than 97 percent of publishing climate scientists agree on the existence and cause of climate change (as does nearly 60 percent of the general population of the United States).

So where did we get the idea that there’s still debate about climate change? A lot of it came from coordinated messaging campaigns by companies and politicians that opposed climate action. Many pushed the narrative that scientists still hadn’t made up their minds about climate change, even though that was misleading. Frank Luntz, a Republican consultant, explained the rationale in an infamous 2002 memo to conservative lawmakers: “Should the public come to believe that the scientific issues are settled, their views about global warming will change accordingly,” he wrote. Questioning consensus remains a common talking point today, and the 97 percent figure has become something of a lightning rod .

To bolster the falsehood of lingering scientific doubt, some people have pointed to things like the Global Warming Petition Project, which urged the United States government to reject the Kyoto Protocol of 1997, an early international climate agreement. The petition proclaimed that climate change wasn’t happening, and even if it were, it wouldn’t be bad for humanity. Since 1998, more than 30,000 people with science degrees have signed it. However, nearly 90 percent of them studied something other than Earth, atmospheric or environmental science, and the signatories included just 39 climatologists. Most were engineers, doctors, and others whose training had little to do with the physics of the climate system.

A few well-known researchers remain opposed to the scientific consensus. Some, like Willie Soon, a researcher affiliated with the Harvard-Smithsonian Center for Astrophysics, have ties to the fossil fuel industry . Others do not, but their assertions have not held up under the weight of evidence. At least one prominent skeptic, the physicist Richard Muller, changed his mind after reassessing historical temperature data as part of the Berkeley Earth project. His team’s findings essentially confirmed the results he had set out to investigate, and he came away firmly convinced that human activities were warming the planet. “Call me a converted skeptic,” he wrote in an Op-Ed for the Times in 2012.

Mr. Luntz, the Republican pollster, has also reversed his position on climate change and now advises politicians on how to motivate climate action.

A final note on uncertainty: Denialists often use it as evidence that climate science isn’t settled. However, in science, uncertainty doesn’t imply a lack of knowledge. Rather, it’s a measure of how well something is known. In the case of climate change, scientists have found a range of possible future changes in temperature, precipitation and other important variables — which will depend largely on how quickly we reduce emissions. But uncertainty does not undermine their confidence that climate change is real and that people are causing it.

Earth’s climate is inherently variable. Some years are hot and others are cold, some decades bring more hurricanes than others, some ancient droughts spanned the better part of centuries. Glacial cycles operate over many millenniums. So how can scientists look at data collected over a relatively short period of time and conclude that humans are warming the planet? The answer is that the instrumental temperature data that we have tells us a lot, but it’s not all we have to go on.

Historical records stretch back to the 1880s (and often before), when people began to regularly measure temperatures at weather stations and on ships as they traversed the world’s oceans. These data show a clear warming trend during the 20th century.

climate change definition research paper

Global average temperature compared with the middle of the 20th century

+0.75°C

–0.25°

climate change definition research paper

Some have questioned whether these records could be skewed, for instance, by the fact that a disproportionate number of weather stations are near cities, which tend to be hotter than surrounding areas as a result of the so-called urban heat island effect. However, researchers regularly correct for these potential biases when reconstructing global temperatures. In addition, warming is corroborated by independent data like satellite observations, which cover the whole planet, and other ways of measuring temperature changes.

Much has also been made of the small dips and pauses that punctuate the rising temperature trend of the last 150 years. But these are just the result of natural climate variability or other human activities that temporarily counteract greenhouse warming. For instance, in the mid-1900s, internal climate dynamics and light-blocking pollution from coal-fired power plants halted global warming for a few decades. (Eventually, rising greenhouse gases and pollution-control laws caused the planet to start heating up again.) Likewise, the so-called warming hiatus of the 2000s was partly a result of natural climate variability that allowed more heat to enter the ocean rather than warm the atmosphere. The years since have been the hottest on record .

Still, could the entire 20th century just be one big natural climate wiggle? To address that question, we can look at other kinds of data that give a longer perspective. Researchers have used geologic records like tree rings, ice cores, corals and sediments that preserve information about prehistoric climates to extend the climate record. The resulting picture of global temperature change is basically flat for centuries, then turns sharply upward over the last 150 years. It has been a target of climate denialists for decades. However, study after study has confirmed the results , which show that the planet hasn’t been this hot in at least 1,000 years, and probably longer.

Scientists have studied past climate changes to understand the factors that can cause the planet to warm or cool. The big ones are changes in solar energy, ocean circulation, volcanic activity and the amount of greenhouse gases in the atmosphere. And they have each played a role at times.

For example, 300 years ago, a combination of reduced solar output and increased volcanic activity cooled parts of the planet enough that Londoners regularly ice skated on the Thames . About 12,000 years ago, major changes in Atlantic circulation plunged the Northern Hemisphere into a frigid state. And 56 million years ago, a giant burst of greenhouse gases, from volcanic activity or vast deposits of methane (or both), abruptly warmed the planet by at least 9 degrees Fahrenheit, scrambling the climate, choking the oceans and triggering mass extinctions.

In trying to determine the cause of current climate changes, scientists have looked at all of these factors . The first three have varied a bit over the last few centuries and they have quite likely had modest effects on climate , particularly before 1950. But they cannot account for the planet’s rapidly rising temperature, especially in the second half of the 20th century, when solar output actually declined and volcanic eruptions exerted a cooling effect.

That warming is best explained by rising greenhouse gas concentrations . Greenhouse gases have a powerful effect on climate (see the next question for why). And since the Industrial Revolution, humans have been adding more of them to the atmosphere, primarily by extracting and burning fossil fuels like coal, oil and gas, which releases carbon dioxide.

Bubbles of ancient air trapped in ice show that, before about 1750, the concentration of carbon dioxide in the atmosphere was roughly 280 parts per million. It began to rise slowly and crossed the 300 p.p.m. threshold around 1900. CO2 levels then accelerated as cars and electricity became big parts of modern life, recently topping 420 p.p.m . The concentration of methane, the second most important greenhouse gas, has more than doubled. We’re now emitting carbon much faster than it was released 56 million years ago .

climate change definition research paper

30 billion metric tons

Carbon dioxide emitted worldwide 1850-2017

Rest of world

Other developed

European Union

Developed economies

Other countries

United States

climate change definition research paper

E.U. and U.K.

climate change definition research paper

These rapid increases in greenhouse gases have caused the climate to warm abruptly. In fact, climate models suggest that greenhouse warming can explain virtually all of the temperature change since 1950. According to the most recent report by the Intergovernmental Panel on Climate Change, which assesses published scientific literature, natural drivers and internal climate variability can only explain a small fraction of late-20th century warming.

Another study put it this way: The odds of current warming occurring without anthropogenic greenhouse gas emissions are less than 1 in 100,000 .

But greenhouse gases aren’t the only climate-altering compounds people put into the air. Burning fossil fuels also produces particulate pollution that reflects sunlight and cools the planet. Scientists estimate that this pollution has masked up to half of the greenhouse warming we would have otherwise experienced.

Greenhouse gases like water vapor and carbon dioxide serve an important role in the climate. Without them, Earth would be far too cold to maintain liquid water and humans would not exist!

Here’s how it works: the planet’s temperature is basically a function of the energy the Earth absorbs from the sun (which heats it up) and the energy Earth emits to space as infrared radiation (which cools it down). Because of their molecular structure, greenhouse gases temporarily absorb some of that outgoing infrared radiation and then re-emit it in all directions, sending some of that energy back toward the surface and heating the planet . Scientists have understood this process since the 1850s .

Greenhouse gas concentrations have varied naturally in the past. Over millions of years, atmospheric CO2 levels have changed depending on how much of the gas volcanoes belched into the air and how much got removed through geologic processes. On time scales of hundreds to thousands of years, concentrations have changed as carbon has cycled between the ocean, soil and air.

Today, however, we are the ones causing CO2 levels to increase at an unprecedented pace by taking ancient carbon from geologic deposits of fossil fuels and putting it into the atmosphere when we burn them. Since 1750, carbon dioxide concentrations have increased by almost 50 percent. Methane and nitrous oxide, other important anthropogenic greenhouse gases that are released mainly by agricultural activities, have also spiked over the last 250 years.

We know based on the physics described above that this should cause the climate to warm. We also see certain telltale “fingerprints” of greenhouse warming. For example, nights are warming even faster than days because greenhouse gases don’t go away when the sun sets. And upper layers of the atmosphere have actually cooled, because more energy is being trapped by greenhouse gases in the lower atmosphere.

We also know that we are the cause of rising greenhouse gas concentrations — and not just because we can measure the CO2 coming out of tailpipes and smokestacks. We can see it in the chemical signature of the carbon in CO2.

Carbon comes in three different masses: 12, 13 and 14. Things made of organic matter (including fossil fuels) tend to have relatively less carbon-13. Volcanoes tend to produce CO2 with relatively more carbon-13. And over the last century, the carbon in atmospheric CO2 has gotten lighter, pointing to an organic source.

We can tell it’s old organic matter by looking for carbon-14, which is radioactive and decays over time. Fossil fuels are too ancient to have any carbon-14 left in them, so if they were behind rising CO2 levels, you would expect the amount of carbon-14 in the atmosphere to drop, which is exactly what the data show .

It’s important to note that water vapor is the most abundant greenhouse gas in the atmosphere. However, it does not cause warming; instead it responds to it . That’s because warmer air holds more moisture, which creates a snowball effect in which human-caused warming allows the atmosphere to hold more water vapor and further amplifies climate change. This so-called feedback cycle has doubled the warming caused by anthropogenic greenhouse gas emissions.

A common source of confusion when it comes to climate change is the difference between weather and climate. Weather is the constantly changing set of meteorological conditions that we experience when we step outside, whereas climate is the long-term average of those conditions, usually calculated over a 30-year period. Or, as some say: Weather is your mood and climate is your personality.

So while 2 degrees Fahrenheit doesn’t represent a big change in the weather, it’s a huge change in climate. As we’ve already seen, it’s enough to melt ice and raise sea levels, to shift rainfall patterns around the world and to reorganize ecosystems, sending animals scurrying toward cooler habitats and killing trees by the millions.

It’s also important to remember that two degrees represents the global average, and many parts of the world have already warmed by more than that. For example, land areas have warmed about twice as much as the sea surface. And the Arctic has warmed by about 5 degrees. That’s because the loss of snow and ice at high latitudes allows the ground to absorb more energy, causing additional heating on top of greenhouse warming.

Relatively small long-term changes in climate averages also shift extremes in significant ways. For instance, heat waves have always happened, but they have shattered records in recent years. In June of 2020, a town in Siberia registered temperatures of 100 degrees . And in Australia, meteorologists have added a new color to their weather maps to show areas where temperatures exceed 125 degrees. Rising sea levels have also increased the risk of flooding because of storm surges and high tides. These are the foreshocks of climate change.

And we are in for more changes in the future — up to 9 degrees Fahrenheit of average global warming by the end of the century, in the worst-case scenario . For reference, the difference in global average temperatures between now and the peak of the last ice age, when ice sheets covered large parts of North America and Europe, is about 11 degrees Fahrenheit.

Under the Paris Climate Agreement, which President Biden recently rejoined, countries have agreed to try to limit total warming to between 1.5 and 2 degrees Celsius, or 2.7 and 3.6 degrees Fahrenheit, since preindustrial times. And even this narrow range has huge implications . According to scientific studies, the difference between 2.7 and 3.6 degrees Fahrenheit will very likely mean the difference between coral reefs hanging on or going extinct, and between summer sea ice persisting in the Arctic or disappearing completely. It will also determine how many millions of people suffer from water scarcity and crop failures, and how many are driven from their homes by rising seas. In other words, one degree Fahrenheit makes a world of difference.

Earth’s climate has always changed. Hundreds of millions of years ago, the entire planet froze . Fifty million years ago, alligators lived in what we now call the Arctic . And for the last 2.6 million years, the planet has cycled between ice ages when the planet was up to 11 degrees cooler and ice sheets covered much of North America and Europe, and milder interglacial periods like the one we’re in now.

Climate denialists often point to these natural climate changes as a way to cast doubt on the idea that humans are causing climate to change today. However, that argument rests on a logical fallacy. It’s like “seeing a murdered body and concluding that people have died of natural causes in the past, so the murder victim must also have died of natural causes,” a team of social scientists wrote in The Debunking Handbook , which explains the misinformation strategies behind many climate myths.

Indeed, we know that different mechanisms caused the climate to change in the past. Glacial cycles, for example, were triggered by periodic variations in Earth’s orbit , which take place over tens of thousands of years and change how solar energy gets distributed around the globe and across the seasons.

These orbital variations don’t affect the planet’s temperature much on their own. But they set off a cascade of other changes in the climate system; for instance, growing or melting vast Northern Hemisphere ice sheets and altering ocean circulation. These changes, in turn, affect climate by altering the amount of snow and ice, which reflect sunlight, and by changing greenhouse gas concentrations. This is actually part of how we know that greenhouse gases have the ability to significantly affect Earth’s temperature.

For at least the last 800,000 years , atmospheric CO2 concentrations oscillated between about 180 parts per million during ice ages and about 280 p.p.m. during warmer periods, as carbon moved between oceans, forests, soils and the atmosphere. These changes occurred in lock step with global temperatures, and are a major reason the entire planet warmed and cooled during glacial cycles, not just the frozen poles.

Today, however, CO2 levels have soared to 420 p.p.m. — the highest they’ve been in at least three million years . The concentration of CO2 is also increasing about 100 times faster than it did at the end of the last ice age. This suggests something else is going on, and we know what it is: Since the Industrial Revolution, humans have been burning fossil fuels and releasing greenhouse gases that are heating the planet now (see Question 5 for more details on how we know this, and Questions 4 and 8 for how we know that other natural forces aren’t to blame).

Over the next century or two, societies and ecosystems will experience the consequences of this climate change. But our emissions will have even more lasting geologic impacts: According to some studies, greenhouse gas levels may have already warmed the planet enough to delay the onset of the next glacial cycle for at least an additional 50,000 years.

The sun is the ultimate source of energy in Earth’s climate system, so it’s a natural candidate for causing climate change. And solar activity has certainly changed over time. We know from satellite measurements and other astronomical observations that the sun’s output changes on 11-year cycles. Geologic records and sunspot numbers, which astronomers have tracked for centuries, also show long-term variations in the sun’s activity, including some exceptionally quiet periods in the late 1600s and early 1800s.

We know that, from 1900 until the 1950s, solar irradiance increased. And studies suggest that this had a modest effect on early 20th century climate, explaining up to 10 percent of the warming that’s occurred since the late 1800s. However, in the second half of the century, when the most warming occurred, solar activity actually declined . This disparity is one of the main reasons we know that the sun is not the driving force behind climate change.

Another reason we know that solar activity hasn’t caused recent warming is that, if it had, all the layers of the atmosphere should be heating up. Instead, data show that the upper atmosphere has actually cooled in recent decades — a hallmark of greenhouse warming .

So how about volcanoes? Eruptions cool the planet by injecting ash and aerosol particles into the atmosphere that reflect sunlight. We’ve observed this effect in the years following large eruptions. There are also some notable historical examples, like when Iceland’s Laki volcano erupted in 1783, causing widespread crop failures in Europe and beyond, and the “ year without a summer ,” which followed the 1815 eruption of Mount Tambora in Indonesia.

Since volcanoes mainly act as climate coolers, they can’t really explain recent warming. However, scientists say that they may also have contributed slightly to rising temperatures in the early 20th century. That’s because there were several large eruptions in the late 1800s that cooled the planet, followed by a few decades with no major volcanic events when warming caught up. During the second half of the 20th century, though, several big eruptions occurred as the planet was heating up fast. If anything, they temporarily masked some amount of human-caused warming.

The second way volcanoes can impact climate is by emitting carbon dioxide. This is important on time scales of millions of years — it’s what keeps the planet habitable (see Question 5 for more on the greenhouse effect). But by comparison to modern anthropogenic emissions, even big eruptions like Krakatoa and Mount St. Helens are just a drop in the bucket. After all, they last only a few hours or days, while we burn fossil fuels 24-7. Studies suggest that, today, volcanoes account for 1 to 2 percent of total CO2 emissions.

When a big snowstorm hits the United States, climate denialists can try to cite it as proof that climate change isn’t happening. In 2015, Senator James Inhofe, an Oklahoma Republican, famously lobbed a snowball in the Senate as he denounced climate science. But these events don’t actually disprove climate change.

While there have been some memorable storms in recent years, winters are actually warming across the world. In the United States, average temperatures in December, January and February have increased by about 2.5 degrees this century.

On the flip side, record cold days are becoming less common than record warm days. In the United States, record highs now outnumber record lows two-to-one . And ever-smaller areas of the country experience extremely cold winter temperatures . (The same trends are happening globally.)

So what’s with the blizzards? Weather always varies, so it’s no surprise that we still have severe winter storms even as average temperatures rise. However, some studies suggest that climate change may be to blame. One possibility is that rapid Arctic warming has affected atmospheric circulation, including the fast-flowing, high-altitude air that usually swirls over the North Pole (a.k.a. the Polar Vortex ). Some studies suggest that these changes are bringing more frigid temperatures to lower latitudes and causing weather systems to stall , allowing storms to produce more snowfall. This may explain what we’ve experienced in the U.S. over the past few decades, as well as a wintertime cooling trend in Siberia , although exactly how the Arctic affects global weather remains a topic of ongoing scientific debate .

Climate change may also explain the apparent paradox behind some of the other places on Earth that haven’t warmed much. For instance, a splotch of water in the North Atlantic has cooled in recent years, and scientists say they suspect that may be because ocean circulation is slowing as a result of freshwater streaming off a melting Greenland . If this circulation grinds almost to a halt, as it’s done in the geologic past, it would alter weather patterns around the world.

Not all cold weather stems from some counterintuitive consequence of climate change. But it’s a good reminder that Earth’s climate system is complex and chaotic, so the effects of human-caused changes will play out differently in different places. That’s why “global warming” is a bit of an oversimplification. Instead, some scientists have suggested that the phenomenon of human-caused climate change would more aptly be called “ global weirding .”

