Earth’s climate is determined by a delicate balance between how much of the Sun’s radiative energy is absorbed in the atmosphere and at the surface and how much thermal infrared radiation Earth emits to space. A positive energy imbalance means the Earth system is gaining energy, causing the planet to heat up. The doubling of the energy imbalance is the topic of a recent study published June 15 in Geophysical Research Letters.
Scientists at NASA and the National Oceanic and Atmospheric Administration compared data from two independent sets of measurements. NASA’s Clouds and the Earth’s Radiant Energy System (CERES) satellite sensors measure how much energy enters and leaves Earth’s system. A global array of ocean floats, called Argo, provide data to enable an accurate estimate of the rate at which the world’s oceans are warming. Since approximately 90 percent of the excess energy from an energy imbalance ends up in the ocean, the overall trends of incoming and outgoing radiation should broadly agree with changes in ocean heat content.
“The two very independent ways of looking at changes in Earth’s energy imbalance are in really, really good agreement, and they’re both showing this very large trend, which gives us a lot of confidence that what we’re seeing is a real phenomenon and not just an instrumental artifact,” said Norman Loeb, lead author for the study and principal investigator for CERES at NASA’s Langley Research Center. “The trends we found were quite alarming in a sense.”
“It’s likely a mix of anthropogenic forcing and internal variability,” said Loeb. “And over this period they’re both causing warming, which leads to a fairly large change in Earth’s energy imbalance. The magnitude of the increase is unprecedented.”
Increases in emissions of greenhouse gases such as carbon dioxide and methane trap heat in the atmosphere, capturing outgoing radiation that would otherwise escape into space. The warming drives other changes, such as the melting of snow and ice, increased water vapor, and cloud changes that can further enhance the warming. Earth’s energy imbalance is the net effect of all these factors.
In order to determine the factors driving the imbalance, the investigators examined changes in clouds, water vapor, trace gases, the output of light from the Sun, Earth’s surface albedo (the amount of light reflected by the surface), atmospheric aerosols, and changes in surface and atmospheric temperature distributions.
The scientists found that the doubling of the energy imbalance is partially the result an increase in greenhouse gases from human activity, also known as anthropogenic forcing. It can also be attributed to increases in water vapor, which traps more outgoing longwave radiation and further contributes to Earth’s energy imbalance. The related decrease in clouds and sea ice also lead to more absorption of solar energy.
The authors also found that a flip of the Pacific Decadal Oscillation (PDO) from a cool phase to a warm phase likely played a major role in the intensification of the energy imbalance. The PDO is a pattern of Pacific climate variability in which a massive wedge of water in the eastern Pacific goes through cool and warm phases. This naturally occurring internal variability in the ocean can have far-reaching effects on weather and climate. An intensely warm PDO phase that began around 2014 and continued until 2020 caused a widespread reduction in cloud coverage over the ocean and a corresponding increase in the absorption of solar radiation.
“The lengthening and highly complementary records from Argo and CERES have allowed us both to pin down Earth’s energy imbalance with increasing accuracy, and to study its variations and trends with increasing insight, as time goes on,” said Gregory Johnson, co-author on the study and physical oceanographer at NOAA’s Pacific Marine Environmental Laboratory. “Observing the magnitude and variations of this energy imbalance are vital to understanding Earth’s changing climate.”
Loeb cautions that the study is only a snapshot relative to long-term climate change, and that it is not possible to predict with any certainty what the coming decades might look like for Earth’s energy budget. The study does conclude, however, that unless the rate of heat uptake subsides, greater changes in climate should be expected.
Every year, a group of scientists affiliated with the Global Carbon Project give Earth something like an annual checkup. Among the key questions they address: how much carbon is stored in the atmosphere, the ocean, and the land? And how much of that carbon has moved from one reservoir to another through fossil fuel burning, deforestation, reforestation, and uptake by the ocean each year?
All of the latest findings—including the data for 2020, a year like few others—are available here, including links to dozens of interesting charts and a peer-reviewed science paper. Ben Poulter, a NASA scientist and member of the Global Carbon Project team, summarized the findings this way: “The economic effects of COVID-19 caused fossil fuel emissions to decrease by 7 percent in 2020, but we continued to see atmospheric CO2 concentrations increase, by 2.5 ppm, or about 5.3 PgC. This means that the remaining carbon budget to avoid 1.5 or 2 degrees warming continues to shrink, and that we need to continue to monitor the land, ocean, and atmosphere to understand where fossil fuel CO2 ends up.”
Below are 10 key findings from the most recent report. (Note: the Global Carbon Project team synthesizes a broad range of data, some of which requires time-consuming processing, quality-control, and analysis. While they do report some 2020 numbers, the most recent full year of data available for others is 2019.)
Due to COVID-19 economic impacts, global fossil CO2 emissions declined by approximately 2.4 billion metric tons in 2020, a record drop. Fossil CO2 emissions declined by 11 percent in the European Union, by 12 percent in the United States, by 9 percent in India, and 2 percent in China.