Extreme weather and natural disasters are part of life on Earth — just ask the dinosaurs. But there is good evidence that climate change has increased the frequency and severity of certain phenomena like heat waves, droughts and floods. Recent research has also allowed scientists to identify the influence of climate change on specific events.

Let’s start with heat waves . Studies show that stretches of abnormally high temperatures now happen about five times more often than they would without climate change, and they last longer, too. Climate models project that, by the 2040s, heat waves will be about 12 times more frequent. And that’s concerning since extreme heat often causes increased hospitalizations and deaths, particularly among older people and those with underlying health conditions. In the summer of 2003, for example, a heat wave caused an estimated 70,000 excess deaths across Europe. (Human-caused warming amplified the death toll .)

Climate change has also exacerbated droughts , primarily by increasing evaporation. Droughts occur naturally because of random climate variability and factors like whether El Niño or La Niña conditions prevail in the tropical Pacific. But some researchers have found evidence that greenhouse warming has been affecting droughts since even before the Dust Bowl . And it continues to do so today. According to one analysis , the drought that afflicted the American Southwest from 2000 to 2018 was almost 50 percent more severe because of climate change. It was the worst drought the region had experienced in more than 1,000 years.

Rising temperatures have also increased the intensity of heavy precipitation events and the flooding that often follows. For example, studies have found that, because warmer air holds more moisture, Hurricane Harvey, which struck Houston in 2017, dropped between 15 and 40 percent more rainfall than it would have without climate change.

It’s still unclear whether climate change is changing the overall frequency of hurricanes, but it is making them stronger . And warming appears to favor certain kinds of weather patterns, like the “ Midwest Water Hose ” events that caused devastating flooding across the Midwest in 2019 .

It’s important to remember that in most natural disasters, there are multiple factors at play. For instance, the 2019 Midwest floods occurred after a recent cold snap had frozen the ground solid, preventing the soil from absorbing rainwater and increasing runoff into the Missouri and Mississippi Rivers. These waterways have also been reshaped by levees and other forms of river engineering, some of which failed in the floods.

Wildfires are another phenomenon with multiple causes. In many places, fire risk has increased because humans have aggressively fought natural fires and prevented Indigenous peoples from carrying out traditional burning practices. This has allowed fuel to accumulate that makes current fires worse .

However, climate change still plays a major role by heating and drying forests, turning them into tinderboxes. Studies show that warming is the driving factor behind the recent increases in wildfires; one analysis found that climate change is responsible for doubling the area burned across the American West between 1984 and 2015. And researchers say that warming will only make fires bigger and more dangerous in the future.

It depends on how aggressively we act to address climate change. If we continue with business as usual, by the end of the century, it will be too hot to go outside during heat waves in the Middle East and South Asia . Droughts will grip Central America, the Mediterranean and southern Africa. And many island nations and low-lying areas, from Texas to Bangladesh, will be overtaken by rising seas. Conversely, climate change could bring welcome warming and extended growing seasons to the upper Midwest , Canada, the Nordic countries and Russia . Farther north, however, the loss of snow, ice and permafrost will upend the traditions of Indigenous peoples and threaten infrastructure.

It’s complicated, but the underlying message is simple: unchecked climate change will likely exacerbate existing inequalities . At a national level, poorer countries will be hit hardest, even though they have historically emitted only a fraction of the greenhouse gases that cause warming. That’s because many less developed countries tend to be in tropical regions where additional warming will make the climate increasingly intolerable for humans and crops. These nations also often have greater vulnerabilities, like large coastal populations and people living in improvised housing that is easily damaged in storms. And they have fewer resources to adapt, which will require expensive measures like redesigning cities, engineering coastlines and changing how people grow food.

Already, between 1961 and 2000, climate change appears to have harmed the economies of the poorest countries while boosting the fortunes of the wealthiest nations that have done the most to cause the problem, making the global wealth gap 25 percent bigger than it would otherwise have been. Similarly, the Global Climate Risk Index found that lower income countries — like Myanmar, Haiti and Nepal — rank high on the list of nations most affected by extreme weather between 1999 and 2018. Climate change has also contributed to increased human migration, which is expected to increase significantly .

Even within wealthy countries, the poor and marginalized will suffer the most. People with more resources have greater buffers, like air-conditioners to keep their houses cool during dangerous heat waves, and the means to pay the resulting energy bills. They also have an easier time evacuating their homes before disasters, and recovering afterward. Lower income people have fewer of these advantages, and they are also more likely to live in hotter neighborhoods and work outdoors, where they face the brunt of climate change.

These inequalities will play out on an individual, community, and regional level. A 2017 analysis of the U.S. found that, under business as usual, the poorest one-third of counties, which are concentrated in the South, will experience damages totaling as much as 20 percent of gross domestic product, while others, mostly in the northern part of the country, will see modest economic gains. Solomon Hsiang, an economist at University of California, Berkeley, and the lead author of the study, has said that climate change “may result in the largest transfer of wealth from the poor to the rich in the country’s history.”

Even the climate “winners” will not be immune from all climate impacts, though. Desirable locations will face an influx of migrants. And as the coronavirus pandemic has demonstrated, disasters in one place quickly ripple across our globalized economy. For instance, scientists expect climate change to increase the odds of multiple crop failures occurring at the same time in different places, throwing the world into a food crisis .

On top of that, warmer weather is aiding the spread of infectious diseases and the vectors that transmit them, like ticks and mosquitoes . Research has also identified troubling correlations between rising temperatures and increased interpersonal violence , and climate change is widely recognized as a “threat multiplier” that increases the odds of larger conflicts within and between countries. In other words, climate change will bring many changes that no amount of money can stop. What could help is taking action to limit warming.

One of the most common arguments against taking aggressive action to combat climate change is that doing so will kill jobs and cripple the economy. But this implies that there’s an alternative in which we pay nothing for climate change. And unfortunately, there isn’t. In reality, not tackling climate change will cost a lot , and cause enormous human suffering and ecological damage, while transitioning to a greener economy would benefit many people and ecosystems around the world.

Let’s start with how much it will cost to address climate change. To keep warming well below 2 degrees Celsius, the goal of the Paris Climate Agreement, society will have to reach net zero greenhouse gas emissions by the middle of this century. That will require significant investments in things like renewable energy, electric cars and charging infrastructure, not to mention efforts to adapt to hotter temperatures, rising sea-levels and other unavoidable effects of current climate changes. And we’ll have to make changes fast.

Estimates of the cost vary widely. One recent study found that keeping warming to 2 degrees Celsius would require a total investment of between $4 trillion and $60 trillion, with a median estimate of $16 trillion, while keeping warming to 1.5 degrees Celsius could cost between $10 trillion and $100 trillion, with a median estimate of $30 trillion. (For reference, the entire world economy was about $88 trillion in 2019.) Other studies have found that reaching net zero will require annual investments ranging from less than 1.5 percent of global gross domestic product to as much as 4 percent . That’s a lot, but within the range of historical energy investments in countries like the U.S.

Now, let’s consider the costs of unchecked climate change, which will fall hardest on the most vulnerable. These include damage to property and infrastructure from sea-level rise and extreme weather, death and sickness linked to natural disasters, pollution and infectious disease, reduced agricultural yields and lost labor productivity because of rising temperatures, decreased water availability and increased energy costs, and species extinction and habitat destruction. Dr. Hsiang, the U.C. Berkeley economist, describes it as “death by a thousand cuts.”

As a result, climate damages are hard to quantify. Moody’s Analytics estimates that even 2 degrees Celsius of warming will cost the world $69 trillion by 2100, and economists expect the toll to keep rising with the temperature. In a recent survey , economists estimated the cost would equal 5 percent of global G.D.P. at 3 degrees Celsius of warming (our trajectory under current policies) and 10 percent for 5 degrees Celsius. Other research indicates that, if current warming trends continue, global G.D.P. per capita will decrease between 7 percent and 23 percent by the end of the century — an economic blow equivalent to multiple coronavirus pandemics every year. And some fear these are vast underestimates .

Already, studies suggest that climate change has slashed incomes in the poorest countries by as much as 30 percent and reduced global agricultural productivity by 21 percent since 1961. Extreme weather events have also racked up a large bill. In 2020, in the United States alone, climate-related disasters like hurricanes, droughts, and wildfires caused nearly $100 billion in damages to businesses, property and infrastructure, compared to an average of $18 billion per year in the 1980s.

Given the steep price of inaction, many economists say that addressing climate change is a better deal . It’s like that old saying: an ounce of prevention is worth a pound of cure. In this case, limiting warming will greatly reduce future damage and inequality caused by climate change. It will also produce so-called co-benefits, like saving one million lives every year by reducing air pollution, and millions more from eating healthier, climate-friendly diets. Some studies even find that meeting the Paris Agreement goals could create jobs and increase global G.D.P . And, of course, reining in climate change will spare many species and ecosystems upon which humans depend — and which many people believe to have their own innate value.

The challenge is that we need to reduce emissions now to avoid damages later, which requires big investments over the next few decades. And the longer we delay, the more we will pay to meet the Paris goals. One recent analysis found that reaching net-zero by 2050 would cost the U.S. almost twice as much if we waited until 2030 instead of acting now. But even if we miss the Paris target, the economics still make a strong case for climate action, because every additional degree of warming will cost us more — in dollars, and in lives.

Veronica Penney contributed reporting.

Illustration photographs by Esther Horvath, Max Whittaker, David Maurice Smith and Talia Herman for The New York Times; Esther Horvath/Alfred-Wegener-Institut

An earlier version of this article misidentified the authors of The Debunking Handbook. It was written by social scientists who study climate communication, not a team of climate scientists.

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Opinion article, enhancing climate change research with open science.

climate change definition research paper

  • 1 Institute for the Oceans and Fisheries, University of British Columbia, Vancouver, BC, Canada
  • 2 Lancaster Environment Centre, Lancaster University, Lancaster, United Kingdom

Climate change research aims to understand global environmental change and how it will impact nature and society. The broad scope of climate change impacts means that successful adaptation and mitigation efforts will require an unprecedented collaboration effort that unites diverse disciplines and is able to rapidly respond to evolving climate issues ( IPCC, 2014 ). However, to achieve this aim, climate change research practices need updating: key research findings remain behind journal paywalls, and scientific progress can be impeded by low levels of reproducibility and transparency ( Ellison, 2010 ; Morueta-Holme et al., 2018 ), individual data ownership ( Hampton et al., 2015 ), and inefficient research workflows ( Lowndes et al., 2017 ). Furthermore, the level of public interest and policy engagement on climate change issues relies on fast communication of academic research to public institutions, with the result that the societal impact of climate change studies will differ according to their public availability and exposure. Here, we argue that by adopting open science (OS) principles, scientists can advance climate change research and accelerate efforts to mitigate impacts; especially for highly vulnerable developing regions of the world where research capacity is limited. We underscore the specific benefits of OS in raising the academic and societal impact of climate change research using citation and media metrics.

OS Facilitates Collaboration and Triage

The pace of climate change combined with a need to address societal and ecological impacts with limited resources mean that climate change research is fast-moving and interdisciplinary. Some fields, such as biological conservation, can be considered triage disciplines that require efficient and rapid decision making ( Bottrill et al., 2008 ). To this end, OS principles can help to minimize scientific uncertainty while increasing collaboration potential. For example, OS encourages data and code sharing ( Ram, 2013 ), assists the peer-review process with fully-reproducible manuscripts ( Lowndes et al., 2017 ), and reduces time to publication with preprints and open access (OA) journals ( Vale, 2015 ). Most scientists agree that publicly-funded research should be freely available ( Dallmeier-Tiessen et al., 2011 ) and several institutions have successfully implemented OS practices to share data and research in open-access archives. For instance, research on climate-driven thermal bleaching events in coral reef ecosystems has benefited hugely from open access to NOAA's large-scale monitoring data (e.g., NOAA CoralWatch; Harris et al., 2017 ). Although comprehensive open data policies have been implemented by some governments (e.g., USA; Obama, 2013 ) and journal groups (e.g., Nature editors, 2018 ), journal policies on data sharing are typically insufficient for adequate reproducibility ( Stodden et al., 2018 ). Nonetheless, these examples demonstrate importance of adopting open data principles; comprehensive uptake of these practices will substantially enhance the application of academic research to climate change issues.

Academic and non-academic communication of climate change may be especially important for developing nations. Most climate change research is published through institutes within the developed world ( McSweeney, 2015 ), yet the greatest impacts will be observed in some of the least developed and most vulnerable regions of the world ( IPCC, 2014 ; Blasiak et al., 2017 ). Inability to access subscription-only publications may inhibit science-based policy in developing countries. For example, inaccessibility of primary research has contributed to low citation rates in policy plans for tropical marine protected areas, implying that environmental management may fall behind current scientific knowledge ( Cvitanovic et al., 2014 ). With the rise in usage of publication repositories such as Sci-Hub ( https://en.wikipedia.org/wiki/Sci-Hub ), which enable users to download PDF versions of paywalled articles, there is clearly a widespread demand for OA research ( Bohannon, 2016 ; Himmelstein et al., 2018 ).

OA Benefits to Research Communication: Citations and Altmetric Data

Open science practices can result in greater public engagement ( Wang et al., 2015 ) and, through OA publications, increase citation rates (“the OA citation advantage”) ( Lawrence, 2001 ; Eysenbach, 2006 ). Using Scopus citation data, we show that the proportion of OA studies increased substantially over time in publications containing “climat* change” in their title, abstract, or keywords between 2007 and 2016 (Scopus; www.scopus.com ), accounting for only 4% in 2007 and increasing to 25% in 2016 (Figure 1 ). However, this varied by journal rank (JR). We categorized journals into four groups, using JRs that are 3-year weighted citation rates obtained from SCImago Journal Rankings (see Figure 1 caption for category breakdown; SCImago 1 ). For the low JR category, OA publications in 2016 accounted for < 20%, while the medium category had the largest OA proportion at 30%. High and very high categories had 23% and 26% OA, respectively. Popular OA journals such as PLoS ONE and Scientific Reports comprised 71 and 24% of OA publications within their JR groups (medium- and high-ranked, respectively), and 15 and 3% of all publications within their groups, respectively. Across all journal ranks, OA climate change studies were cited more than closed studies (Figure 2A ), indicating that adopting OA could lead to earlier and increased citations of climate change research, and thus accelerate scientific progression by building upon existing science at a faster rate ( Eysenbach, 2006 ; Lowndes et al., 2017 ). Though we used SCImago Journal Rankings to keep consistency with the Scopus citation database, such citation-based metrics are coarse measures of journal research quality, and do not represent research impact for individual papers ( Lariviere et al., 2016 ) or non-academic audiences.

www.frontiersin.org

Figure 1 . Increasing prevalence of open access (OA) climate studies published between 2007 and 2016. Proportional increase in OA climate change publications (black line) and across four journal ranking categories (colored lines; low = 0.14–0.93, medium = 0.93–1.5, high = 1.5–2.2, very high = 2.2–18.1). Publications were extracted from Scopus ( www.scopus.com ) for articles and reviews published between 2007 and 2016 containing the term “climat* change” in title, abstract, or keywords. We further restricted publications to those journals with >100 total citation records (i.e., journals which regularly published climate change research, n = 225). Journal rankings are 3-year weighted citation rates (SCImago Journal Rankings; www.scimagojr.com ), ranging from 0.14 to 18.13. Bins are the 25th, 50th, and 75th quantiles of the journal rank distribution.

www.frontiersin.org

Figure 2 . Citations, communication, and media influence of closed and open access climate change studies published between 2007 and 2016. Points are predicted mean number of citations (A) , news mentions (B) , twitter mentions (C) , and policy mentions (D) in four journal ranking categories, controlling for effects of publication year and journal on citations/mentions. Dashed lines are mean citations/mentions controlling for journal rank, publication year and journal name. Citations were extracted from Scopus for the same studies in Figure 1 . News, twitter and policy mentions were extracted from Altmetric ( www.altmetric.com ) for study DOIs in Figure 1 . Citations and mentions were averaged for each journal in each year, and fitted to linear mixed effects models with journal ranking bin (4 bins represented by the 25th, 50th, and 75th quantiles) and access (open/closed) as fixed effects and year and journal as random intercepts. Citations and mentions were log 10 transformed for normality and presented on a log 10 scale. All analyses were conducted in R 3.4.4 ( R Core Team, 2018 ).

Beyond academic citation advantages, OA climate change research can have a greater societal impact when studies are communicated to non-academic audiences by mainstream news and social media, as well as used by policymakers ( Wang et al., 2015 ; Bornmann et al., 2016 ). In “mentions” of climate change studies in online news sources, Twitter feeds, and policy documents ( www.altmetric.com ), we show that OA studies were communicated more frequently (Figures 2B–D ), likely due to those studies being more accessible to non-academic audiences. Despite the positive OA effect, the most widely-communicated papers were high impact and closed access papers (e.g., 88% of studies with >100 news mentions were closed access). High-ranking journals such as Nature and Science are often promoted with academic press releases, highlighting how paywalls can limit public understanding and engagement of academic knowledge ( Parker, 2013 ). Nonetheless, higher news and Twitter activity for OA studies—irrespective of journal rank—supports a longstanding perception that open research is more widely disseminated and discussed online ( Wang et al., 2015 ; Côté and Darling, 2018 ).

Policy documents cited open studies more often than closed, and this difference was consistent across JRs (Figure 2D ). Thus, when policymakers lack institutional access to paywalled journals, the OA effect may result in greater uptake of primary research into policy. However, because Altmetric tracks major policy groups in North America and Europe ( Bornmann et al., 2016 ), we note that these policy trends may be biased toward academic authors working for international organizations (e.g., Food and Agriculture Organization of the United Nations, World Bank, Intergovernmental Panel on Climate Change). While our results show a positive trend toward OA (Figure 1 ) and higher OA mentions in policy documents (Figure 2D ), important research still remains behind paywalls and there is evidence that subscription-only publishing models can limit the uptake of current scientific knowledge by policymakers (e.g., Cvitanovic et al., 2014 ; Fuller et al., 2014 ; Rafidimanantsoa et al., 2018 ). For example, OA may be especially important for small-scale, low-impact studies which are relevant for local policy but do not receive much media attention.

Transitioning to Open Climate Change Research

Core OS principles are simply the open sharing of data, code, and papers throughout the research process ( Hampton et al., 2015 ; McKiernan et al., 2016 ). Such practices have reformed entire disciplines (e.g., preprints in mathematics, open genome data in genetics; Nielsen, 2011 ), but the transition to OS for climate change research is incomplete. For climate change scientists, who must respond to evolving environmental changes with research that has considerable societal impact, the open sharing of data, code, and research outputs could be transformative (e.g., Lowndes et al., 2017 ). Because of the success of OS in other fields, tools for OS are already freely available (Table 1 ). For example, several preprint and data repositories target climate change fields (e.g., MarXiv for marine science), while existing version control and coding tools have been adapted for an OS workflow in environmental research (e.g., RStudio and Github, Lowndes et al., 2017 ).

www.frontiersin.org

Table 1 . Recommendations to advance climate change research with open science tools.

Despite the clear benefits of OS in enhancing research output and communication to stakeholders, considerable barriers to OS uptake persist, including closed publishing, fear of being “scooped,” and clarity of data ownership ( Nosek et al., 2015 ). Research outputs—usually publications—are already required by most granting agencies, where OA publishing costs are typically covered by grants and institutions ( Dallmeier-Tiessen et al., 2011 ). Furthermore, most climate change research is funded by developed countries yet may focus on climate issues in developing countries that often lack the institutional capacity for journal subscriptions and OA fees ( van Helden, 2012 ; McSweeney, 2015 ). Thus, to incentivize OS climate change research, we propose funding bodies should require grant holders to openly publish datasets, papers and code, and mandate active dissemination of climate change findings to stakeholders rather than passive dissemination by publication.

Scientists across disciplines have argued, convincingly, for improving research practices by adopting OS principles ( Hampton et al., 2015 ; Nosek et al., 2015 ; McKiernan et al., 2016 ). We extend these arguments to show that adoption of OS practices, such as OA publications, OS workflows, and sharing data, is particularly needed to improve the academic and societal impact of climate change research. Given that global efforts to combat climate change impacts will require both rapid collaborative research and communication among academics, policymakers and the public, climate change research is in urgent need of strong OS stewardship.

Data Availability

Journal citations and mentions were extracted from Scopus ( www.scopus.com ) and Altmetric ( www.altmetric.com ). We provide our queried search terms and R coding scripts at github.com/travistai2/open-science-cc.

Author Contributions

TT conceived the idea. TT and JR contributed equally to data analysis and writing.

TT is grateful for funding support from OceanCanada Partnership and MEOPAR.

Conflict of Interest Statement

The 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.

Acknowledgments

We thank Jonathan Adams for discussions about altmetrics, and William Cheung, Andres Cisneros, Cameron Freshwater, Nick Graham, Laura Kehoe, Sally Keith and Rashid Sumaila for useful comments on the manuscript.

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Keywords: interdisciplinary, science communication, open access, reproducibility, citation metrics, altmetrics

Citation: Tai TC and Robinson JPW (2018) Enhancing Climate Change Research With Open Science. Front. Environ. Sci . 6:115. doi: 10.3389/fenvs.2018.00115

Received: 03 June 2018; Accepted: 20 September 2018; Published: 11 October 2018.

Reviewed by:

Copyright © 2018 Tai and Robinson. 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: Travis C. Tai, [email protected]

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Open Access

Peer-reviewed

Research Article

Talking about Climate Change and Global Warming

Contributed equally to this work with: Maurice Lineman, Yuno Do

Affiliation College of Natural Sciences, Department of Biological Sciences, Pusan National University, Busan, South Korea

* E-mail: [email protected]

  • Maurice Lineman, 
  • Yuno Do, 
  • Ji Yoon Kim, 
  • Gea-Jae Joo

PLOS

  • Published: September 29, 2015
  • https://doi.org/10.1371/journal.pone.0138996
  • Reader Comments

Fig 1

The increasing prevalence of social networks provides researchers greater opportunities to evaluate and assess changes in public opinion and public sentiment towards issues of social consequence. Using trend and sentiment analysis is one method whereby researchers can identify changes in public perception that can be used to enhance the development of a social consciousness towards a specific public interest. The following study assessed Relative search volume (RSV) patterns for global warming (GW) and Climate change (CC) to determine public knowledge and awareness of these terms. In conjunction with this, the researchers looked at the sentiment connected to these terms in social media networks. It was found that there was a relationship between the awareness of the information and the amount of publicity generated around the terminology. Furthermore, the primary driver for the increase in awareness was an increase in publicity in either a positive or a negative light. Sentiment analysis further confirmed that the primary emotive connections to the words were derived from the original context in which the word was framed. Thus having awareness or knowledge of a topic is strongly related to its public exposure in the media, and the emotional context of this relationship is dependent on the context in which the relationship was originally established. This has value in fields like conservation, law enforcement, or other fields where the practice can and often does have two very strong emotive responses based on the context of the problems being examined.

Citation: Lineman M, Do Y, Kim JY, Joo G-J (2015) Talking about Climate Change and Global Warming. PLoS ONE 10(9): e0138996. https://doi.org/10.1371/journal.pone.0138996

Editor: Hayley J. Fowler, Newcastle University, UNITED KINGDOM

Received: August 18, 2014; Accepted: September 8, 2015; Published: September 29, 2015

Copyright: © 2015 Lineman et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

Data Availability: All relevant data are within the paper.

Funding: This study was financially supported by the 2015 Post-Doc Development Program of Pusan National University.

Competing interests: The authors have declared that no competing interests exist.

Introduction

Identifying trends in the population, used to be a long and drawn out process utilizing surveys and polls and then collating the data to determine what is currently most popular with the population [ 1 , 2 ]. This is true for everything that was of merit to the political organizations present, regarding any issue of political or public interest.

Recently, the use of the two terms ‘Climate Change’ and ‘Global Warming’ have become very visible to the public and their understanding of what is happening with respect to the climate [ 3 ]. The public response to all of the news and publicity about climate has been a search for understanding and comprehension, leading to support or disbelief. The two terms while having similarity in meaning are used in slightly different semantic contexts. The press in order to expand their news readership/viewer lists has chosen to use this ambiguity to their favor in providing news to the public [ 4 ]. Within the news releases, the expression ‘due to climate change’ has been used to explain phenomological causality.

These two terms “global warming–(GW)” and “climate change–(CC)” both play a role in how the public at large views the natural world and the changes occurring in it. They are used interactively by the news agencies, without a thought towards their actual meaning [ 3 , 4 ]. Therefore, the public in trying to identify changes in the news and their understanding of those changes looks for the meaning of those terms online. The extent of their knowledge can be examined by assessing the use of the terms in online search queries. Information searches using the internet are increasing, and therefore can indicate public or individual interest.

Internet search queries can be tracked using a variety of analytic engines that are independent of, or embedded into, the respective search engines (google trend, naver analytics) and are used to determine the popularity of a topic in terms of internet searches [ 5 ]. The trend engines will look for selected keywords from searches, keywords chosen for their relevance to the field or the query being performed.

The process of using social media to obtain information on public opinion is a practice that has been utilized with increasing frequency in modern research for subjects ranging from politics [ 6 , 7 ] to linguistics [ 8 – 10 ] complex systems [ 11 , 12 ] to environment [ 13 ]. This variety of research belies the flexibility of the approach, the large availability of data availability for mining in order to formulate a response to public opinion regarding the subject being assessed. In modern society understanding how the public responds regarding complex issues of societal importance [ 12 ].

While the two causally connected terms GW and CC are used interchangeably, they describe entirely different physical phenomena [ 14 ]. These two terms therefore can be used to determine how people understand the parallel concepts, especially if they are used as internet search query terms in trend analysis. However, searching the internet falls into two patterns, searches for work or for personal interest, neither of which can be determined from the trend engines. The By following the searches, it is possible to determine the range of public interest in the two terms, based on the respective volumes of the search queries. Previously in order to mine public opinion on a subject, government agencies had to revert to polling and surveys, which while being effective did not cover a very large component of the population [ 15 – 17 ].

Google trend data is one method of measuring popularity of a subject within the population. Individuals searching for a topic use search keywords to obtain the desired information [ 5 , 18 ]. These keywords are topic sensitive, and therefore indicate the level of knowledge regarding the searched topic. The two primary word phrases here “climate change” and “global warming” are unilateral terms that indicate a level of awareness about the issue which is indicative of the individuals interest in that subject [ 5 , 19 , 20 ]. Google trend data relates how often a term is searched, that is the frequency of a search term can be identified from the results of the Google® trend analysis. While frequency is not a direct measure of popularity, it does indicate if a search term is common or uncommon and the value of that term to the public at large. The relationship between frequency and popularity lies in the volume of searches by a large number of individuals over specific time duration. Therefore, by identifying the number of searches during a specific period, it is possible to come to a proximate understanding of how popular or common a term is for the general population [ 21 ]. However, the use of trend data is more appropriately used to identify awareness of an issue rather than its popularity.

This brings us to sentiment analysis. Part of the connection between the search and the populations’ awareness of an issue can be measured using how they refer to the subject in question. This sentiment, is found in different forms of social media, or social networking sites sites i.e. twitter®, Facebook®, linked in® and personal blogs [ 7 , 22 – 24 ]. Thus, the original information, which was found on the internet, becomes influenced by personal attitudes and opinions [ 25 ]and then redistributed throughout the internet, accessible to anyone who has an internet connection and the desire to search. This behavior affects the information that now provides the opportunity to assess public sentiment regarding the prevailing attitudes regarding environmental issues [ 26 , 27 ]. To assess this we used Google® and Twitter® data to understand public concerns related to climate change and global warming. Google trend was used to trace changes in interest between the two phenomena. Tweets (comments made on Twitter®) were analyzed to identify negative or positive emotional responses.

Comparatively, twitter data is more indicative of how people refer to topics of interest [ 28 – 31 ], in a manner that is very linguistically restricted. As well, twitter is used as a platform for verbal expression of emotional responses. Due to the restrictions on tweet size (each tweet can only be 140 characters in length), it is necessary to be more direct in dealing with topics of interest to the tweeter. Therefore, the tweets are linguistically more emotionally charged and can be used to define a level of emotional response by the tweeter.

The choice of target words for the tweets and for the Google trend searches were the specific topic phrases [ 32 , 33 ]. These were chosen because of the descriptive nature of the phrases. Scientific literature is very specific in its use and therefore has very definitive meanings. The appropriation of these words by the population as a method for describing their response to the variation in the environment provides the basis for the choice as target words for the study. The classification of the words as being positive versus negative lies in the direction provided by Frank Lutz. This politicization of a scientific word as a means of directing public awareness, means the prescription of one phrase (climate change) as being more positive than the other (global warming).

Global warming is defined as the long-term trend of increasing average global temperatures; alternatively, climate change is defined as a change in global or regional climate patterns, in particular a change apparent from the mid to late 20 th century onwards and attributed to the increased levels of atmospheric carbon dioxide arising from the use of fossil fuels. Therefore, the search keywords were chosen based on their scientific value and their public visibility. What is important about the choice of these search terms is that due to their scientific use, they describe a distinctly identifiable state. The more specific these words are, the less risk of the algorithm misinterpreting the keyword and thus having the results misinterpreted [ 34 – 36 ].

The purpose of the following study was to identify trends within search parameters for two specific sets of trend queries. The second purpose of the study was to identify how the public responds emotionally to those same queries. Finally, the purpose of the study was to determine if the two had any connections.

Data Collection

Public awareness of the terms climate change and global warming was identified using Google Trends (google.com/trends) and public databases of Google queries [ 37 ]. To specify the exact searches we used the two terms ‘climate change’ and ‘global warming’ as query phrases. Queries were normalized using relative search volume (RSV) to the period with the highest proportion of searches going to the focal terms (i.e. RSV = 100 is the period with the highest proportion for queries within a category and RSV = 50 when 50% of that is the highest search proportion). Two assumptions were necessary for this study. The first is, of the two terms, climate change and global warming, that which draws more search results is considered more interesting to the general population. The second assumption is that changes in keyword search patterns are indicators of the use of different forms of terminology used by the public. To analyze sentiments related to climate change and global warming, tweets containing acronyms for climate change and global warming were collected from Twitter API for the period from October 12 to December 12, 2013. A total of 21,182 and 26,462 tweets referencing the terms climate change and global warming were collected respectively. When duplicated tweets were identified, they were removed from the analysis. The remaining tweets totaled 8,465 (climate change) and 8,263 (global warming) were compiled for the sentiment analysis.

Data Analysis

In Twitter® comments are emotionally loaded, due to their textually shortened nature. Sentiment analysis, which is in effect opinion mining, is how opinions in texts are assessed, along with how they are expressed in terms of positive, neutral or negative content [ 36 ]. Nasukawa and Yi [ 10 ]state that sentiment analysis identifies statements of sentiment and classifies those statements based on their polarity and strength along with their relationship to the topic.

Sentiment analysis was conducted using Semantria® software ( www.semantria.com ), which is available as an MS Excel spreadsheet application plugin. The plugin is broken into parts of speech (POS), the algorithm within the plugin then identifies sentiment-laden phrases and then scores them from -10 to 10 on a logarithmic scale, and finally the scores for each POS are tabulated to identify the final score for each phrase. The tweets are then via statistical inferences tagged with a numerical value from -2 to 2 and given a polarity, which is classified as positive, neutral or negative [ 36 ]. Semantria®, the program utilized for this study, has been used since 2011 to perform sentiment analyses [ 7 , 22 ].

For the analysis, an identity column was added to the dataset to enable analysis of individual tweets with respect to sentiment. A basic sentiment analysis was conducted on the dataset using the Semantria® plugin. The plugin uses a cloud based corpus of words tagged with sentimental connotations to analyze the dataset. Through statistical inference, each tweet is tagged with a sentiment value from -2 to +2 and a polarity of (i) negative, (ii) neutral, or (iii) positive. Positive nature increases with increasing positive sentiment. The nature of the language POS assignation is dependent upon the algorithmic classification parameters defined by the Semantria® program. Determining polarity for each POS is achieved using the relationship between the words as well as the words themselves. By assigning negative values to specific negative phrases, it limits the use of non-specific negation processes in language; however, the program has been trained to assess non-specific linguistic negations in context.

A tweet term frequency dictionary was computed using the N-gram method from the corpus of climate change and global warming [ 38 ]. We used a combination of unigrams and bigrams, which has been reported to be effective [ 39 ]. Before using the N-gram method, typological symbols were removed using the open source code editor (i.e. Notepad) or Microsoft Words’ “Replace” function.

Differences in RSV’s for the terms global warming and climate change for the investigation period were identified using a paired t-test. Pettitt and Mann-Kendall tests were used to identify changes in distribution, averages and the presence of trends within the weekly RSV’s. The Pettitt and MK tests, which assume a stepwise shift in the mean (a break point) and are sensitive to breaks in the middle of a time series, were applied to test for homogeneity in the data [ 40 ]. Temporal trends within the time series were analyzed with Spearman’s non-parametric correlation analysis. A paired t-test and Spearman’s non-parametric correlation analysis were conducted using SPSS software (version 17.0 SPSS In corp. Chicago IL) and Pettitt and MK tests were conducted using XLSTAT (version 7.0).

To determine the accuracy and reliability of the Sentiment analysis, a Pearson’s chi-square analysis was performed. This test identifies the difference ratio for each emotional response group, and then compares them to determine reliance and probability of interactions between the variables, in this case the terms global warming and climate change.

According to Google trend ( Fig 1 ) from 2004–2014, people searched for the term global warming (n = 8,464; mean ± S.D = 25.33 ± 2.05) more frequently than climate change (n = 8,283; mean ± S.D. = 7.97±0.74). Although the Intergovernmental Panel on Climate Change (IPCC) published its Fourth Assessment Report in 2007 and was awarded the Nobel Prize, interest in the term global warming as used in internet searches has decreased significantly since 2010 (K = 51493, t = 2010-May-23, P<0.001). Further the change in RSV also been indicative of the decreased pattern (Kendall’s tau = -0.336, S = -44563, P<0.001). The use of the term “climate change” has risen marginally since 2006 (K = 38681, t = 2006-Oct-08, P<0.001), as indicated by a slight increase (Kendall’s tau = -0.07, S = 9068, P<0.001). These findings show that the difference in usage of the two terms climate change and global warming has recently been reduced.

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https://doi.org/10.1371/journal.pone.0138996.g001

The sentiment analysis of tweets ( Fig 2 ) shows that people felt more negative about the term global warming (sentiment index = -0.21±0.34) than climate change (-0.068±0.36). Global warming tweets reflecting negative sentiments via descriptions such as, “bad, fail, crazy, afraid and catastrophe,” represented 52.1% of the total number of tweets. As an example, the tweet, “Supposed to snow here in the a.m.! OMG. So sick of already, but Saturday says 57 WTF!” had the lowest score at -1.8. Another observation was that 40.7% of tweets, including “agree, recommend, rescue, hope, and contribute,” were regarded as neutral. While 7.2% of tweets conveyed positive messages such as, “good, accept, interesting, and truth.” One positive global warming tweet, read, “So if we didn’t have global warming, would all this rain be snow!”. The results from the Pearson’s chi-square analysis showed that the relationship between the variables was significant (Pearson’s chi-square –763.98, d.f. = 2, P<0.001). Negative climate change tweets represented 33.1% of the total while neutral tweets totaled 49.8%, while positive climate change tweets totaled 17.1%.

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https://doi.org/10.1371/journal.pone.0138996.g002

Understandably, global warming and climate change are the terms used most frequently to describe each phenomenon, respectively, as revealed by the N-gram analysis ( Table 1 ). When people tweeted about global warming, they repeatedly used associated such as, “ice, snow, Arctic, and sea.” In contrast, tweets referring to climate change commonly used, “report, IPCC, world, science, environment, and scientist.” People seem to think that climate change as a phenomenon is revealed by scientific investigation.