The global atmospheric CO2 concentration rose by 2.5 parts per million (ppm) in 2020 to reach 412 ppm averaged over the year. That puts it 48 percent above pre-industrial levels, 16 percent above 1990 levels, and 3 percent above 2015 levels.
The growth rate in atmospheric CO2 concentration in 2020 was near the 2019 growth rate, despite slightly lower anthropogenic emissions.
The land and oceans combined to absorb more than half of the CO2 emitted to the atmosphere (54 percent in 2020). While this can slow global warming, it leads to ocean acidification.
Total CO2 emissions from human activities (fossil CO2 burning and land-use change) were around 40 billion metric tons in 2020. That compares to 43 billion tons in 2019.
The growth of forests on abandoned farmland removed nearly 11 billion metric tons of CO2 in 2020. However, deforestation caused the equivalent of 16 billion tons of CO2 emissions.
Land-use change emissions rose in 2020, predominantly in tropical regions. These emissions came from several areas, particularly Latin America, Sub-Saharan Africa, and Southeast Asia.
Many economic sectors that produce fossil fuel carbon emissions returned to pre-COVID levels by the end of 2020, including the residential and power sectors. One exception was ground transportation, where declines persisted throughout 2020.
Countries have a broad range of per capita emissions reflecting their national circumstances. The United States has the highest per capita emissions.
Five years since the adoption of the Paris Agreement, growth in global fossil CO2 emissions have begun to falter. For the decade prior to 2020 (2010-2019), fossil CO2 emissions were decreasing significantly in 24 countries with growing economies.
If you follow science news, this will probably sound familiar.
In May 2019, when atmospheric carbon dioxide reached its yearly peak, it set a record. The May average concentration of the greenhouse gas was 414.7 parts per million (ppm), as observed at NOAA’s Mauna Loa Atmospheric Baseline Observatory in Hawaii. That was the highest seasonal peak in 61 years, and the seventh consecutive year with a steep increase, according to NOAA and the Scripps Institution of Oceanography.
The Mauna Loa Observatory has been measuring carbon dioxide since 1958. The remote location (high on a volcano) and scarce vegetation make it a good place to monitor carbon dioxide because it does not have much interference from local sources of the gas. (There are occasional volcanic emissions, but scientists can easily monitor and filter them out.) Mauna Loa is part of a globally distributed network of air sampling sites that measure how much carbon dioxide is in the atmosphere.
The broad consensus among climate scientists is that increasing concentrations of carbon dioxide in the atmosphere are causing temperatures to warm, sea levels to rise, oceans to grow more acidic, and rainstorms, droughts, floods and fires to become more severe. Here are six less widely known but interesting things to know about carbon dioxide.
The rate of increase is accelerating.
For decades, carbon dioxide concentrations have been increasing every year. In the 1960s, Mauna Loa saw annual increases around 0.8 ppm per year. By the 1980s and 1990s, the growth rate was up to 1.5 ppm year. Now it is above 2 ppm per year. There is “abundant and conclusive evidence” that the acceleration is caused by increased emissions, according to Pieter Tans, senior scientist with NOAA’s Global Monitoring Division.
Scientists have detailed records of atmospheric carbon dioxide that go back 800,000 years.
To understand carbon dioxide variations prior to 1958, scientists rely on ice cores. Researchers have drilled deep into icepack in Antarctica and Greenland and taken samples of ice that are thousands of years old. That old ice contains trapped air bubbles that make it possible for scientists to reconstruct past carbon dioxide levels. The video below, produced by NOAA, illustrates this data set in beautiful detail. Notice how the variations and seasonal “noise” in the observations at short time scales fade away as you look at longer time scales.
CO2 is not evenly distributed.
Satellite observations show carbon dioxide in the air can be somewhat patchy, with high concentrations in some places and lower concentrations in others. For instance, the map below shows carbon dioxide levels for May 2013 in the mid-troposphere, the part of the atmosphere where most weather occurs. At the time there was more carbon dioxide in the northern hemisphere because crops, grasses, and trees hadn’t greened up yet and absorbed some of the gas. The transport and distribution of CO2 throughout the atmosphere is controlled by the jet stream, large weather systems, and other large-scale atmospheric circulations. This patchiness has raised interesting questions about how carbon dioxide is transported from one part of the atmosphere to another—both horizontally and vertically.
Despite the patchiness, there is still lots of mixing.
In this animation from NASA’s Scientific Visualization Studio, big plumes of carbon dioxide stream from cities in North America, Asia, and Europe. They also rise from areas with active crop fires or wildfires. Yet these plumes quickly get mixed as they rise and encounter high-altitude winds. In the visualization, reds and yellows show regions of higher than average CO2, while blues show regions lower than average. The pulsing of the data is caused by the day/night cycle of plant photosynthesis at the ground. This view highlights carbon dioxide emissions from crop fires in South America and Africa. The carbon dioxide can be transported over long distances, but notice how mountains can block the flow of the gas.
Carbon dioxide peaks during the Northern Hemisphere spring.
You’ll notice that there is a distinct sawtooth pattern in charts that show how carbon dioxide is changing over time. There are peaks and dips in carbon dioxide caused by seasonal changes in vegetation. Plants, trees, and crops absorb carbon dioxide, so seasons with more vegetation have lower levels of the gas. Carbon dioxide concentrations typically peak in April and May because decomposing leaves in forests in the Northern Hemisphere (particularly Canada and Russia) have been adding carbon dioxide to the air all winter, while new leaves have not yet sprouted and absorbed much of the gas. In the chart and maps below, the ebb and flow of the carbon cycle is visible by comparing the monthly changes in carbon dioxide with the globe’s net primary productivity, a measure of how much carbon dioxide vegetation consume during photosynthesis minus the amount they release during respiration. Notice that carbon dioxide dips in Northern Hemisphere summer.
It isn’t just about what is happening in the atmosphere.
Most of Earth’s carbon—about 65,500 billion metric tons—is stored in rocks. The rest resides in the ocean, atmosphere, plants, soil, and fossil fuels. Carbon flows between each reservoir in the carbon cycle, which has slow and fast components. Any change in the cycle that shifts carbon out of one reservoir puts more carbon into other reservoirs. Any changes that put more carbon gases into the atmosphere result in warmer air temperatures. That’s why burning fossil fuels or wildfires are not the only factors determining what happens with atmospheric carbon dioxide. Things like the activity of phytoplankton, the health of the world’s forests, and the ways we change the landscapes through farming or building can play critical roles as well. Read more about the carbon cycle here.
Lately, it feels like we’re hearing about wildfires erupting in the western United States more often. But how have wildfire occurrences changed over the decades?
Researchers with the NASA-funded Rehabilitation Capability Convergence for Ecosystem Recovery (RECOVER) have analyzed more than 40,000 fires from Colorado to California between 1950 to 2017 to learn how wildfire frequency, size, location, and a few other traits have changed.
Here are six trends they have observed in the western United States:
1. There are more fires.
Over the past six decades, there has been a steady increase in the number of fires in the western U.S. In fact, the majority of western fires—61 percent—have occurred since 2000 (shown in the graph below).
Those fires are also burning more acres of land. The average annual amount of acres burned has been steadily increasing since 1950. The number of megafires—fires that burn more than 100,000 acres (156 square miles)—has increased in the past two decades.
Source: NASA RECOVER / Keith Weber
The recent increase in fire frequency and size is likely related to a few reasons, including the rise of global temperatures since the start of the new millennia. Seventeen of the 18 warmest years on record have occurred since 2001.
Global temperatures can affect local fire conditions. Amber Soja, a wildfire expert at NASA’s Langley Research Center, said fire-weather conditions—high temperatures, low relative humidity, high wind speed, and low precipitation—can increase dryness and make vegetation in the west easier to burn. “Those fire conditions all fall under weather and climate,” said Soja. “The weather will change as Earth warms, and we’re seeing that happen.”
3. A small percentage of the West has burned.
Even though fire frequency and size has increased, only a small percentage of western lands— 11 percent—has burned since 1950. In this map, wildfires are shown in orange. Private lands are shown in purple while public lands are clear (no color). The location of wildfires was random; that is, there was no bias toward fires affecting private or public land.
Keith Weber, a professor at Idaho State University who led the analysis, was surprised at the 11 percent figure. There’s no clear reason yet for why more of the region hasn’t burned. “Some of the 89% may not burn because it has low susceptibility—not dry enough or it has low fuel (vegetation),” said Weber. “Some areas may be really ripe for a fire, but they have not had an ignition source yet.”
4. The same areas keep burning.
How has only 11 percent of the west burned, yet the annual number of acres burned and the frequency of fire increased? It turns out that many fires are occurring in areas that have already experienced fires, known as burn-on-burn effects. About 3 percent—almost a third of the burned land—has seen repeated fire activity.
The map here shows the locations of repeated fire activity. While you can’t see it at this map’s resolution, some areas have experienced as many as 11 fires since 1950. In those areas, fires occurred about every seven years, said Weber, which is about the amount of time it takes for an ecosystem to build up enough vegetation to burn again.
5. Recent fires are burning more coniferous forests than other types of landscape.
Since 2000, wildfires have shifted from burning shrub-lands to burning conifers. The Southern Rocky Mountains Ponderosa Pine Woodland landscape has experienced the most acres burned—more than 3 million.
The reason might lie within the tree species. Ponderosa Pine is a fire-adapted species. With its thick and flaky bark, the tree can withstand low-intensity surface fires. It also drops branches lower as they age, which deters fire from climbing up the tree and burning their green needles. “The fire will remove forest undergrowth, but will be just fine for the pines,” said Weber. “We are starting to see Ponderosa Pines thrive in those areas.”
Source: National Park Service
6. Wildfires are going to have a big impact on our future.
Research suggests that global warming is predicted to increase the number of very large fires (more than 50,000 acres) in the western United States by the middle of the century (2041-2070).
The map below shows the projected increase in the number of “very large fire weeks”—periods where conditions will be conducive to very large fires—by mid-century (2041-2070) compared to the recent past (1971-2000). The projections are based on scenarios where carbon dioxide emissions continue to increase.
Wildfires are expected to further stress our nation’s “aging and deteriorating infrastructure.”