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https://doi.org/10.1371/journal.pone.0138996.t001

Internet searches are one way of understanding the popularity of an idea or meme within the public at large. Within that frame of reference, the public looks at these two terms global warming and climate change and their awareness of the roles of the two phenomena [ 41 ]. From 2004 to 2008, the search volumes for the term global warming far exceeded the term climate change. The range for the term global warming in Relative search volumes (RSV) was more than double that of climate change in this period ( Fig 1 ). From 2008 on the RSV’s began to steadily decrease until in 2014 when the RSV’s for the term global warming were nearly identical to those for the term climate change. From 2008 there was an increase in the RSVs for CC until 2010 at which point the RSVs also began to decline for the term climate change. The decline in the term climate change for the most part paralleled that of the term global warming from 2010 on to the present.

While we are seeing the increases and decreases in RSVs for both the terms global warming and climate change, the most notable changes occur when the gap between the terms was the greatest, from 2008 through to 2010. During this period, there was a very large gap found between the RSVs for the terms global warming and climate change; however, searches for the term climate change was increasing while searches for the tem global warming were decreasing. The counter movement of the RSV’s for the two terms shows that there is a trend happening with respect to term recognition. At this point, there was an increase in the use of the CC term while there was a corresponding decrease in the use of the GW term. The change in the use of the term could have been due to changes in the publicity of the respective terms, since at this point, the CC term was being used more visibly in the media, and therefore the CC term was showing up in headlines and the press, resulting in a larger number of searches for the CC term. Correspondingly, the decrease in the use of the GW term is likely due to the changes in how the term was perceived by the public. The public press determines how a term is used, since they are the body that consistently utilizes a term throughout its visible life. The two terms, regardless of how they differ in meaning, are used with purpose in a scientific context, yet the public at large lacks this definition and therefore has no knowledge of the variations in the terms themselves [ 42 ]. Therefore, when searching for a term, the public may very well, choose the search term that they are more comfortable with, resulting in a search bias, since they do not know the scientific use of the term.

The increase in the use of the CC term, could be a direct result of the release of the fourth assessment report for the IPCC in 2007 [ 43 ]. The publicity related to the release of this document, which was preceded by the release of the Al Gore produced documentary “An Inconvenient Truth”, both of which were followed by the selection by the Nobel committee of Al Gore and the IPCC scientists for the Nobel Prize in 2007 [ 43 ]. These three acts individually may not have created the increased media presence of the CC term; however, at the time the three events pushed the CC term and increased its exposure to the public which further drove the public to push for positive environmental change at the political level [ 44 , 45 ]. This could very well have resulted in the increases in RSV’s for the CC term. This point is more likely to depict accurately the situation, since in 2010 the use of the two terms decline at almost the same rate, with nearly the same patterns.

Thus with respect to trend analysis, what is interesting is that RSVs are paralleling the press for specific environmental events that have predetermined value according to the press. The press in increasing the visibility of the term may drive the increases in the RSV’s for that term. Prior to 2007, the press was using the GW term indiscriminately whenever issues affecting the global climate arose; however, after the movie, the report and then the Nobel prize the terminology used by the press switched and the CC term became the word du jour. This increased the visibility of the word to the public, thereby it may be that increasing public awareness of the word, but not necessarily its import, is the source for the increases in RSV’s between 2008 and 2010.

The decline in the RSV’s then is a product of the lack of publicity about the issue. As the terms become more familiar, there would be less necessity to drive the term publicly into the spotlight; however, occasionally events/situations arise that refocus the issue creating a resurgence in the terms even though they have reached their peak visibility between 2008 and 2010.

Since these terms have such an impact on the daily lives of the public via local regional national and global weather it is understandable that they have an emotional component to them [ 46 ]. Every country has its jokes about the weather, where they come up with cliché’s about the weather (i.e. if you don’t like the weather wait 10minutes) that often show their discord and disjunction with natural climatological patterns [ 47 ]. Furthermore, some sectors of society (farmers) have a direct relationship with the climate and their means of living; bad weather is equal to bad harvests, which means less money. To understand how society represents this love hate relationship with the weather, the twitter analysis was performed. Twitter, a data restricted social network system, has a limited character count to relay information about any topic the sender chooses to relate. These tweets can be used to assess the sentiment of the sender towards a certain topic. As stated previously, the sentiment is defined by the language of the tweet within the twitter system. Sentiment analysis showed that the two terms differed greatly. Based on the predefined algorithm for the sentiment analysis, certain language components carried a positive sentiment, while others carried a negative sentiment. Tweets about GW and CC were subdivided based on their positive, neutral and negative connotations within the tweet network. These emotions regardless of their character still play a role in how humans interacts with surroundings including other humans [ 48 , 49 ] As seen in Fig 2 the different terms had similar distributions, although with different ranges in the values. Global warming showed a much smaller positive tweet value than did climate change. Correspondent to this the respective percentage of positive sentiments for CC was more than double that of GW. Comparatively, the neutral percentiles were more similar for each term with a small difference. However, the negative sentiments for the two terms again showed a greater disparity, with negative statements about GW nearly double those of climate change.

These differences show that there is a perceptive difference in how the public relates to the two terms Global Warming and Climate Change [ 50 , 51 ]. Climate change is shown in a more positive light than global warming simply based on the tweets produced by the public. The difference in how people perceive climate change and global warming is possibly due to the press, personal understanding of the terms, or level of education. While this in itself is indefinable, since by nature tweets are linguistically restrictive, the thing to take from it is that there is a measurable difference in how individuals respond to climatological changes that they are experiencing daily. These changes have a describable effect on how the population is responding to the publicity surrounding the two terms to the point where it can be used to manipulate governmental policy [ 52 ].

Sentiment analysis is a tool that can be used to determine how the population feels about a topic; however, the nature of the algorithm makes it hard to effectively determine how this is being assessed. For the current study, the sentiment analysis showed that there was a greater negative association with the term global warming than with the term climate change. This difference, which while being an expression of individual like or dislike at the time the tweet was created, denotes that the two terms were either not understood in their true form, or that individuals may have a greater familiarity with one term over the other, which may be due to a longer exposure to the term (GW) or the negative press associated with the term (GW).

Conclusions

Trend analysis identified that the public is aware of the terminology used to describe climatological variation. The terminology showed changes in use over time with global warming starting as the more well-known term, and then its use decreased over time. At the same time, the more definitive term climate change had less exposure early on; however, with the increase of press exposure, the public became increasingly aware of the term and its more accurate definition. This increase appeared to be correspondent with the increasing publicity around three very powerful press exposure events (a documentary, a scientific report release and a Nobel Prize). The more the term was used the more people came to use it, this included searches on the internet.

Comparatively sentiment analysis showed that the two terms had differential expressions in the population. With climate change being seen in a more positive frame than global warming. The use of sentiment analysis as a tool to evaluate how the population is responding to a feature is an important tool. However, it is a tool that measures, it does not define.

Social network systems and internet searches are effective tools in identifying changes in both public awareness and public perception of an issue. However, in and of itself, these are bell ringers they can be used to determine the importance of an issue, but not the rationale behind the why it is important. This is an important fact to remember when using analytical tools that evaluate social network systems and their use by the public.

Acknowledgments

This study was financially supported by the 2015 Post-Doc. Development Program of Pusan National University

Author Contributions

Conceived and designed the experiments: YD GJJ. Performed the experiments: ML YD. Analyzed the data: ML YD. Contributed reagents/materials/analysis tools: JK YD. Wrote the paper: ML YD GJJ.

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  • Research in practice
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  • Published: 06 April 2022

Tracking the impacts of climate change on human health via indicators: lessons from the Lancet Countdown

  • Claudia Di Napoli   ORCID: orcid.org/0000-0002-4901-3641 1 , 2 ,
  • Alice McGushin 3 ,
  • Marina Romanello 3 ,
  • Sonja Ayeb-Karlsson 4 , 5 , 6 ,
  • Wenjia Cai 7 ,
  • Jonathan Chambers 8 ,
  • Shouro Dasgupta 9 , 10 , 11 ,
  • Luis E. Escobar 12 ,
  • Ilan Kelman 3 , 4 , 13 ,
  • Tord Kjellstrom 14 ,
  • Dominic Kniveton 5 ,
  • Yang Liu 15 ,
  • Zhao Liu 7 ,
  • Rachel Lowe 16 , 17 , 18 ,
  • Jaime Martinez-Urtaza 19 ,
  • Celia McMichael 20 ,
  • Maziar Moradi-Lakeh 21 ,
  • Kris A. Murray 22 , 23 ,
  • Mahnaz Rabbaniha 24 ,
  • Jan C. Semenza 25 ,
  • Liuhua Shi 15 ,
  • Meisam Tabatabaei 26 , 27 ,
  • Joaquin A. Trinanes 28 ,
  • Bryan N. Vu 15 ,
  • Chloe Brimicombe 2 &
  • Elizabeth J. Robinson 9  

BMC Public Health volume  22 , Article number:  663 ( 2022 ) Cite this article

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In the past decades, climate change has been impacting human lives and health via extreme weather and climate events and alterations in labour capacity, food security, and the prevalence and geographical distribution of infectious diseases across the globe. Climate change and health indicators (CCHIs) are workable tools designed to capture the complex set of interdependent interactions through which climate change is affecting human health. Since 2015, a novel sub-set of CCHIs, focusing on climate change impacts, exposures, and vulnerability indicators (CCIEVIs) has been developed, refined, and integrated by Working Group 1 of the “ Lancet Countdown: Tracking Progress on Health and Climate Change”, an international collaboration across disciplines that include climate, geography, epidemiology, occupation health, and economics.

This research in practice article is a reflective narrative documenting how we have developed CCIEVIs as a discrete set of quantifiable indicators that are updated annually to provide the most recent picture of climate change’s impacts on human health. In our experience, the main challenge was to define globally relevant indicators that also have local relevance and as such can support decision making across multiple spatial scales. We found a hazard, exposure, and vulnerability framework to be effective in this regard. We here describe how we used such a framework to define CCIEVIs based on both data availability and the indicators’ relevance to climate change and human health. We also report on how CCIEVIs have been improved and added to, detailing the underlying data and methods, and in doing so provide the defining quality criteria for Lancet Countdown CCIEVIs.

Conclusions

Our experience shows that CCIEVIs can effectively contribute to a world-wide monitoring system that aims to track, communicate, and harness evidence on climate-induced health impacts towards effective intervention strategies. An ongoing challenge is how to improve CCIEVIs so that the description of the linkages between climate change and human health can become more and more comprehensive.

Peer Review reports

Climate change affects global health via multiple direct and indirect pathways [ 1 , 2 ]. Every year, disasters involving weather- and climate-related hazards result in thousands of deaths worldwide and contribute to the global burden of disease [ 3 , 4 ]. Direct health consequences may derive from changes in temperature and precipitation, and human exposure to heatwaves, wildfires, floods, and droughts [ 5 , 6 , 7 , 8 , 9 ]. These include an increase in cardiovascular mortality during events of extreme heat; a higher incidence of chronic kidney disease among outdoor workers in hot areas; and fatalities and multiple negative health consequences of fire smoke inhalation during wildfires [ 10 , 11 , 12 , 13 ]. Indirect impacts may be triggered by climate change-induced environmental and ecosystem alterations, such as crop failures, reduced marine food capture, geographic range expansion of disease vectors, and reduced labour capacity [ 14 , 15 , 16 , 17 ]. Other indirect effects may be mediated through social systems and responses to climate change. Examples include reduced labour capacity in vulnerable occupations due to increasing heat, migration of populations related to sea-level rise and food insecurity, as well as other drivers of human mobility [ 18 , 19 , 20 ].

As understanding and awareness grow regarding the health dimension of climate change, there is an increased demand for feasible and efficient tools that can inform climate change mitigation and adaptation strategies to deliver benefits to human health at all levels. Climate-health linkages are complex, with multiple interactions, synergies, and feedback loops. Understanding their characteristics and relationships is challenging and poses a level of complexity that could render the development of such tools unfeasible. A reasonable alternative is to deploy a subset of measures describing the key aspects of the state, extent and change of the climate-health system [ 21 ].

Climate change and health indicators (CCHIs) are summary measures, often quantitative, that represent the heterogeneous set of factors and relationships linking climate change with health [ 22 , 23 , 24 , 25 , 26 ]. Their purpose is three-fold. First, to assess long-term trends and changes in climate-induced impacts on population health. Second, to effectively communicate relevant evidence to researchers, health professionals, policymakers, and the general public. Third, to support and monitor decision making for successful intervention strategies and plans that address the human health consequences of climate change.

In the past decade, CCHIs have represented the building blocks for a variety of initiatives tracking the climate-health system across multiple spatial and temporal scales [ 24 , 25 ]. One high profile and current CCHI initiative is the “ Lancet Countdown: Tracking Progress on Health and Climate Change”. Since 2015 the Lancet Countdown has been providing a global monitoring system that tracks the complex, reflective ways in which climate change affects health [ 27 , 28 , 29 , 30 , 31 , 32 ]. It accomplishes this through a set of CCHIs spanning five domains of the climate-health system: climate change impacts, exposures, and vulnerability; adaptation planning and resilience for health; mitigation actions and health co-benefits; economics and finance; and public and political engagement.

One of the five Lancet Countdown thematic working groups, namely Working Group 1 (WG1), is tasked with identifying and tracking indicators that trace the links across “climate change impacts, exposures, and vulnerability”. This research in practice article aims to provide guidance for the development and tracking of CCHIs through a detailed case study of how WG1 authors developed these indicators, referred to from here on as climate change impacts, exposures, and vulnerability indicators (CCIEVIs), that contribute to the Lancet Countdown’s overall objectives. As such, this article is equally relevant for those individuals and policy makers wanting to develop their own climate change and health indicators specific to a country or region, and those wanting a more detailed understanding of specific indicators used in the Lancet Countdown, and particularly WG1. Hereafter, we describe (i) the aim, design, and characteristics of CCIEVIs, as employed in the Lancet Countdown, (ii) their main components, (iii) underlying data and methods, and (iv) plans for future improvements.

Climate change impacts, exposures, and vulnerability indicators

Designing ccievis.

Lancet Countdown CCIEVIs have been developed to monitor the health impacts related to anthropogenic climate change, their trends to date, and the extent to which progress (or backsliding) is occurring over time. They are presented from a historical starting date up to the year for which the most recent complete data are available. The indicators provide evidence along different dimensions – some indicators are positioned closer to the climate change hazard and some closer to the health impact – as to whether, and to what extent, climate change is affecting health, for better or for worse. In the Lancet Countdown, CCIEVIs start tracking climate change-health linkages from the 1980s, and are grouped into five thematic clusters: heat; weather and climate extremes; infectious diseases; food security of terrestrial and marine assets; global mean sea-level rise.

To ensure that CCIEVIs are reproducible, relevant, and useful to scientific and policy-making communities, they must satisfy five quality criteria that mirror the World Meteorological Organisation guidelines for climate indicators [ 33 ]:

Representativeness: a CCIEVI should track an aspect of both climate change and health, particularly focusing on the relationship between the two; it should do this across a timescale and geographical coverage sufficient for long-term global trends to be observed.

Relevance: a CCIEVI should be clear and understandable to a broad range of audiences; global CCIEVIs may also have value at local (i.e. national, regional) level for policy and decision makers.

Robustness: a CCIEVI should use data and methods that are robust, reliable, and valid to track the relevant aspect of climate change and health; data from publicly available databases, and especially those developed by international organisations, governmental bodies, or academic institutions, are preferred; methods should be supported by a high standard of evidence from the scientific literature.

Reproducibility: a CCIEVI should be calculated using an internationally agreed and published scientific method as well as open-access and quality-controlled data; the methodology underlying an indicator should be clearly laid out, including details of the process of data collection and processing, which must be done in a systematic and unbiased way; statistical analysis of the data should be carried out to support the interpretation of the data.

Timeliness: a CCIEVI should be calculated regularly, with a short lag between the end of the period under consideration and the publication of the data; the calculation must be practicable with existing and future resources.

WG1 indicators have evolved over the five years that they have been reported in the annual Lancet Countdown publications so to fulfil above-listed criteria. For example, in 2020, there were twelve WG1 Lancet Countdown CCIEVIs, almost double the number of when they first appeared in 2016 [ 28 , 32 ]. As well as new indicators being introduced each year, established indicators are revised to reflect the latest evidence in the scientific literature and the needs of new and emerging stakeholders [ 30 , 31 ]. The revision of Lancet Countdown CCIEVIs occurs annually by experts across a broad range of relevant disciplines, including climate, geography, epidemiology, and occupation health. The publication of an appendix alongside each Lancet Countdown annual report ensures that the data and methods underlying every CCIEVI are explicitly cited and described along with possible caveats.

Main components of CCIEVIs

Lancet Countdown CCIEVIs focus on three main components: hazard, exposure, and vulnerability. These are defined according to the terminology provided by the United Nations Office for Disaster Risk Reduction and the Intergovernmental Panel on Climate Change [ 34 , 35 ]. Specifically:

Hazard is any physical event with the potential to cause disruption or damage in vulnerable and exposed elements;

Exposure is who or what is present in the area where a hazard may occur;

Vulnerability is the factors or constraints of an economic, social, physical, or geographic nature, which reduce the ability of exposed elements to prepare for and cope with the impact of the hazard.

In Lancet Countdown CCIEVIs, each of these components represents a dimension or layer of the multi-facet climate-health system. For the hazards, the Lancet Countdown CCIEVIs target extremes in temperature and precipitation, namely heatwaves, drought, and floods, as well as phenomena mediated by these two environmental variables, i.e. wildfires and suitability of vector-borne diseases (malaria, dengue). Similarly, health hazards in the ocean system are identified in changes of global mean sea level as well as sea surface temperature and salinity. The second layer tracks the exposure of populations, cultivated areas, and health systems to hazards. This reflects the purpose of representing climate change-health pathways that are both direct (e.g. on people) and indirect (e.g. food mediated). Lancet Countdown CCIEVIs also track vulnerabilities to extreme heat, incorporating the proportion of the population over 65 years of age, the prevalence of predisposing chronic diseases (i.e. diabetes and cardiovascular, respiratory, and renal disease), the proportion of the population living in urban areas, and the number of outdoor workers. In the 2021 report, the vulnerability of children younger than 1 year to life-threatening heatwaves was added [ 36 ]. Lancet Countdown CCIEVIs also consider vulnerability linked to the capacity of local health services to respond to public health risks and emergencies.