Smoke from wildfires is expected to impair outdoor recreational activities.
Wildfires on rangelands are expected to disrupt the U.S.’s agricultural productivity, creating challenges to livestock health, declining crop yields and quality, and affecting sustainable food security and price stability.
Increased wildfire activity is “expected to decrease the ability of U.S. forests to support economic activity, recreation, and subsistence activities.”
More about the source data:
Unless otherwise stated in the article, these data come from NASA’s Rehabilitation Capability Convergence for Ecosystem Recovery. RECOVER is an online mapping tool that pulls together data on 26 different variables useful for fires managers, such as burn severity, land slope, vegetation, soil type, and historical wildfires. In the past, fire managers might need several days or weeks to assemble and present such a large amount of information. RECOVER does so in five minutes, with the help of sophisticated server technologies that gather data from a multitude of sources. Funded by NASA’s Applied Science Program, RECOVER provides these data on specific fires to help fire managers to start rehabilitation plans earlier and implement recovery efforts quickly.
In October 2018, the Intergovernmental Panel on Climate Change (IPCC) released yet another sobering report about the planetary disruption happening because of all the carbon human activity puts into the atmosphere.
Many parts of the world are already seeing rising sea levels, hotter temperatures, more extreme precipitation and droughts, more acidic oceans, and faster rates of extinctions, the scientists said. And without dramatic reductions in carbon emissions to keep warming below 1.5 degrees Celsius, the problems are going to get far worse.
In just one sentence, Gavin Schmidt, director of NASA’s Goddard Institute for Space Studies, captured the essence of the report. “The key thing to remember is that it’s clear that the best time to have reduced emissions was 25 years ago,” he said during an interview with PBS News Hour. “But the second best time to reduce emissions is right now.”
Like any good scientist, Schmidt is always quick to give credit where credit is due. In this case, he noted that what he said on television was a riff on something that renowned Kenyan marathoner Eliud Kipchoge once said. “The best time to plant a tree was 25 years ago. The second-best time to plant a tree is today,” The New York Times quoted the marathon world record holder as saying.
If you are wondering what the IPCC authors based their findings on, there is no shortage of information to explore. Each chapter of the report has a supplementary information section with dozens of references that detail the evidence the scientists used to draw their conclusions.
According to an ongoing temperature analysis conducted by scientists at NASA’s Goddard Institute for Space Studies, the average global temperature on Earth has increased by about 0.8° Celsius (1.4° Fahrenheit) since 1880. Two-thirds of the warming has occurred since 1975, at a rate of roughly 0.15-0.20°C per decade. Read more about this map here.
A posthumous plea from Sellers arrived in July 2018 in the form of an article in PNAS. The topic was one that he cared deeply about: building a better space-based system for observing and understanding the carbon cycle and its climate feedbacks.
As NASA’s Patrick Lynch reported, Sellers wrote the paper along with colleagues at NASA’s Jet Propulsion Laboratory and the University of Oklahoma. Work on the paper began in 2015, and Sellers continued working with his collaborators up until about six weeks before he died. They carried on the research and writing of the paper until its publication in July 2018.
The carbon cycle refers to the constant flow of carbon between rocks, water, the atmosphere, plants, soil, and fossil fuels. Climate change feedbacks—natural effects that may amplify or diminish the human emissions of greenhouse gases—are one of the most poorly understood aspects of climate science.
Here is how Sellers and colleagues characterized the current state of the carbon cycle in the PNAS article:
“It is quite remarkable and telling that human activity has significantly altered carbon cycling at the planetary scale. The atmospheric concentrations of carbon dioxide (CO2) and methane (CH4) have dramatically exceeded their envelope of the last several million years.”
They also explain in detail how we have altered the carbon cycle:
“The perturbation by humans occurs first and foremost through the transfer of carbon from geological reservoirs (fossil fuels) into the active land–atmosphere–ocean system and, secondarily, through the transfer of biotic carbon from forests, soils, and other terrestrial storage pools (e.g., industrial timber) into the atmosphere.”
Scientists understand the broad outlines of how this works relatively well. What worried Sellers was the potential curve balls the climate might throw at us with unanticipated feedbacks. They addressed some of the the challenges in understanding how climate change might affect concentrations of carbon dioxide and methane through feedbacks.
For carbon dioxide:
“While experimental studies consistently show increases in plant growth rates under elevated CO2 (termed carbon dioxide fertilization), the extrapolation of even the largest-scale experiments to ecosystem carbon storage is problematic, and some ecologists have argued that the physiological response could be eliminated entirely by restrictions due to limitation by nutrients or micronutrients. However, there is recent evidence from the atmosphere that suggests increasing CO2 enhances terrestrial carbon storage, leading to the continued increase in land uptake paralleling CO2 concentrations.”
As we detailed in a separate story, the situation is even more complicated for methane. Sellers and his colleagues explained some of the challenges in understanding the feedbacks that affect that potent greenhouse gas this way:
“Atmospheric methane is currently at three times its preindustrial levels, which is clearly driven by anthropogenic emissions, but equally clearly, some of the change is because of carbon-cycle–climate feedbacks. Atmospheric CH4 rose by about 1 percent per year in the 1970s and 1980s, plateaued in the 1990s, and resumed a steady rise after 2006. Why did the plateau occur? These trends in atmospheric methane concentration are not understood. They may be due to changes in climate: increases in temperature, shifts in the precipitation patterns, changes to wetlands, or proliferation in the carbon availability to methane-producing bacteria.”
The consequences of the gaps in understanding could be significant.
“Terrestrial tropical ecosystem feedbacks from the El Nino drove an ∼2-PgC increase in global CO2 emissions in 2015. If emissions excursions such as this become more frequent or persistent in the future, agreed-upon mitigation commitments could become ineffective in meeting climate stabilization targets. Earth system models disagree wildly about the magnitude and frequency of carbon–climate feedback events, and data to this point have been astonishingly ineffective at reducing this uncertainty.”
NASA’s current missions and partnership missions in orbit. Credit: NASA
Sellers and his colleagues do offer a solution. It has much to do with satellites.
“Space-based observations provide the global coverage, spatial and temporal sampling, and suite of carbon cycle observations required to resolve net carbon fluxes into their component fluxes (photosynthesis, respiration, and biomass burning). These space-based data substantially reduce ambiguity about what is happening in the present and enable us to falsify models more effectively than previous datasets could, leading to more informed projections.”
Hydrologist Matt Rodell of NASA’s Goddard Spaceflight Center has been living with first-of-its-kind data from the Gravity Recovery and Climate Experiment (GRACE) for 16 years. That data shows big changes of mass in specific spots on Earth, primarily the result of the movement of water and ice, but it doesn’t tell them what causes those changes. That’s where Matt and the GRACE team come in, painstakingly connecting these observed changes to the loss of ice sheets, depleting aquifers, and climate change. It’s a problem they’re still working on, getting closer every day. Matt explains the years-long process in his own words.
Ominous beginning: Garbage data from a new satellite
Six months after GRACE launched in March 2002, we got our first look at the data fields. They had these big vertical, pole-to-pole stripes that obscured everything. We’re like, holy cow this is garbage. All this work and it’s going to be useless.
But it didn’t take the science team long to realize that they could use some pretty common data filters to remove the noise, and after that they were able to clean up the fields and we could see quite a bit more of the signal. We definitely breathed a sigh of relief. Steadily over the course of the mission, the science team became better and better at processing the data, removing errors, and some of the features came into focus. Then it became clear that we could do useful things with it.
And then trends emerged
It only took a couple of years. By 2004, 2005, the science team working on mass changes in the Arctic and Antarctic could see the ice sheet depletion of Greenland and Antarctica. We’d never been able before to get the total mass change of ice being lost. It was always the elevation changes – there’s this much ice, we guess – but this was like wow, this is the real number.
Not long after that we started to see, maybe, that there were some trends on the land, although it’s a little harder on the land because with terrestrial water storage — the groundwater, soil moisture, snow, and everything. There’s inter-annual variability, so if you go from a drought one year to wet a couple years later, it will look like you’re gaining all this water, but really, it’s just natural variability.
By around 2006, there was a pretty clear trend over Northern India. At the GRACE science team meeting, it turned out another group had noticed that as well. We were friendly with them, so we decided to work on it separately. Our research ended up being published in 2009, a couple years after the trends had started to become apparent. By the time we looked at India, we knew that there were other trends around the world. Slowly not just our team but all sorts of teams, all different scientists around the world, were looking at different apparent trends and diagnosing them and trying to decide if they were real and what was causing them.
A world of big blobs of red and blue
I think the map, the global trends map, is the key. By 2010 we were getting the broad-brush outline, and I wanted to tell a story about what is happening in that map. For me the easiest way was to just look at the data around the continents and talk about the major blobs of red or blue that you see and explain each one of them and not worry about what country it’s in or placing it in a climate region or whatever. We can just draw an outline around these big blobs. Water is being gained or lost. The possible explanations are not that difficult to understand. It’s just trying to figure out which one is right.
Not everywhere you see as red or blue on the map is a real trend. It could be natural variability in part of the cycle where freshwater is increasing or decreasing. But some of the blobs were real trends. If it’s lined up in a place where we know that there’s a lot of agriculture, that they’re using a lot of water for irrigation, there’s a good chance it’s a decreasing trend that’s caused by human-induced groundwater depletion.
And then, there’s the question: are any of the changes related to climate change? There have been predictions of precipitation changes, that they’re going to get more precipitation in the high latitudes and more precipitation as rain as opposed to snow. Sometimes people say that the wet get wetter and the dry get dryer. That’s not always the case, but we’ve been looking for that sort of thing. These are large-scale features that are observed by a relatively new satellite system and we’re lucky enough to be some of the first to try and explain them.
What kept me up at night
The past couple years when I’d been working the most intensely on the map, the best parts of my time in the office were when I was working on it. Because I’m a lab chief, I spend about half my time on managerial and administrative things. But I love being able to do the science, and in particular this, looking at the GRACE data, trying to diagnose what’s happening, has been very enjoyable and fulfilling. We’ve been scrutinizing this map going on eight, nine years now, and I really do have a strong connection to it.