Data and methods for developing CCIEVIs

Hazards, exposures, and vulnerabilities change over time and across space as do their interactions as a result of changes in the climate. Consequently, climate change amplifies or diminishes existing health impacts, and induces or suppresses new ones with a high degree of spatial and temporal variability. Geographic information systems, and more specifically geospatial data, have long been identified as useful analytical tools for monitoring the health-climate system at all spatial levels, from local to global [ 37 ].

Many relevant sources for CCIEVIs provide the data as rasters, i.e. grids of cells. This allows information, such as variations in climate-relevant variables like air temperature, to be represented in a spatially continuous and consistent way across the Earth’s surface. Usefully, rasters can be stacked to assess cumulative hazards in a given region or to produce spatially resolved time series of a given variable.

Environmental data are nowadays available as rasters across a wide range of spatial and temporal scales. This makes them an ideal tool for constructing relationships between hazards, exposed elements, and associated vulnerability anywhere in the world, low and middle income countries included [ 38 ]. For instance, one of the Lancet Countdown CCIEVIs tracks the climatic suitability for the transmission of Plasmodium falciparum , the parasite causing malaria. Following the work by Grover-Kopec and colleagues [ 39 ], the indicator tracks the number of months per year suitable for malaria transmission as the coincidence of precipitation accumulation greater than 80 mm, average temperature between 18 °C and 32 °C, and relative humidity greater than 60%. The number of suitable months in a year is calculated for each grid cell by overlaying precipitation, temperature, and humidity raster layers across twelve months. Year-by-year changes in the number of suitable months generate a time series, which can be spatially aggregated (e.g. for highland vs lowland areas).

CCIEVIs can be explored via an online visualization platform. For example, maps represent a powerful way to visualise and communicate climate information, and harness the latter for action [ 40 ]. CCIEVIs can be displayed as a set of world-wide maps that can be navigated via an interactive, user-friendly interface to show year-specific data or to highlight countries or geographical areas of interest, as has been done by the Lancet Countdown [ 41 ].

Perspectives and improvements for CCIEVIs

Climate change is an evolving phenomenon and our understanding of it is also evolving. Only with the most up-to-date data, methodologies, and expertise, CCIEVIs can provide the quantitative underpinnings of a compelling narrative on the health impacts that climate change imposes across the globe. Based on our experience from the Lancet Countdown, we here report the main points CCIEVIs might consider to be meaningful and useful in their purpose.

Partnership: The cross-cutting nature of CCIEVIs demands a combination of skills, knowledge and data that span across institutions, disciplines, countries, and geographical regions. Creating and maintaining a long-term collaboration among a group of diverse experts is crucial to guaranteeing the robustness and reliability of developed CCIEVIs over time. Partnership can also foster the development of CCIEVI-based initiatives in locations where these are currently missing but are perceived as useful [ 42 , 43 , 44 , 45 , 46 ]. Over the long term, partnerships can expand the areas where CCIEVIs drive decision-making as well as increase workforce preparedness via the inclusion of climate change curriculum into health professional education [ 47 , 48 , 49 , 50 ].

Iterative process: To satisfy the timeliness criteria of CCIEVIs, their design and utility must be revised periodically [ 21 ]. Every year, Lancet Countdown CCIEVIs indicators undergo a thorough quality check and improvement process before being considered for the annual report. In this process, independent experts assess the quality and suitability of each indicator and provide constructive feedback to aid their development and improvement. Additionally, new CCIEVIs can be developed and added to the original indicators suite to provide a more complete description of the climate-health system.

Going local : Because CCIEVIs are provided as geospatial data, they are down-scalable and able to identify priority areas for public health intervention across the globe. Location-specific CCIEVIs can assist local health departments in tracking variations in community exposure and vulnerability to climate change, uncovering health impacts at regional or sub-regional level (including their linkages to the surrounding urban/natural environment), designing interventions to enhance community resilience and evaluating the effectiveness of implemented interventions [ 51 , 52 ]. With this motivation, the Lancet Countdown CCIEVIs structure has been replicated to produce the Australia and China reports with indicators being provided at national to regional and city scales [ 53 , 54 ]. These local reports may serve as a guidance for disseminating progress on CCIEVIs from other countries in a uniform format. Every year, the Lancet Countdown report is also followed by a range of resources, such as briefs for policymakers and translations of the executive summaries, tailored to specific cities, countries and geographical areas [ 55 , 56 , 57 ].

Data : CCIEVIs typically have to rely on different sources for the provision of health, climate, and demographics data. They therefore may well differ in the spatial resolution, the time period covered, and the reference baseline used for the definition of extreme events. Notwithstanding this, consistency in climate data and demographics among Lancet Countdown CCIEVIs has been attempted and achieved wherever possible so that indicators are compatible and comparable with each other. It is worth noting that protocols are in place for the effective archival, management, analysis, delivery, and use of climate data, whereas health data standardisation is an open topic [ 58 , 59 ]. The collection and reporting of public health data, for instance, vary greatly across nations and healthcare organizations [ 60 ]. Data monitoring on disease incidence or health outcomes that are standardised at a global level is in general lacking. In most cases, health impacts are therefore tracked using epidemiological models rather than measured records. Furthermore, their geographical resolution is generally lower than climate data (i.e. country level rather than at raster cell level). Unlike climate data, health records are generally not publicly available. Creating open-access online databases for public health data would help foster knowledge exchange between the climate and the public health community, as well as promote its applicability at all scales.

CCIEVIs can be used as a starting point for broader explorations of the linkages between climate change and health that have relevance for policy making. Here we suggest six specific areas for future research, many of which are currently being pursued by WG1 authors.

Looking to the future: In the Lancet Countdown, CCIEVIs are used to track the current health impacts of climate change, and contextualise them with respect to the recent past. These indicators therefore hint at how the future will likely evolve without efforts to mitigate and adapt to climate change. However, the methodology described in this research in practice article is time-agnostic and can be applied to indicators predictive of the future. Designing, developing and implementing future projections of CCIEVIs presents additional research challenges, such as the choice of the climate prediction model and its verification, but has the potential to provide actionable information to support policymaking [ 61 ].

Identifying hotspots : Climate change often affects people and places in multiple ways. We define climate-health hotspots as locations where climate affects people negatively through multiple pathways. This definition builds upon work already established in the climatology field on mapping hotspots , i.e. geographical areas where the combined occurrence of multiple weather extremes (compound events) is observed [ 62 , 63 ]. As adverse health effects occur in populations that are already at risk from climate extremes and lack adequate health infrastructure [ 64 , 65 , 66 ], the overlay method described in this work can be adapted to identify climate-health hotspots and trace them overtime by overlapping multiple layers of hazard, exposure and vulnerability. It can also be applied to different CCIEVIs to assess multiple climate change impacts in a single location. This could allow for a one health perspective if CCIEVIs at the human–animal–environment interface are implemented [ 67 , 68 ]. For instance, concerning food security of terrestrial assets, a CCIEVI tracking changes in crop yield potential due to rising temperatures could be overlayed with CCIEVIs monitoring animal-source foods and/or the effects of environmental changes on diseases of livestock and crops. In either case, the identification and tracking of climate-health hotspots and incorporation into a monitoring system, could improve further public health action, particularly in the context of reducing the likelihood of systemic health crises [ 69 ].

Inequality and inequity: Global indicators generally do not provide a nuanced picture of the differentiated impacts of climate change on health by country and region, or across populations within countries. Disaggregating indicators by relevant socioeconomic and occupational characteristics (e.g. income, gender, race or ethnicity, disability, occupation, and age) is necessary to measure inequalities in exposure to climatic risk factors and health outcomes attributable to these exposures. For heatwaves, for instance, these may consider pregnancy and mental health [ 70 , 71 ]. With this approach, climate change and climate-related risk factors are positioned as both environmental and social determinants of health.

Mental health: Mental health issues are closely intertwined with people’s geographical, social, environmental, financial, and political context, and any climate-induced impact on physical health is also a risk factor for mental health [ 72 ]. As such, factors linking climate change to health may affect mental health and underlying mental health status can affect the capacity to adapt to (or to mitigate) climate change. The description of the climate-health system, therefore, requires the inclusion of CCIEVIs that address the mental health consequences of climate change. A growing body of literature is currently examining these linkages [ 73 , 74 , 75 , 76 , 77 , 78 , 79 ], inspiring the development of future CCIEVIS on the topic.

Attribution : In recent years, formal methods have been developed to identify changes in the occurrence of adverse health outcomes, and to determine the extent to which those changes may be attributed to climate change [ 80 ]. Multi-step attribution, for instance, consists of a) attributing an observed change in a variable of interest to a change in climate or other environmental variable and b) attributing the change in climate to external drivers such as greenhouse gas emissions [ 81 ]. The 2020 Lancet Countdown report showcased the attribution of 76 extreme weather events to climate change, and the effects that a selection of these have had on the health of the population [ 32 ]. Future CCIEVIs could be based on attribution to quantitatively understand how climate change makes extreme events associated with health outcomes more likely.

Embracing the unquantifiable : There are important aspects of the relationship between climate change and health that cannot be quantified or are very difficult to quantify meaningfully. One example of this is human migration [ 18 ]. Local-level studies find worsening food security where people move away from places affected by sea level rise and saltwater intrusion, or adverse psychosocial impacts of disrupted lives, social networks and livelihoods for those on the move in a changing climate. It is extremely challenging, however, to attribute human mobility to climate change (and quantify its magnitude), or to develop a global CCIEVI that can quantify links between climate impacts, human mobility, and health.

In developing climate change impacts, exposures, and vulnerability indicators (CCIEVIs), several key challenges have to be addressed around the idea of representing highly complex systems by a discrete set of quantifiable indicators that are updated annually. One challenge is to find globally relevant indicators, with data available across a sufficiently large number of countries, that also have local relevance.

A hazard, exposure, and vulnerability framework has proven to be an effective structure within the Lancet Countdown initiative. Around such a framework, Lancet Countdown CCIEVIs have been crafted, based on both data availability and the indicators’ relevance to climate change and human health. Over time these indicators have been improved and added to, as data become available and methodologies are refined. An ongoing challenge is how to incorporate those linkages between climate change and health that are clear and well documented but at the same time difficult to measure, quantify, and fit within this framework.

Together, Lancet Countdown CCIEVIs present a compelling visualisation of how climate change is increasingly exposing people to the negative health impacts of climate change, both direct and indirect, across land and water. Ultimately the impact of climate change on human health depends not only on the exposure and vulnerability of populations to climate hazards, but also on the extent to which individuals and countries are able to adapt and build resilience. By unveiling the challenges underpinning the evolution of the climate-health system in time and in space, CCIEVIs can be the pillars of a new public health that safeguards and advances global wellbeing in the decades to come.

Availability of data and materials

Not applicable.

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Acknowledgments

The authors are grateful to the three anonymous reviewers and the editor for their constructive comments.

This work is supported by an unrestricted grant from the Wellcome Trust (209734/Z/17/Z).

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School of Agriculture, Policy and Development, University of Reading, Reading, UK

Claudia Di Napoli

Department of Geography and Environmental Science, University of Reading, Reading, UK

Claudia Di Napoli & Chloe Brimicombe

Institute for Global Health, University College London, London, UK

Alice McGushin, Marina Romanello & Ilan Kelman

Institute for Risk and Disaster Reduction, University College London, London, UK

Sonja Ayeb-Karlsson & Ilan Kelman

School of Global Studies, University of Sussex, Brighton Falmer, UK

Sonja Ayeb-Karlsson & Dominic Kniveton

United Nations University, Institute for Environment and Human Security, Bonn, Germany

Sonja Ayeb-Karlsson

Ministry of Education Key Laboratory for Earth System modeling, Department of Earth System Science, Tsinghua University, Beijing, 100084, China

Wenjia Cai & Zhao Liu

Institute for Environmental Science, University of Geneva, Geneva, Switzerland

Jonathan Chambers

Grantham Research Institute on Climate Change and the Environment, London School of Economics and Political Science (LSE), London, UK

Shouro Dasgupta & Elizabeth J. Robinson

Centro Euro-Mediterraneo sui Cambiamenti Climatici (CMCC), Venice, Italy

Shouro Dasgupta

Università Ca’ Foscari, Venice, Italy

Department of Fish and Wildlife Conservation, Virginia Tech, Blacksburg, VA, USA

Luis E. Escobar

University of Agder, Kristiansand, Norway

Ilan Kelman

Health and Environment International Trust, Nelson, New Zealand

Tord Kjellstrom

Rollins School of Public Health, Emory University, Atlanta, USA

Yang Liu, Liuhua Shi & Bryan N. Vu

Barcelona Supercomputing Center, Barcelona, Spain

Rachel Lowe

Catalan Institution for Research and Advanced Studies (ICREA), Barcelona, Spain

Centre on Climate Change & Planetary Health and Centre for Mathematical Modelling of Infectious Diseases, London School of Hygiene & Tropical Medicine, London, UK

Department of Genetics and Microbiology, Universitat Autònoma de Barcelona (UAB), Barcelona, Spain

Jaime Martinez-Urtaza

School of Geography, Earth and Atmospheric Sciences, The University of Melbourne, Melbourne, Australia

Celia McMichael

Preventive Medicine and Public Health Research Center, Psychosocial Health Research Institute, Iran University of Medical Sciences, Tehran, Iran

Maziar Moradi-Lakeh

MRC Centre for Global Infectious Disease Analysis, Imperial College London, London, UK

Kris A. Murray

MRC Unit The Gambia At London School of Hygiene and Tropical Medicine, Atlantic Boulevard, Fajara, The Gambia

Iranian Fisheries Science Research Institute, Agricultural Research, Education, and Extension Organisation, Tehran, Iran

Mahnaz Rabbaniha

Heidelberg Institute of Global Health, University of Heidelberg, Heidelberg, Germany

Jan C. Semenza

Higher Institution Centre of Excellence (HICoE), Institute of Tropical Aquaculture and Fisheries (AKUATROP), Universiti Malaysia Terengganu, 21030, Kuala Nerus, Terengganu, Malaysia

Meisam Tabatabaei

Henan Province Forest Resources Sustainable Development and High-value Utilization Engineering Research Center, School of Forestry, Henan Agricultural University, Zhengzhou, 450002, China

Department of Electronics and Computer Science, Universidade de Santiago de Compostela, Santiago, Spain

Joaquin A. Trinanes

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CDN, AM, MR, SAK, WC, JC, SD, LEE, IK, TK, DK, YL, ZL, RL, JMU, CM, MML, KAM, MR2, JCS, LS, MT, JAT, BNV, CB, and EJR authors planned and designed this manuscript. CDN prepared the first draft; CDN, AM, MR, SAK, WC, JC, SD, LEE, IK, TK, DK, YL, ZL, RL, JMU, CM, MML, KAM, MR2, JCS, LS, MT, JAT, BNV, CB, and EJR provided input into subsequent drafts. CDN, AM, MR, SAK, WC, JC, SD, LEE, IK, TK, DK, YL, ZL, RL, JMU, CM, MML, KAM, MR2, JCS, LS, MT, JAT, BNV, CB, and EJR authors read and approved the final manuscript.

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Di Napoli, C., McGushin, A., Romanello, M. et al. Tracking the impacts of climate change on human health via indicators: lessons from the Lancet Countdown. BMC Public Health 22 , 663 (2022). https://doi.org/10.1186/s12889-022-13055-6

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climate change definition research paper

Climate change refers to significant changes in global temperature, precipitation, wind patterns and other measures of climate that occur over several decades or longer.

The seas are rising. The foods we eat and take for granted are threatened.  Ocean acidification  is increasing. Ecosystems are changing, and for some, that could spell the end of certain regions the way we have known them. And while some species are adapting, for others, it’s not that easy.

Evidence suggests many of these extreme  climate changes  are connected to rising levels of carbon dioxide and other greenhouse gases in the Earth’s atmosphere — more often than not, the result of human activities.

Search below for key terms and definitions related to climate change.

Aerosols  are small suspended particles in a gas. Scientists can detect them in the atmosphere. They range in size from one nanometer (one billionth of a meter) to 100 micrometers (one millionth of a meter).

Antarctic sea ice

Antarctic sea ice   is nearly a geographic opposite of its Arctic counterpart because Antarctica is a landmass covered in ice surrounded by an ocean, and the Arctic is an ocean of sea ice surrounded by land.

Anthropogenic

Anthropogenic  describes a process or result generated by human beings.

Aquaculture

Aquaculture  uses a body of water for the cultivation of plants and animals. (Compare to agriculture, which uses land to cultivate plants and animals.) Ponds, lakes, rivers, and the ocean serve as places to breed, rear and harvest aquatic species.

Aquifer  is water-bearing rock from which water can be pumped.

Arctic sea ice

Arctic sea ice  is an integral part of the Arctic Ocean and an important indicator of climate change. During winter’s dark months, sea ice will typically cover the majority of the Arctic Ocean.

Biofuels  are renewable fuels derived from biological materials, such as algae and plants, that can be regenerated. This distinguishes them from fossil fuels, which are considered nonrenewable. Example of biofuels are ethanol, methanol and biodiesel.

Biogenic emissions

Biogenic emissions  are emissions generated by living things.

Biological productivity

Biological productivity  is a measure of the amount of plant and animal growth in a defined region and time.

Carbon  is a configuration of molecules and an elemental building block of all organisms on Earth.

Carbon cycle

Carbon cycle  describes the process by which living things absorb carbon from the atmosphere, sediments and soil, or food. To complete the cycle, carbon returns to the atmosphere in the form of carbon dioxide or methane by respiration, combustion or decay.

Carbon dioxide

Carbon dioxide  is the gas that accounts for about 84 percent of total U.S. greenhouse gas emissions. In the U.S. the largest source of carbon dioxide (98 percent) emissions is combustion of fossil fuels. Combustion can be from mobile (vehicles) or stationary sources (power plants). As energy use increases, so do carbon dioxide emissions.