What kept me up at night was finding the right explanations and the evidence to support our hypotheses – or evidence to say that this hypothesis is wrong and we need to consider something else. In some cases, you have a strong feeling you know what’s happening but there’s no published paper or data that supports it. Or maybe there is anecdotal evidence or a map that corroborates what you think but is not enough to quantify it. So being able to come up with defendable explanations is what kept me up at night. I knew the reviewers, rightly, couldn’t let us just go and be completely speculative. We have to back up everything we say.
A tangled mix of answers
The world is a complicated place. I think it helped, in the end, that we categorized these changes as natural variability or as a direct human impact or a climate change related impact. But then there can be a mix of those – any of those three can be combined, and when they’re combined, that’s when it’s more difficult to disentangle them and say this one is dominant or whatever. It’s often not obvious. Because these are moving parts and particularly with the natural variability, you know it’s going to take another 15 years, probably the length of the GRACE Follow-On mission, before we become completely confident about some of these. So it’ll be interesting to return to this in 15 years and see which ones we got right and which ones we got wrong.
NASA Earth Observatory readers may recognize this image of a long trail of clouds — an atmospheric river — reaching across the Pacific Ocean toward California. It appeared first as an Image of the Day about how these moisture superhighways fueled a series of drought-busting rain and snow storms.
More recently, we were pleased to see that image on the cover of the Fourth National Climate Assessment — a major report issued by the U.S. Global Research Program. That image was one of many from Earth Observatory that appeared in the report. Since the authors did not give much background about the images, here is a quick rundown of how they were created and how they fit with some of the key points on our changing climate.
Hurricanes in the Atlantic
Found in Chapter 1: Our Globally Changing Climate
What the image shows:
Three hurricanes — Katia, Irma, and Jose — marching across the Atlantic Ocean on September 6, 2017.
What the report says about tropical cyclones and climate change:
The frequency of the most intense hurricanes is projected to increase in the Atlantic and the eastern North Pacific. Sea level rise will increase the frequency and extent of extreme flooding associated with coastal storms, such as hurricanes.
How the image was made:
The Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite collected the data. Earth Observatory staff combined several scenes, taken at different times, to create this composite. Original source of the image: Three Hurricanes in the Atlantic
The North Pole
Found in Chapter 2: Physical Drivers of Climate Change
What the image shows:
Clouds swirl over sea ice, glaciers, and green vegetation in the Northern Hemisphere, as seen on a spring day from an angle of 70 degrees North, 60 degrees East.
What the report says about climate change and the Arctic:
Over the past 50 years, near-surface air temperatures across Alaska and the Arctic have increased at a rate more than twice as fast as the global average. It is very likely that human activities have contributed to observed Arctic warming, sea ice loss, glacier mass loss, and a decline in snow extent in the Northern Hemisphere.
How it was made:
Ocean scientist Norman Kuring of NASA’s Goddard Space Flight Center pieced together this composite based on 15 satellite passes made by VIIRS/Suomi NPP on May 26, 2012. The spacecraft circles the Earth from pole to pole, so it took multiple passes to gather enough data to show an entire hemisphere without gaps. Original source of the image: The View from the Top
Found in Chapter 3: Detection and Attribution of Climate Change
What the image shows:
Columbia Glacier in Alaska, one of the most rapidly changing glaciers in the world.
What the report says about Alaskan glaciers and climate change:
The collective ice mass of all Arctic glaciers has decreased every year since 1984, with significant losses in Alaska.
How the image was made:
NASA Earth Observatory visualizers made this false-color image based on data collected in 1986 by the Thematic Mapper on Landsat 5. The image combines shortwave-infrared, near-infrared, and green portions of the electromagnetic spectrum. With this combination, snow and ice appears bright cyan, vegetation is green, clouds are white or light orange, and open water is dark blue. Exposed bedrock is brown, while rocky debris on the glacier’s surface is gray. Original source of the image: World of Change: Columbia Glacier
Found in: Intro to Chapter 4: Climate Models, Scenarios, and Projections
What the image shows:
Sea ice hugging the Russian coastline and cloud streets streaming over the Bering Sea.
What the report says about clouds and climate change:
Climate feedbacks are the largest source of uncertainty in quantifying climate sensitivity — that is, how much global temperatures will change in response to the addition of more greenhouse gases to the atmosphere.
How it was made:
The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this natural-color image on January 4, 2012. The LANCE/EOSDIS MODIS Rapid Response Team generated the image, and NASA Earth Observatory staff cropped and labeled it. Original source of the image: Cloud streets over the Bering Sea
Found in Intro to Chapter 5: Large-scale circulation and climate variability
What it shows:
Two extratropical cyclones, the cause of most winter storms, churned near each other off the coast of South Africa in 2009.
What the report says about extratropical storms and climate change:
There is uncertainty about future changes in winter extratropical cyclones. Activity is projected to change in complex ways, with increases in some regions and seasons and decreases in others. There has been a trend toward earlier snowmelt and a decrease in snowstorm frequency on the southern margins of snowy areas. Winter storm tracks have shifted northward since 1950 over the Northern Hemisphere.
How the image was made:
The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite captured this natural-color image. The LANCE/EOSDIS MODIS Rapid Response Team generated the image and NASA Earth Observatory staff cropped and labeled it. Original source of the image: Cyclonic Clouds over the South Atlantic Ocean
Sea of Sand
Found in: Chapter 6: Temperature Changes in the United States
What the image shows: Large, linear sand dunes alternating with interdune salt flats in the Rub’ al Khali in the Sultanate of Oman.
What the report says about drought, dust storms, and climate change:
The human effect on droughts is complicated. There is little evidence for a human influence on precipitation deficits, but a lot of evidence for a human fingerprint on surface soil moisture deficits — starting with increased evapotranspiration caused by higher temperatures. Decreases in surface soil moisture over most of the United States are likely as the climate warms. Assuming no change to current water resources management, chronic hydrological drought is increasingly possible by the end of the 21st century. Changes in drought frequency or intensity will also play an important role in the strength and frequency of dust storms.
How it was made: An astronaut on the International Space Station took the photograph with a Nikon D3S digital camera using a 200 millimeter lens on May 16, 2011. Original source of the image: Ar Rub’ al Khali Sand Sea, Arabian Peninsula
Flooding on the Missouri River
Found in Chapter 7: Precipitation Change in the United States
What the image shows:
Sediment-rich flood water lingering on the Missouri River in July 2011.
What the report says about precipitation, floods, and climate change:
Detectable changes in flood frequency have occurred in parts of the United States, with a mix of increases and decreases in different regions. Extreme precipitation, one of the controlling factors in flood statistics, is observed to have generally increased and is projected to continue to do. However, scientists have not yet established a significant connection between increased river flooding and human-induced climate change.
How the image was made:
The Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite captured the data for this natural-color image. NASA Earth Observatory staff processed, cropped, and labeled the image. Original source of the image: Flooding near Hamburg, Iowa
Smoke and Fire
Found in Chapter 8: Droughts, Floods, and Wildfires
What the image shows: Smoke streaming from the Freeway fire in the Los Angeles metro area on November 16, 2008.
What the report says about wildfires and climate change: The incidence of large forest fires in the western United States and Alaska has increased since the early 1980s and is projected to further increase as the climate warms, with profound changes to certain ecosystems. However, other factors related to climate change — such as water scarcity or insect infestations — may act to stifle future forest fire activity by reducing growth or otherwise killing trees.
How it was made: The MODIS Rapid Response Team made this image based on data collected by NASA’s Aqua satellite. Original source of the image: Fires in California
The Colorado River and Grand Canyon Found in Chapter 10: Changes in Land Cover and Terrestrial Biogeochemistry
What the image shows:
The Grand Canyon in northern Arizona.
What the report says about climate change and the Colorado River: The southwestern United States is projected to experience significant decreases in surface water availability, leading to runoff decreases in California, Nevada, Texas, and the Colorado River headwaters, even in the near term. Several studies focused on the Colorado River basin showed that annual runoff reductions in a warmer western U.S. climate occur through a combination of evapotranspiration increases and precipitation decreases, with the overall reduction in river flow exacerbated by human demands on the water supply.
How the image was made:
On July 14, 2011, the ASTER sensor on NASA’s Terra spacecraft collected the data used in this 3D image. NASA Earth Observatory staff made the image by draping an ASTER image over a digital elevation model produced from ASTER stereo data. Original source of the image: Grand New View of the Grand Canyon
Arctic Sea Ice
Found in Chapter 11: Arctic Changes and their Effects on Alaska and the Rest of the United States
What the image shows: A clear view of the Arctic in June 2010. Clouds swirl over sea ice, snow, and forests in the far north.
What the report says about sea ice and climate change: Since the early 1980s, annual average Arctic sea ice has decreased in extent between 3.5 percent and 4.1 percent per decade, become 4.3 to 7.5 feet (1.3 and 2.3 meters) thinner. The ice melts for at least 15 more days each year. Arctic-wide ice loss is expected to continue through the 21st century, very likely resulting in nearly sea ice-free late summers by the 2040s.
How it was made: Earth Observatory staff used data from several MODIS passes from NASA’s Aqua satellite to make this mosaic. All of the data were collected on June 28, 2010. Original source of the image: Sunny Skies Over the Arctic
Crack in the Larsen C Ice Shelf
Found in Chapter 12: Sea Level Rise
What the image shows:
This photograph shows a rift in the Larsen C Ice Shelf as observed from NASA’s DC-8 research aircraft. An iceberg the size of Delaware broke off from the ice shelf in 2017.
What the report says about ice shelves in Antarctica and climate change?
Floating ice shelves around Antarctica are losing mass at an accelerating rate. Mass loss from floating ice shelves does not directly affect global mean sea level — because that ice is already in the water — but it does lead to the faster flow of land ice into the ocean.