Carbon sequestration

Carbon sequestration  is the process of removing carbon from the atmosphere and storing it in a fixed molecule in soil, oceans or plants. An organism or landscape that stores carbon is called a  carbon sink . An organism or landscape that emits carbon is called a  carbon source . For example, soils contain inorganic carbon (calcium carbonate) and organic carbon (humus) and can be either a source or a sink for atmospheric carbon dioxide, depending on how landscapes are managed. Because large amounts of carbon are stored in soils, small changes to soil can have major impacts on atmospheric carbon dioxide.

Climate change adaptation

Climate change adaptation  refers to the adjustments societies or ecosystems make to limit the negative effects of climate change or to take advantage of opportunities provided by a changing climate. Adaptation can range from farmers planting more  drought-resistant crops  to coastal communities evaluating how best to protect themselves from sea level.

Climate forcing

Climate forcing  refers to how climate affects the physical, chemical and biological attributes of a region.

Climate science

Climate science  studies how changing climates affect the natural order on a global level. Rising global temperatures bring with them the potential to raise sea levels to raise sea levels, change precipitation and local climate conditions.

Coastal Wetlands

Coastal wetlands include saltwater and freshwater wetlands located within coastal watersheds — specifically USGS 8-digit hydrologic unit watersheds which drain into the Atlantic Ocean, Pacific Ocean, or Gulf of Mexico.

Dimethylsulfide

Dimethylsulfide  is the most abundant biological sulfur compound emitted to the atmosphere, mostly from phytoplankton, and encourages cloud formation.

Ecosystem services

Ecosystem services  are the benefits or “services” of an ecosystem to human life, such as clean water and the decomposition of organic matter.

Electrolytes

Electrolytes  are chemical substances containing free ions that conduct electricity.

Emissions  are substances released into the air and are measured by their concentrations, or parts per million, in the atmosphere.

Feedstock  is raw material, usually plant or agricultural waste, that can be processed into fuel or energy.

Glaciers  and ice caps form on land . Glaciers accumulate snow, which over time becomes compressed into ice. On average, glaciers worldwide have been losing mass since at least the 1970s.

Global temperature

Global temperature  is an average of air temperature recordings from weather stations on land and sea as well as some satellite measurements. Worldwide, 2006-2015 was the warmest decade on record since thermometer-based observations began nearly 150 years ago.

Global warming

In the early 1960s scientists recognized that carbon dioxide in the atmosphere was increasing. Later they discovered that methane, nitrous oxide and other gases were rising. Because these gases trap heat and warm the Earth, as a greenhouse traps heat from the sun, scientists concluded that increasing levels of “greenhouse gases” would increase  global warming .

Global Warming Potential (GWP)

Global Warming Potential  (GWP) is the ability of a greenhouse gas to absorb heat compared to carbon dioxide over a specified period of time, from 20 to 500 years. The timeframe is important because each gas has a different rate at which it is removed from the atmosphere. For each time period, carbon dioxide is always set at “1”, and other greenhouse gases are compared to carbon dioxide for the same timeframe. For example, the sulfur hexafluoride’s GWP at 20 years is 15,100, meaning it has 15,100 times more warming potential than carbon dioxide in that timeframe.

Greenhouse gases

The main  greenhouse gases  are water vapor (H2O), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6). Water vapor is the most plentiful at about one percent. The next most plentiful is carbon dioxide at 0.04 percent. The effect of human activity on global water vapor concentrations is too small to be important. The effects of human activity on the other greenhouse gases, however, is large and very important. These gases are increasing faster than they are removed from the atmosphere.

A heat dome is when hot ocean air gets trapped over a large area, resulting in dangerously high temperatures. It occurs when high atmospheric pressure forms over a region, pushing air down, which heats as the air compresses. This forms a “lid” that seals to create a dome of trapped heat, setting the stage for heat waves.

Hydrologic cycle

Hydrologic cycle  is the process by which water moves around the earth. The cycle includes evaporation, precipitation, runoff, condensation, transpiration and infiltration.

Hydrologic model

Hydrologic model  is a computer analysis of large amounts of historical data. It helps predict how variables such as temperature, rain, and carbon dioxide levels might affect the hydrologic cycle.

Ice loss  refers to the retreat of sea ice and land ice mass from its historic extents. This retreat of sea ice and land ice is one of two major causes of the current sea level rise.

An  ice sheet  forms on land and extends over tens of thousands of miles. Greenland and Antarctica have vast ice sheets that together contain more than 99 percent of the freshwater ice on Earth. In Greenland, today’s record summer melts bring rapid and widespread ice sheet loss. In Antarctica, the melt is slower and more localized for now.

An  ice shelf  forms from the outflow of land ice and floats on the sea at the land’s edge. It creates a barrier that slows the flow of land ice into the ocean. In the last thirty years, both rapid disintegration of ice shelves and ice shelf collapses have been observed along Canada and the Antarctic Peninsula.

Methane  is a gas and represents about 8 percent of total U.S. greenhouse gas emissions. The largest sources are wood burning in stoves and fireplaces, livestock digestive systems, and decomposition in landfills.

Mesoscale  is a measure of distance useful for local winds, thunderstorms and tornadoes. It ranges from a few to a few hundred miles.

A  micron , also called a micrometer, is one millionth of a meter, or a thousandth of a millimeter. It is a common measure for particulate matter in the atmosphere. Particles measuring only 2.5 microns (approximately 1/30th the average width of a human hair) lodge deeply into the lungs.

Mitigation potential

Mitigation potential  is a measurement of the amount of carbon that can be stored in order to balance the release of carbon. It is important in discussions about power plants and vehicles.

Nano  refers to nanometer, one billionth of a meter or a hundred-thousandth of a millimeter.

Nitrous oxide

Nitrous oxide  is one of six gases addressed by the Kyoto Protocol international agreement and the main regulator of stratospheric ozone. Animal waste and nitrogen fertilization of soil are the largest contributors. Nitrogen emissions have nearly 300 times the global warming potential of carbon dioxide over 100 years.

Ocean acidification

Ocean acidification  is the change in ocean chemistry due to decreasing pH levels, or increasing acidity, in seawater.

Ground level ozone  is a gas produced through reactions between nitrous oxides (NOX) and volatile organic compounds (VOCs) when burning coal, gasoline and other fuels. VOCs are found in solvents, paints, hairsprays and more common items. Ozone consists of three oxygen atoms and is the main component of smog.

Stratospheric ozone  is a gas found in a layer from six to 25 miles above the Earth’s surface. It acts as a barrier to global warming. Specifically, the ozone layer keeps 95-99% of the sun’s ultraviolet radiation from striking the Earth.

Ozone forming potential

Ozone forming potential  is a measure of the reactivity of an individual chemical compound to the presence of other chemicals that form ozone together.

Particulate matter

Particulate matter  (PM-10) are aerosols including dust, soot and tiny bits of solid materials that are released and move around in the air. Sources are burning of diesel fuels, incineration of garbage, mixing and applying fertilizers and pesticides, road construction, steel making, mining, field burning, forest fires, fireplaces and woodstoves. PM causes eye, nose and throat irritation and respiratory problems.

Polar Vortex

The polar vortex is a large area of low pressure and cold air around Earth’s North Pole.  The phenomenom typically goes unnoticed by those of us living in lower latitudes except for when, every once in a while, the air pressure and winds shift.

Primary production

Primary production  is the production of organic compounds from atmospheric or aquatic carbon dioxide, principally through the process of photosynthesis.

Renewable energy

Renewable energy  is energy from sources that will renew themselves within our lifetime. Renewable energy sources include wind, sun, water, biomass (vegetation) and geothermal heat.

Sea ice , both  Antarctic  and  Arctic  seas, forms from salty ocean water. Overall, the Earth has lost a mass of sea ice the size of Maryland each year since 1979.

Sea level  is the average level between high tide and low tide where the surface of the sea meets a shoreline.

Sea level rise

Sea level rise  describes an increase in the average level between high tide and low tide where the surface of the sea meets a shoreline.

Seed particles

Seed particles  are tiny solid or liquid particles that provide a non-gaseous surface. The surface allows water to make the transition from a vapor to a liquid.

Sediment data

Sediment data  are materials and measurements obtained from taking a vertical core of lake bottom sediment and analyzing the layers.

Sensitivity analysis

Sensitivity analysis  is an interpretation of different sources of variation in the output of a predictive model.

Solar Cycle

The  solar cycle  describes the sun’s activity over its eleven-year period of movement and related variations. The cycle was first determined in 1843 by German astronomer Heinrich Schwabe. Scientists are trying to determine how much solar variations affect the temperature of Earth’s atmosphere.

Solar Power

Solar power   refers to the energy harnessed from the sun, which can then be transformed into different types of energy, including thermal and electric.

Stratosphere

Stratosphere  is a layer of the atmosphere nine to 31 miles above the Earth. Ozone in the stratosphere filters out harmful sun rays, including a type of sunlight called ultraviolet B. This type of light causes health and environmental damage.

Synoptic  is used to describe a large-scale weather system more than 200 miles across.

Thermochemical technologies

Thermochemical technologies  are methods of capturing the energy potential of biomass.

Thermodynamic modules

Thermodynamic modules  are the portions of models that predict changes in aerosols due to temperature.

Tillage  refers to cultivation of the soil to improve production of crops.

Trace gases

Trace gases  make up only one percent of the atmosphere. Most of the atmosphere is made up of nitrogen (78 percent by volume) and oxygen (21 percent by volume).

Transpiration

Transpiration  is the evaporation of water into the atmosphere from the leaves and stems of plants. It accounts for approximately 90 percent of all evaporating water.

Transportation Control Measures

Transportation Control Measures  describe travel demand management measures to help reduce air pollutants from transportation sources.

Volatile organic compounds

Volatile organic compounds , or volatile organic carbon, are chemical compounds from solids or liquids that are emitted as gases. VOCs are emitted by thousands of man-made sources including paints, lacquers, cleaning supplies, pesticides, building materials, furnishings, copiers, correction fluids, adhesives, permanent markers, cleaners and disinfectants, fuels, crude oil and cosmetics. Natural sources are trees, termites, cows (ruminants) and agricultural cultivation.

Water column

Water column  is the full depth of a lake from the surface to the bottom.

Wildfires  are unplanned burns in any natural environment, like a forest or a grassland. Wildfire can spread quickly, burning through most anything in their path, causing  injury and death to people and animals .

Experts Answer 8 Important Wildfire Questions

5 Ways to Prevent and Prepare for Wildfires

Wildland-urban interface

The wildland-urban interface is where the wilderness meets a well-populated area. A wildfire that crosses this divide becomes more dangerous because there is a higher chance of burning people’s homes and releasing toxic materials that can cause significant harm to humans and animals. It can also directly lead to more deaths. According to UC Davis researchers, wildfires are crossing the wildland-urban interface more frequently .

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  • v.33(2); Fall 2010

Climate Change: The Evidence and Our Options

Glaciers serve as early indicators of climate change. Over the last 35 years, our research team has recovered ice-core records of climatic and environmental variations from the polar regions and from low-latitude high-elevation ice fields from 16 countries. The ongoing widespread melting of high-elevation glaciers and ice caps, particularly in low to middle latitudes, provides some of the strongest evidence to date that a large-scale, pervasive, and, in some cases, rapid change in Earth's climate system is underway. This paper highlights observations of 20th and 21st century glacier shrinkage in the Andes, the Himalayas, and on Mount Kilimanjaro. Ice cores retrieved from shrinking glaciers around the world confirm their continuous existence for periods ranging from hundreds of years to multiple millennia, suggesting that climatological conditions that dominate those regions today are different from those under which these ice fields originally accumulated and have been sustained. The current warming is therefore unusual when viewed from the millennial perspective provided by multiple lines of proxy evidence and the 160-year record of direct temperature measurements. Despite all this evidence, plus the well-documented continual increase in atmospheric greenhouse gas concentrations, societies have taken little action to address this global-scale problem. Hence, the rate of global carbon dioxide emissions continues to accelerate. As a result of our inaction, we have three options: mitigation, adaptation, and suffering.

Climatologists, like other scientists, tend to be a stolid group. We are not given to theatrical rantings about falling skies. Most of us are far more comfortable in our laboratories or gathering data in the field than we are giving interviews to journalists or speaking before Congressional committees. Why then are climatologists speaking out about the dangers of global warming? The answer is that virtually all of us are now convinced that global warming poses a clear and present danger to civilization ( “Climate Change,” 2010 ).

That bold statement may seem like hyperbole, but there is now a very clear pattern in the scientific evidence documenting that the earth is warming, that warming is due largely to human activity, that warming is causing important changes in climate, and that rapid and potentially catastrophic changes in the near future are very possible. This pattern emerges not, as is so often suggested, simply from computer simulations, but from the weight and balance of the empirical evidence as well.

THE EVIDENCE

Figure 1 shows northern hemisphere temperature profiles for the last 1,000 years from a variety of high-resolution climate recorders such as glacier lengths ( Oerlemans, 2005 ), tree rings ( Briffa, Jones, Schwerngruber, Shiyatov, & Cook, 2002 ; Esper, Cook, & Schweingruber, 2002 ), and combined sources that include some or all of the following: tree rings, sediment cores, ice cores, corals, and historical records ( Crowley & Lowery, 2000 ; Jones, Briffa, Barnett, & Tett, 1998 ; Mann, Bradley, & Hughes, 1999 ; Moberg, Sonechkin, Holmgrem, Datsenko, & Karlen, 2005 ). The heavy gray line is a composite of all these temperatures ( Mann & Jones, 2003 ), and the heavy black line depicts actual thermometer readings back to 1850 (see National Research Council, 2006 , for a review of surface temperature reconstructions). Although the various curves differ from one another, their general shapes are similar. Each data source shows that average northern hemisphere temperatures remained relatively stable until the late 20th century. It is the agreement of these diverse data sets and the pattern that make climatologists confident that the warming trend is real.

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A variety of temperature records over the last 1,000 years, based on a variety of proxy recorders such as tree rings, ice cores, historical records, instrumental data, etc., shows the extent of the recent warming. The range of temperature projected by Meehl et al. (2007) to 2100 AD is shown by the shaded region, and the average of the range is depicted by the filled circle.

Because these temperature numbers are based on northern hemisphere averages, they do not reflect regional, seasonal, and altitudinal variations. For example, the average temperature in the western United States is rising more rapidly than in the eastern part of the country, and on average winters are warming faster than summers ( Meehl, Arblaster, & Tebaldi, 2007 ). The most severe temperature increases appear to be concentrated in the Arctic and over the Antarctic Peninsula as well as within the interior of the large continents. This variability complicates matters, and adds to the difficulty of convincing the public, and even scientists in other fields, that global warming is occurring. Because of this, it may be useful to examine another kind of evidence: melting ice.

Retreat of Mountain Glaciers

The world's mountain glaciers and ice caps contain less than 4% of the world's ice cover, but they provide invaluable information about changes in climate. Because glaciers are smaller and thinner than the polar ice sheets, their ratio of surface area to volume is much greater; thus, they respond more quickly to temperature changes. In addition, warming trends are amplified at higher altitudes where most glaciers are located ( Bradley, Keimig, Diaz, & Hardy, 2009 ; Bradley, Vuille, Diaz, & Vergara, 2006 ). Thus, glaciers provide an early warning system of climate change; they are our “canaries in the coal mine.”

Consider the glaciers of Africa's Mount Kilimanjaro ( Figure 2 ). Using a combination of terrestrial photogrammetric maps, satellite images, and aerial photographs, we have determined that the ice fields on Kibo, the highest crater on Kilimanjaro, have lost 85% of their coverage since 1912 ( Thompson, Brecher, Mosley-Thompson, Hardy, & Mark, 2009 ).

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The retreat of glaciers on Mount Kilimanjaro can be seen in the photographs from 1912, 1970, 2000, and 2006; from 1912 to 2006, 85% of the ice has disappeared.

Figure 3 shows a series of aerial photographs of Furtwängler glacier, in the center of Kibo crater, taken between 2000 and 2007, when the glacier split into two sections. As Furtwängler recedes, it is also thinning rapidly, from 9.5 m in 2000 to 4.7 m in 2009 (for more images of Furtwängler's retreat, see http://www.examiner.com/examiner/x-10722-Orlando-Science-Policy-Examiner∼y2009m11d2-Mt-Kilimanjaros-Furtwängler-Glacier-in-retreat ). If you connect the dots on the changes seen to date and assume the same rate of loss in the future, within the next decade many of the glaciers of Kilimanjaro, a Swahili word meaning “shining mountain,” will have disappeared.

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Deterioration of the Furtwängler glacier in the center of Kibo crater on Mount Kilimanjaro. Since 2000 the ice field has decreased in size and thickness and has divided in two.

The Quelccaya ice cap, which is located in southern Peru adjacent to the Amazon Basin, is the largest tropical ice field on Earth. Quelccaya has several outlet glaciers, glaciers that extend from the edges of an ice cap like fingers from a hand. The retreat of one of these, Qori Kalis, has been studied and photographed since 1963. At the beginning of this study, Qori Kalis extended 1,200 m out from the ice cap, and there was no melt water at the end ( Figure 4 , map top left). By the summer of 2008, Qori Kalis had retreated to the very edge of Quelccaya, leaving behind an 84-acre lake, 60 m deep. Over the years, a boulder near the base camp has served as a benchmark against which to record the changes in the position of the edge of the ice. In 1977 the ice was actually pushing against the boulder ( Figure 5 , top), but by 2006 a substantial gap had appeared and been filled by a lake ( Figure 5 , bottom). Thus, the loss of Quelccaya's ice is not only on the Qori Kalis glacier but also on the margin of the ice cap itself. Since 1978, about 25% of this tropical ice cap has disappeared.

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Retreat of the Qori Kalis outlet glacier on the Quelccaya ice cap. Each line shows the extent of the ice. The photos along the bottom provide a pictorial history of the melting of the Qori Kalis outlet glacier and the formation of a lake. The retreat of Qori Kalis is similar to the loss of several Peruvian glaciers, as shown in the graph insert.

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Top: photo taken in 1978 shows a margin of the Quelccaya ice cap pushing against a boulder. Bottom: the same margin is shown in a 2005 photo. The ice has receded and has been replaced by a small lake. The boulder shown in the top photo is located in the center of the white circle to the right.

The Himalayan Mountains are home to more than 15,000 glaciers. Unfortunately, only a few of these glaciers have been monitored over an extended period, so reliable ground observations that are crucial for determining regional retreat rates do not yet exist. However, a recent study of an ice core from the Naimona'nyi glacier in the southwestern Himalayas ( Kehrwald et al., 2008 ) shows that ice is disappearing from the top of the glacier, as shown by the lack of the radioactive bomb layers from the 1950s and early 1960s that appear in all Tibetan and Himalayan ice core records ( Thompson, 2000 ; Thompson et al., 1990 , 1997 , 2006 ).

Glaciologists at the Institute of Tibetan Plateau Research in Beijing have been monitoring 612 glaciers across the High Asian region since 1980. These scientists found that from 1980 to 1990, 90% of these glaciers were retreating; from 1990 to 2005, the proportion of retreating glaciers increased to 95% ( Yao, Pu, Lu, Wang, & Yu, 2007 ).

A study of 67 glaciers in Alaska from the mid-1950s to the mid-1990s shows that all are thinning ( Arendt, Echelmeyer, Harrison, Lingle, & Valentine, 2002 ). In northern Alaska's Brooks Range, 100% of the glaciers are in retreat, and in southeastern Alaska 98% are shrinking ( Molnia, 2007 ). Glacier National Park in Montana contained more than 100 glaciers when it was established in 1910. Today, just 26 remain, and at the current rate of decrease it is estimated that by 2030 there will be no glaciers in Glacier National Park ( Hall & Fagre, 2003 ). The oldest glacier photos come from the Alps. Ninety-nine percent of the glaciers in the Alps are retreating, and 92% of Chile's Andean glaciers are retreating ( Vince, 2010 ).

The pattern described here is repeated around the world. Mountain glaciers nearly everywhere are retreating.

Loss of Polar Ice

Satellite documentation of the area covered by sea ice in the Arctic Ocean extends back three decades. This area, measured each September, decreased at a rate of about 8.6% per decade from 1979 to 2007. In 2007 alone, 24% of the ice disappeared. In 2006 the Northwest Passage was ice free for the first time in recorded history.

As noted earlier, polar ice sheets are slower to respond to temperature rise than the smaller mountain glaciers, but they, too, are melting. The Greenland ice sheet has also experienced dramatic ice melt in recent years. There has been an increase in both the number and the size of lakes in the southern part of the ice sheet, and crevices can serve as conduits (called moulins) that transport meltwater rapidly into the glacier. Water has been observed flowing through these moulins down to the bottom of the ice sheet where it acts as a lubricant that speeds the flow of ice to the sea ( Das et al., 2008 ; Zwally et al., 2002 ).

The ice in Antarctica is also melting. The late John Mercer, a glacial geologist at The Ohio State University, long ago concluded that the first evidence of global warming due to increasing carbon dioxide (CO 2 ) would be the breakup of the Antarctic ice shelves ( Mercer, 1978 ). Mean temperatures on the Antarctic Peninsula have risen 2.5° C (4.5° F) in the last 50 years, resulting in the breakup of the ice shelves in just the way Mercer predicted. One of the most rapid of these shelf deteriorations occurred in 2002, when the Larsen B, a body of ice over 200 m deep that covered an area the size of Rhode Island, collapsed in just 31 days (see images http://earthobservatory.nasa.gov/IOTD/view.php?id = 2351). An ice shelf is essentially an iceberg attached to land ice. Just as an ice cube does not raise the water level in a glass when it melts, so a melting ice shelf leaves sea levels unchanged. But ice shelves serve as buttresses to glaciers on land, and when those ice shelves collapse it speeds the flow of the glaciers they were holding back into the ocean, which causes sea level to rise rapidly.

Just days before this paper went to press, a giant ice island four times the size of Manhattan broke off the Petermann glacier in Greenland. This event alone does not prove global climate change, because half of the ice loss from Greenland each year comes from icebergs calving from the margins. It is the fact that this event is part of a long-term trend of increasing rates of ice loss, coupled with the fact that temperature is increasing in this region at the rate of 2° C (3.6° F) per decade, that indicates that larger scale global climate change is underway.

The loss of ice in the Arctic and Antarctic regions is especially troubling because these are the locations of the largest ice sheets in the world. Of the land ice on the planet, 96% is found on Greenland and Antarctica. Should all this ice melt, sea level would rise over 64 m ( Church et al., 2001 ; Lemke et al., 2007 ), and of course the actual sea level would be much higher due to thermal expansion of the world's oceans as they warm.

Although research shows some variability in the rate of ice loss, it is clear that mountain glaciers and polar ice sheets are melting, and there is no plausible explanation for this but global warming. Add to this the laboratory evidence and the meteorological measurements, and the case for global warming cannot be denied. So what causes global temperatures to rise?

CAUSES OF GLOBAL WARMING

Climatologists strive to reconstruct past climate variations on regional and global scales, but they also try to determine the mechanisms, called forcers , that drive climate change. Climatologists recognize two basic categories of forcers. Natural forcers are recurring processes that have been around for millions of years; anthropogenic forcers are more recent processes caused by human activity.

One familiar natural forcer is the earth's orbit around the sun, which gives us our seasons. In the northern hemisphere, June is warm because the sun's rays fall more directly on it, and the sun appears high in the sky; in the southern hemisphere, June is cool because the sun's rays hit the earth at a deep angle, and the sun appears low in the sky.

Less obvious natural forcers include short- and long-term changes in the atmosphere and ocean. For example, when Mount Pinatubo erupted in the Philippines in 1991, it spewed millions of tons of sulfuric gases and ash particles high into the atmosphere, blocking the sun's rays. This lowered global temperatures for the next few years. Another natural forcer is the linked oceanic and atmospheric system in the equatorial Pacific Ocean known as the El Niño-Southern Oscillation (ENSO). ENSO occurs every 3 to 7 years in the tropical Pacific and brings warm, wet weather to some regions and cool, dry weather to other areas.

Other natural forcers include periodic changes in energy from the sun. These include the 11- to 12-year sunspot cycle and the 70- to 90-year Wolf-Gleissberg cycle, a modulation of the amplitude of the 11-year solar cycle. These changes in solar energy can affect atmospheric temperature across large regions for hundreds of years and may have caused the “medieval climate anomaly” in the northern hemisphere that lasted from about 1100 AD to 1300 AD. Solar cycles may also have played a role in the cause of the “little ice age” in North America and Europe during the 16th to 19th centuries. These changes in climate, which are often cited by those who dismiss global warming as a normal, cyclical event, affected large areas, but not the Earth as a whole. The medieval climate anomaly showed warmth that matches or exceeds that of the past decade in some regions, but it fell well below recent levels globally ( Mann et al., 2009 ).

The most powerful natural forcers are variations in the orbit of the Earth around the Sun, which last from 22,000 to 100,000 years. These “orbital forcings” are partly responsible for both the ice ages (the glacial periods during which large regions at high and midddle latitudes are covered by thick ice sheets), and for the warm interglacial periods such as the present Holocene epoch which began about 10,000 years ago.

There is consensus among climatologists that the warming trend we have been experiencing for the past 100 years or so cannot be accounted for by any of the known natural forcers. Sunspot cycles, for example, can increase the sun's output, raising temperatures in our atmosphere. We are seeing a temperature increase in the troposphere, the lower level of our atmosphere, and a temperature decrease in the stratosphere, the upper level. But this is the exact opposite of what we would get if increased solar energy were responsible. Similarly, global temperatures have increased more at night than during the day, again the opposite of what would occur if the sun were driving global warming. In addition, temperatures have risen more in winter than in summer. This, too, is the opposite of what would be expected if the sun were responsible for the planet's warming. High latitudes have warmed more than low latitudes, and because we get more radiation from the sun at low latitudes, we again would expect the opposite if the sun were driving these changes. Thus, changes in solar output cannot account for the current period of global warming ( Meehl et al., 2007 ). ENSO and other natural forcers also fail to explain the steady, rapid rise in the earth's temperature. The inescapable conclusion is that the rise in temperature is due to anthropogenic forces, that is, human behavior.

The relatively mild temperatures of the past 10,000 years have been maintained by the greenhouse effect, a natural phenomenon. As orbital forcing brought the last ice age to an end, the oceans warmed, releasing CO 2 into the atmosphere, where it trapped infrared energy reflected from the earth's surface. This warmed the planet. The greenhouse effect is a natural, self-regulating process that is absolutely essential to sustain life on the planet. However, it is not immutable. Change the level of greenhouse gases in the atmosphere, and the planet heats up or cools down.

Greenhouse gases are captured in ice, so ice cores allow us to see the levels of greenhouse gases in ages past. The longest ice core ever recovered (from the European Project for Ice Coring in Antarctica) takes us 800,000 years back in time, and includes a history of CO 2 and methane levels preserved in bubbles in the ice ( Loulergue et al., 2008 ; Lüthi et al., 2008 ). The CO 2 and methane curves illustrated in Figure 6 show that the modern levels of these gases are unprecedented in the last 800 millennia.

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Concentrations of carbon dioxide (CO 2 ) and methane (CH 4 ) over the last 800,000 years (eight glacial cycles) from East Antarctic ice cores. Data from Loulergue et al. (2008) and Lüthi et al. (2008) . The current concentrations of CO 2 and CH 4 are also shown ( Forster et al., 2007 ).

Globally, CO 2 concentrations have varied between 180 and 190 parts per million per volume (ppmv) during glacial (cold) periods and between 270 and 290 ppmv during interglacial (warm) periods. However, since the onset of the Industrial Revolution, when fossil fuel use (chiefly coal and oil) began to burgeon, CO 2 concentration has increased about 38% over the natural interglacial levels ( Forster et al., 2007 ). Between 1975 and 2005, CO 2 emissions increased 70%, and between 1999 and 2005 global emissions increased 3% per year ( Marland, Boden, & Andres, 2006 ). As of this writing, the CO 2 concentration in the atmosphere is 391 ppmv (Mauna Loa CO 2 annual mean data from the National Oceanic and Atmospheric Administration, 2010 ), a level not seen at any time in 800,000 years. Climatologists have identified no natural forcers that could account for this rapid and previously unseen rise in CO 2 .

Methane raises temperature even more than CO 2 , and the amount of methane in the atmosphere, like that of CO 2 , is also at a level not seen in 800 millennia. Two thirds of current emissions of methane are by-products of human activity, things like the production of oil and natural gas, deforestation, decomposition of garbage and sewage, and raising farm animals.

Many people find it difficult to believe that human activity can affect a system as large as Earth's climate. After all, we are so tiny compared to the planet. But every day we tiny human beings drive cars; watch television; turn on lamps; heat or cool our houses and offices; eat food transported to us by planes, ships, and trucks; clear or burn forests; and behave in countless other ways that directly or indirectly release greenhouse gases into the air. Together, we humans emitted eight billion metric tons of carbon to our planet's atmosphere in 2007 alone ( Boden, Marland, & Andres, 2009 ). (CO 2 weighs 3.66 times more than carbon; that means we released 29.3 billion metric tons of CO 2 .) The evidence is overwhelming that human activity is responsible for the rise in CO 2 , methane, and other greenhouse gas levels, and that the increase in these gases is fueling the rise in mean global temperature.

A global temperature rise of a few degrees may not seem such a bad thing, especially to people living in harsh, cold climates. But global warming does not mean merely that we will trade parkas for T-shirts or turn up the air conditioning. A warming planet is a changing planet, and the changes will have profound consequences for all species that live on it, including humans. Those changes are not just something our children and grandchildren will have to deal with in the future; they are taking place now, and are affecting millions of people.

EFFECTS OF GLOBAL WARMING

One effect of global warming that everyone has heard about is a rise in sea levels. About half of this rise is due to thermal expansion: Ocean temperatures are rising, and as water warms it expands. Put a nearly full cup of water in a microwave and heat it, and the water will spill over the cup.

In addition to thermal expansion, the oceans are rising because ice is melting, and most of that water inevitably finds its way to the sea. So far, most of that water has come from mountain glaciers and ice caps ( Meier et al., 2007 ). As global temperatures increase, sea level rise will mainly reflect polar ice melt. So far, ocean rise has been measured in millimeters, but there is enough water in the Greenland ice sheet alone to raise sea levels by about 7 m, West Antarctica over 5 m, and East Antarctica about 50 m ( Lemke et al., 2007 ). If the Earth were to lose just 8% of its ice, the consequences for some coastal regions would be dramatic. The lower part of the Florida peninsula and much of Louisiana, including New Orleans, would be submerged, and low-lying cities, including London, New York, and Shanghai, would be endangered (to see the effects of various magnitudes of sea level rise in the San Francisco Bay area, go to http://cascade.wr.usgs.gov/data/Task2b-SFBay/data.shtm ).

Low-lying continental countries such as the Netherlands and much of Bangladesh already find themselves battling flooding more than ever before. Many small island nations in the western Pacific (e.g., Vanuatu) are facing imminent destruction as they are gradually overrun by the rising ocean. Indonesia is an island nation, and many of its 17,000 islands are just above sea level. At the 2007 United Nations Climate Change Conference in Bali, Indonesian environmental minister Rachmat Witoelar stated that 2,000 of his country's islands could be lost to sea level rise by 2030. At current rates of sea level rise, another island nation, the Republic of Maldives, will become uninhabitable by the end of the century ( http://unfcc.int/resource/docs/napa/mdv01.pdf ). In 2008, the president of that country, Mohamed Nasheed, announced that he was contemplating moving his people to India, Sri Lanka, and Australia ( Schmidle, 2009 ). One of the major effects of continued sea level rise will be the displacement of millions of people. Where millions of climate refugees will find welcome is unclear. The migration of large numbers of people to new territories with different languages and cultures will be disruptive, to say the least.

In addition to the danger of inundation, rising sea levels bring salt water into rivers, spoil drinking wells, and turn fertile farmland into useless fields of salty soil. These effects of global warming are occurring now in places like the lowlands of Bangladesh ( Church et al., 2001 ).

People on dry land need the fresh water that is running into the sea. In the spring, melting ice from mountain glaciers, ice caps, and snowfields furnish wells and rivers that provide fresh water for drinking, agriculture, and hydroelectric power. For example, in the dry season, people in large areas of India, Nepal, and southern China depend on rivers fed by Himalayan glaciers. The retreat of these glaciers threatens the water supply of millions of people in this part of the world. Peru relies on hydroelectric power for 80% of its energy ( Vergara et al., 2007 ), a significant portion of which comes from mountain streams that are fed by mountain glaciers and ice fields. In Tanzania, the loss of Mount Kilimanjaro's fabled ice cover would likely have a negative impact on tourism, which is the country's primary source of foreign currency. The glaciers and snow packs in the Rocky Mountains are essential for farming in California, one of the world's most productive agricultural areas.

Global warming is expanding arid areas of the Earth. Warming at the equator drives a climate system called the Hadley Cell. Warm, moist air rises from the equator, loses its moisture through rainfall, moves north and south, and then falls to the Earth at 30° north and south latitude, creating deserts and arid regions. There is evidence that over the last 20 years the Hadley Cell has expanded north and south by about 2° latitude, which may broaden the desert zones ( Seidel, Fu, Randel, & Reichler, 2008 ; Seidel & Randel, 2007 ). If so, droughts may become more persistent in the American Southwest, the Mediterranean, Australia, South America, and Africa.

Global warming can also have effects that seem paradoxical. Continued warming may change ocean currents that now bring warm water to the North Atlantic region, giving it a temperate climate. If this happens, Europe could experience a cooling even as other areas of the world become warmer.

Accelerating Change

It is difficult to assess the full effects of global warming, and harder still to predict future effects. Climate predictions are made with computer models, but these models have assumed a slow, steady rate of change. Our best models predict a temperature rise in this century of between 2.4° and 4.5° C (4.3° and 8.1° F), with an average of about 3° C (5.4° F; Meehl et al., 2007 ; Figure 1 ). But these models assume a linear rise in temperature. Increasingly, computer models have underestimated the trends because, in fact, the rate of global temperature rise is accelerating. The average rise in global temperature was 0.11° F per decade over the last century ( National Oceanic and Atmospheric Administration, 2009 ). Since the late 1970s, however, this rate has increased to 0.29° F per decade, and 11 of the warmest years on record have occurred in the last 12 years. May, 2010, was the 303rd consecutive month with a global temperature warmer than its 20th-century average ( National Oceanic and Atmospheric Administration, 2010 ).

The acceleration of global temperature is reflected in increases in the rate of ice melt. From 1963 to 1978, the rate of ice loss on Quelccaya was about 6 m per year. From 1991 to 2006, it averaged 60 m per year, 10 times faster than the initial rate ( Thompson et al., 2006 ). A recent paper by Matsuo and Heki (2010) reports uneven ice loss from the high Asian ice fields, as measured by the Gravity Recovery and Climate Experiment satellite observations between 2003 and 2009. Ice retreat in the Himalayas slowed slightly during this period, and loss in the mountains to the northwest increased markedly over the last few years. Nevertheless, the average rate of ice melt in the region was twice the rate of four decades before. In the last decade, many of the glaciers that drain Greenland and Antarctica have accelerated their discharge into the world's oceans from 20% to 100% ( Lemke et al., 2007 ).

Increasing rates of ice melt should mean an increasing rate of sea level rise, and this is in fact the case. Over most of the 20th century, sea level rose about 2 mm per year. Since 1990, the rate has been about 3 mm per year.

So, not only is Earth's temperature rising, but the rate of this change is accelerating. This means that our future may not be a steady, gradual change in the world's climate, but an abrupt and devastating deterioration from which we cannot recover.

Abrupt Climate Change Possible

We know that very rapid change in climate is possible because it has occurred in the past. One of the most remarkable examples was a sudden cold, wet event that occurred about 5,200 years ago, and left its mark in many paleoclimate records around the world.

The most famous evidence of this abrupt weather change comes from Otzi, the “Tyrolean ice man” whose remarkably preserved body was discovered in the Eastern Alps in 1991 after it was exposed by a melting glacier. Forensic evidence suggests that Otzi was shot in the back with an arrow, escaped his enemies, then sat down behind a boulder and bled to death. We know that within days of Otzi's dying there must have been a climate event large enough to entomb him in snow; otherwise, his body would have decayed or been eaten by scavengers. Radiocarbon dating of Otzi's remains revealed that he died around 5,200 years ago ( Baroni & Orombelli, 1996 ).

The event that preserved Otzi could have been local, but other evidence points to a global event of abrupt cooling. Around the world organic material is being exposed for the first time in 5,200 years as glaciers recede. In 2002, when we studied the Quelccaya ice cap in southern Peru, we found a perfectly preserved wetland plant. It was identified as Distichia muscoides , which today grows in the valleys below the ice cap. Our specimen was radiocarbon dated at 5,200 years before present ( Thompson et al., 2006 ). As the glacier continues to retreat, more plants have been collected and radiocarbon dated, almost all of which confirm the original findings ( Buffen, Thompson, Mosley-Thompson, & Huh, 2009 ).

Another record of this event comes from the ice fields on Mount Kilimanjaro. The ice dating back 5,200 years shows a very intense, very sudden decrease in the concentration of heavy oxygen atoms, or isotopes, in the water molecules that compose the ice ( Thompson et al., 2002 ). Such a decrease is indicative of colder temperatures, more intense snowfall, or both.

The Soreq Cave in Israel contains speleothems that have produced continuous climate records spanning tens of thousands of years. The record shows that an abrupt cooling also occurred in the Middle East about 5,200 years ago, and that it was the most extreme climatic event in the last 13,000 years ( Bar-Matthews, Ayalon, Kaufman, & Wasserburg, 1999 ).

One way that rapid climate change can occur is through positive feedback. In the physical sciences, positive feedback means that an event has an effect which, in turn, produces more of the initial event. The best way to understand this phenomenon as it relates to climate change is through some very plausible examples:

Higher global temperatures mean dryer forests in some areas, which means more forest fires, which means more CO 2 and ash in the air, which raises global temperature, which means more forest fires, which means …

Higher global temperatures mean melting ice, which exposes darker areas (dirt, rock, water) that reflect less solar energy than ice, which means higher global temperatures, which means more melting ice, which means …

Higher global temperatures mean tundra permafrost melts, releasing CO 2 and methane from rotted organic material, which means higher global temperature, which means more permafrost melting, which means …

Positive feedback increases the rate of change. Eventually a tipping point may be reached, after which it could be impossible to restore normal conditions. Think of a very large boulder rolling down a hill: When it first starts to move, we might stop it by pushing against it or wedging chocks under it or building a barrier, but once it has reached a certain velocity, there is no stopping it. We do not know if there is a tipping point for global warming, but the possibility cannot be dismissed, and it has ominous implications. Global warming is a very, very large boulder.

Even if there is no tipping point (or we manage to avoid it), the acceleration of warming means serious trouble. In fact, if we stopped emitting greenhouse gases into the atmosphere tomorrow, temperatures would continue to rise for 20 to 30 years because of what is already in the atmosphere. Once methane is injected into the troposphere, it remains for about 8 to 12 years ( Prinn et al., 1987 ). Carbon dioxide has a much longer residence: 70 to 120 years. Twenty percent of the CO 2 being emitted today will still affect the earth's climate 1,000 years from now ( Archer & Brovkin, 2008 ).

If, as predicted, global temperature rises another 3° C (5.4° F) by the end of the century, the earth will be warmer than it has been in about 3 million years ( Dowsett et al., 1994 ; Rahmstorf, 2007 ). Oceans were then about 25 m higher than they are today. We are already seeing important effects from global warming; the effects of another 3° C (5.4° F) increase are hard to predict. However, such a drastic change would, at the very least, put severe pressure on civilization as we know it.

OUR OPTIONS

Global warming is here and is already affecting our climate, so prevention is no longer an option. Three options remain for dealing with the crisis: mitigate, adapt, and suffer.

Mitigation is proactive, and in the case of anthropogenic climate change it involves doing things to reduce the pace and magnitude of the changes by altering the underlying causes. The obvious, and most hotly debated, remedies include those that reduce the volume of greenhouse gas emissions, especially CO 2 and methane. Examples include not only using compact fluorescent lightbulbs, adding insulation to our homes, and driving less, but societal changes such as shutting down coal-fired power plants, establishing a federal carbon tax (as was recently recommended by the National Academy of Sciences), and substantially raising minimum mileage standards on cars ( National Research Council, 2010 ). Another approach to mitigation that has received widespread attention recently is to enhance the natural carbon sinks (storage systems) through expansion of forests. Some have suggested various geo-engineering procedures (e.g., Govindasamy & Caldeira, 2000 ; Wigley, 2006 ). One example is burying carbon in the ocean or under land surfaces ( Brewer, Friederich, Peltzer, & Orr, 1999 ). Geo-engineering ideas are intriguing, but some are considered radical and may lead to unintended negative consequences ( Parkinson, 2010 ).

Adaptation is reactive. It involves reducing the potential adverse impacts resulting from the by-products of climate change. This might include constructing sea barriers such as dikes and tidal barriers (similar to those on the Thames River in London and in New Orleans), relocating coastal towns and cities inland, changing agricultural practices to counteract shifting weather patterns, and strengthening human and animal immunity to climate-related diseases.

Our third option, suffering, means enduring the adverse impacts that cannot be staved off by mitigation or adaptation. Everyone will be affected by global warming, but those with the fewest resources for adapting will suffer most. It is a cruel irony that so many of these people live in or near ecologically sensitive areas, such as grasslands (Outer Mongolia), dry lands (Sudan and Ethiopia), mountain glaciers (the Quechua of the Peruvian Andes), and coastal lowlands (Bangledesh and the South Sea island region). Humans will not be the only species to suffer.

Clearly mitigation is our best option, but so far most societies around the world, including the United States and the other largest emitters of greenhouse gases, have done little more than talk about the importance of mitigation. Many Americans do not even accept the reality of global warming. The fossil fuel industry has spent millions of dollars on a disinformation campaign to delude the public about the threat, and the campaign has been amazingly successful. (This effort is reminiscent of the tobacco industry's effort to convince Americans that smoking poses no serious health hazards.) As the evidence for human-caused climate change has increased, the number of Americans who believe it has decreased. The latest Pew Research Center (2010) poll in October, 2009, shows that only 57% of Americans believe global warming is real, down from 71% in April, 2008.

There are currently no technological quick fixes for global warming. Our only hope is to change our behavior in ways that significantly slow the rate of global warming, thereby giving the engineers time to devise, develop, and deploy technological solutions where possible. Unless large numbers of people take appropriate steps, including supporting governmental regulations aimed at reducing greenhouse gas emissions, our only options will be adaptation and suffering. And the longer we delay, the more unpleasant the adaptations and the greater the suffering will be.

Sooner or later, we will all deal with global warming. The only question is how much we will mitigate, adapt, and suffer.

Acknowledgments

This paper is based on the Presidential Scholar's Address given at the 35th annual meeting of the Association for Behavior Analysis International, Phoenix, Arizona. I am grateful to Bill Heward for inviting me to give the address. I thank Mary Davis for her help editing the text and figures. I wish to thank all the field and laboratory team members from the Byrd Polar Research Center who have worked so diligently over the years. I am especially indebted to the hard work of our current research team: Ellen Mosley-Thompson, Henry Brecher, Mary Davis, Paolo Gabrielli, Ping-Nan Lin, Matt Makou, Victor Zagorodnov, and all of our graduate students. Funding for our research over the years has been provided by the National Science Foundation's Paleoclimate Program, the National Oceanic and Atmospheric Administration's Paleoclimatology and Polar Programs, the National Aeronautic and Space Administration, Gary Comer Foundation, and The Ohio State University's Climate, Water and Carbon Program. This is Byrd Polar Research Center Publication 1402.

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National Academies Press: OpenBook

Climate Change: Evidence and Causes: Update 2020 (2020)

Chapter: conclusion, c onclusion.

This document explains that there are well-understood physical mechanisms by which changes in the amounts of greenhouse gases cause climate changes. It discusses the evidence that the concentrations of these gases in the atmosphere have increased and are still increasing rapidly, that climate change is occurring, and that most of the recent change is almost certainly due to emissions of greenhouse gases caused by human activities. Further climate change is inevitable; if emissions of greenhouse gases continue unabated, future changes will substantially exceed those that have occurred so far. There remains a range of estimates of the magnitude and regional expression of future change, but increases in the extremes of climate that can adversely affect natural ecosystems and human activities and infrastructure are expected.

Citizens and governments can choose among several options (or a mixture of those options) in response to this information: they can change their pattern of energy production and usage in order to limit emissions of greenhouse gases and hence the magnitude of climate changes; they can wait for changes to occur and accept the losses, damage, and suffering that arise; they can adapt to actual and expected changes as much as possible; or they can seek as yet unproven “geoengineering” solutions to counteract some of the climate changes that would otherwise occur. Each of these options has risks, attractions and costs, and what is actually done may be a mixture of these different options. Different nations and communities will vary in their vulnerability and their capacity to adapt. There is an important debate to be had about choices among these options, to decide what is best for each group or nation, and most importantly for the global population as a whole. The options have to be discussed at a global scale because in many cases those communities that are most vulnerable control few of the emissions, either past or future. Our description of the science of climate change, with both its facts and its uncertainties, is offered as a basis to inform that policy debate.

A CKNOWLEDGEMENTS

The following individuals served as the primary writing team for the 2014 and 2020 editions of this document:

  • Eric Wolff FRS, (UK lead), University of Cambridge
  • Inez Fung (NAS, US lead), University of California, Berkeley
  • Brian Hoskins FRS, Grantham Institute for Climate Change
  • John F.B. Mitchell FRS, UK Met Office
  • Tim Palmer FRS, University of Oxford
  • Benjamin Santer (NAS), Lawrence Livermore National Laboratory
  • John Shepherd FRS, University of Southampton
  • Keith Shine FRS, University of Reading.
  • Susan Solomon (NAS), Massachusetts Institute of Technology
  • Kevin Trenberth, National Center for Atmospheric Research
  • John Walsh, University of Alaska, Fairbanks
  • Don Wuebbles, University of Illinois

Staff support for the 2020 revision was provided by Richard Walker, Amanda Purcell, Nancy Huddleston, and Michael Hudson. We offer special thanks to Rebecca Lindsey and NOAA Climate.gov for providing data and figure updates.

The following individuals served as reviewers of the 2014 document in accordance with procedures approved by the Royal Society and the National Academy of Sciences:

  • Richard Alley (NAS), Department of Geosciences, Pennsylvania State University
  • Alec Broers FRS, Former President of the Royal Academy of Engineering
  • Harry Elderfield FRS, Department of Earth Sciences, University of Cambridge
  • Joanna Haigh FRS, Professor of Atmospheric Physics, Imperial College London
  • Isaac Held (NAS), NOAA Geophysical Fluid Dynamics Laboratory
  • John Kutzbach (NAS), Center for Climatic Research, University of Wisconsin
  • Jerry Meehl, Senior Scientist, National Center for Atmospheric Research
  • John Pendry FRS, Imperial College London
  • John Pyle FRS, Department of Chemistry, University of Cambridge
  • Gavin Schmidt, NASA Goddard Space Flight Center
  • Emily Shuckburgh, British Antarctic Survey
  • Gabrielle Walker, Journalist
  • Andrew Watson FRS, University of East Anglia

The Support for the 2014 Edition was provided by NAS Endowment Funds. We offer sincere thanks to the Ralph J. and Carol M. Cicerone Endowment for NAS Missions for supporting the production of this 2020 Edition.

F OR FURTHER READING

For more detailed discussion of the topics addressed in this document (including references to the underlying original research), see:

  • Intergovernmental Panel on Climate Change (IPCC), 2019: Special Report on the Ocean and Cryosphere in a Changing Climate [ https://www.ipcc.ch/srocc ]
  • National Academies of Sciences, Engineering, and Medicine (NASEM), 2019: Negative Emissions Technologies and Reliable Sequestration: A Research Agenda [ https://www.nap.edu/catalog/25259 ]
  • Royal Society, 2018: Greenhouse gas removal [ https://raeng.org.uk/greenhousegasremoval ]
  • U.S. Global Change Research Program (USGCRP), 2018: Fourth National Climate Assessment Volume II: Impacts, Risks, and Adaptation in the United States [ https://nca2018.globalchange.gov ]
  • IPCC, 2018: Global Warming of 1.5°C [ https://www.ipcc.ch/sr15 ]
  • USGCRP, 2017: Fourth National Climate Assessment Volume I: Climate Science Special Reports [ https://science2017.globalchange.gov ]
  • NASEM, 2016: Attribution of Extreme Weather Events in the Context of Climate Change [ https://www.nap.edu/catalog/21852 ]
  • IPCC, 2013: Fifth Assessment Report (AR5) Working Group 1. Climate Change 2013: The Physical Science Basis [ https://www.ipcc.ch/report/ar5/wg1 ]
  • NRC, 2013: Abrupt Impacts of Climate Change: Anticipating Surprises [ https://www.nap.edu/catalog/18373 ]
  • NRC, 2011: Climate Stabilization Targets: Emissions, Concentrations, and Impacts Over Decades to Millennia [ https://www.nap.edu/catalog/12877 ]
  • Royal Society 2010: Climate Change: A Summary of the Science [ https://royalsociety.org/topics-policy/publications/2010/climate-change-summary-science ]
  • NRC, 2010: America’s Climate Choices: Advancing the Science of Climate Change [ https://www.nap.edu/catalog/12782 ]

Much of the original data underlying the scientific findings discussed here are available at:

  • https://data.ucar.edu/
  • https://climatedataguide.ucar.edu
  • https://iridl.ldeo.columbia.edu
  • https://ess-dive.lbl.gov/
  • https://www.ncdc.noaa.gov/
  • https://www.esrl.noaa.gov/gmd/ccgg/trends/
  • http://scrippsco2.ucsd.edu
  • http://hahana.soest.hawaii.edu/hot/

Image

Climate change is one of the defining issues of our time. It is now more certain than ever, based on many lines of evidence, that humans are changing Earth's climate. The Royal Society and the US National Academy of Sciences, with their similar missions to promote the use of science to benefit society and to inform critical policy debates, produced the original Climate Change: Evidence and Causes in 2014. It was written and reviewed by a UK-US team of leading climate scientists. This new edition, prepared by the same author team, has been updated with the most recent climate data and scientific analyses, all of which reinforce our understanding of human-caused climate change.

Scientific information is a vital component for society to make informed decisions about how to reduce the magnitude of climate change and how to adapt to its impacts. This booklet serves as a key reference document for decision makers, policy makers, educators, and others seeking authoritative answers about the current state of climate-change science.

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Research Paper on Climate Change

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  • Review Article
  • Published: 07 March 2024

River water quality shaped by land–river connectivity in a changing climate

  • Li Li   ORCID: orcid.org/0000-0002-1641-3710 1 ,
  • Julia L. A. Knapp   ORCID: orcid.org/0000-0003-0885-7829 2   na1 ,
  • Anna Lintern   ORCID: orcid.org/0000-0002-2121-0301 3   na1 ,
  • G.-H. Crystal Ng 4   na1 ,
  • Julia Perdrial   ORCID: orcid.org/0000-0002-2581-9341 5 , 6   na1 ,
  • Pamela L. Sullivan   ORCID: orcid.org/0000-0001-8780-8501 7   na1 &
  • Wei Zhi   ORCID: orcid.org/0000-0001-5485-1095 1 , 8   na1  

Nature Climate Change volume  14 ,  pages 225–237 ( 2024 ) Cite this article

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  • Biogeochemistry

River water quality is crucial to ecosystem health and water security, yet its deterioration under climate change is often overlooked in climate risk assessments. Here we review how climate change influences river water quality via persistent, gradual shifts and episodic, intense extreme events. Although distinct in magnitude, intensity and duration, these changes modulate the structure and hydro-biogeochemical processes on land and in rivers, hence reshaping land–river connectivity and the quality of river waters. To advance understanding of and forecasting capabilities for water quality in future climates, it is essential to perceive land and rivers as interconnected systems. It is also vital to prioritize research under climate extremes, where the dynamics of water quality often challenge existing theories and models and call for shifts in conceptual paradigms.

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Acknowledgements

L.L. acknowledges the support of funding from US National Science Foundation (US NSF, EAR-2012669, 2012123, 2121621, 1911960) and US Department of Energy Environmental System Science programme (US DOE ESS, DE-SC0020146). P.L.S. acknowledges the support of funding from US NSF (EAR-2231723, 1911967) and US DOE ESS (DE-SC0023312). J.P. acknowledges the support of funding from National Science Foundation (EAR-2012123). G.-H.C.N. acknowledges the support of funding from US NSF (EAR-1759071) and US DOE ESS (DE-SC0020196). L.L. acknowledges students in the Li Reactive Water Group for feedback on an early version of the paper, and M. Wu for artistic suggestions on figures.

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These authors contributed equally: Julia L. A. Knapp, Anna Lintern, G.-H. Crystal Ng, Julia Perdrial, Pamela L. Sullivan, Wei Zhi.

Authors and Affiliations

Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, PA, USA

Li Li & Wei Zhi

Department of Earth Sciences, Durham University, Durham, UK

Julia L. A. Knapp

Department of Civil Engineering, Monash University, Clayton, Victoria, Australia

Anna Lintern

Department of Earth and Environmental Sciences, University of Minnesota-Twin Cities, Minneapolis, MN, USA

G.-H. Crystal Ng

GUND Institute, University of Vermont, Burlington, VT, USA

Julia Perdrial

Department of Geography and Geosciences, University of Vermont, Burlington, VT, USA

College of Earth Ocean, and Atmospheric Science, Oregon State University, Corvallis, OR, USA

Pamela L. Sullivan

The National Key Laboratory of Water Disaster Prevention, Yangtze Institute for Conservation and Development, Key Laboratory of Hydrologic-Cycle and Hydrodynamic-System of Ministry of Water Resources, Hohai University, Nanjing, China

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Li, L., Knapp, J.L.A., Lintern, A. et al. River water quality shaped by land–river connectivity in a changing climate. Nat. Clim. Chang. 14 , 225–237 (2024). https://doi.org/10.1038/s41558-023-01923-x

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