How it was made:
NASA scientist John Sonntag took the photo on November 10, 2016, during an Operation IceBridge flight. Original source of the image: Crack on Larsen C
The Gulf of Mexico
Found in Chapter 13: Ocean Acidification and Other Changes
What the image shows:
Suspended sediment in shallow coastal waters in the Gulf of Mexico near Louisiana.
What the report says about the Gulf of Mexico:
The western Gulf of Mexico and parts of the U.S. Atlantic Coast (south of New York) are currently experiencing significant sea level rise caused by the withdrawal of groundwater and fossil fuels. Continuation of these practices will further amplify sea level rise.
How the image was made:
The MODIS instrument on NASA’s Aqua satellite captured this natural-color image on November 10, 2009. Original source of the image: Sediment in the Gulf of Mexico
What the image shows: A fall scene showing farmland in the Page Valley of Virginia, between Shenandoah National Park and Massanutten Mountain.
What the report says about farming and climate change: Since 1901, the consecutive number of frost-free days and the length of the growing season have increased for the seven contiguous U.S. regions used in this assessment. However, there is important variability at smaller scales, with some locations actually showing decreases of a few days to as much as one to two weeks. However, plant productivity has not increased, and future consequences of the longer growing season are uncertain.
How the image was made: On October 21, 2013, the Operational Land Imager (OLI) on Landsat 8 captured a natural-color image of these neighboring ridges. The Landsat image has been draped over a digital elevation model based on data from the ASTER sensor on the Terra satellite. Original source of the image: Contrasting Ridges in Virginia
What the image shows: A tight arc of clouds stretching from Hawaii to California, which is a visible manifestation of an atmospheric river of moisture flowing into western states.
What the report says about atmospheric rivers and climate change:
The frequency and severity of land-falling atmospheric rivers on the U.S. West Coast will increase as a result of increasing evaporation and the higher atmospheric water vapor content that occurs with increasing temperature. Atmospheric rivers are narrow streams of moisture that account for 30 to 40 percent of the typical snow pack and annual precipitation along the Pacific Coast and are associated with severe flooding events.
How it was made: On February 20, 2017, the VIIRS on Suomi NPP captured this natural-color image of conditions over the northeastern Pacific. NASA Earth Observatory data visualizers stitched together two scenes to make the image. Original source of the image: River in the Sky Keeps Flowing Over the West
A June snowstorm just topped off the already thick layer of white stuff atop the Sierra Nevadas. California’s snow water equivalent rose to a heaping 170 percent of normal. But not so long ago, the state was in the midst of a deep drought; its mountains were bare and brown, and water levels plummeted in reservoirs.
Throughout, satellites were watching. Check out the California drought and its aftermath in a video from NASA Earth Observatory:
Haze over northeastern China on January 14, 2013. Image by NASA Earth Observatory, using data Terra MODIS data from LANCE MODIS Rapid Response.
In the winter of 2013, thick haze enveloped northern China for several weeks. On January 12, 2013, the peak of that bad-air episode, the air quality index (AQI) rose to a staggering 775—off the U.S. Environmental Protection Agency scale—according to a U.S. air quality sensor in Beijing.
Extra pollution from cars, homes, and factories in the winter often sets the stage for outbreaks of air pollution in China. But a March 2017 study in Science Advances suggests that a loss of Arctic sea ice in 2012 and increased Eurasian snowfall the winter before may have helped fuel the extreme event.
Snow and ice cover can affect weather patterns because both affect albedo, a measure of how much solar radiation the surface reflects in comparison to how much incoming solar radiation it receives. In September 2012, sea ice covered less area than at any other time since 1979. Meanwhile, Eurasia had unusually high snow cover in December 2012, the second most on a record that dates back to 1967.
Normally, winds blow air pollution away from eastern China, which is home to Beijing and several other large cities. But in January 2013, winds died down to a whisper and air pollution piled up. By analyzing decades of data collected by ground-based weather stations, 15 years of satellite data on aerosols, and computer simulations of the atmosphere, the researchers concluded that unusual sea ice and snow conditions triggered a shift in China’s winter monsoon, stilling the winds that normally ventilate Beijing.
A press release from Georgia Tech explained the connection in more detail:
“The reductions in sea ice and increases in snowfall have the effect of damping the climatological pressure ridge structure over China,” explained Yuhang Wang. “That flattens the temperature and pressure gradients and moves the East Asian Winter Monsoon to the east, decreasing wind speeds and creating an atmospheric circulation that makes the air in China more stagnant.”
If correct, this might explain why efforts to reduce air pollution in recent years have not stopped extreme haze events from happening. “Emissions in China have been decreasing over the last four years, but the severe winter haze is not getting better,” said Wang. “Mostly, that’s because of a very rapid change in the high polar regions.”
This is not the first study that connects changes in the Arctic to severe haze in China. Research published in August 2015 in Atmospheric Oceanic Science Letters argued that a decline in Arctic sea ice intensifies haze in eastern China. And a study published in Nature Climate Change in April 2017 came to a similar conclusion. The latter study projected a 50 percent increase in the frequency of extreme haze events and an 80 percent increase in their persistence in the near future.
In 2012, Arctic sea ice extent was unusually low in September. New research suggests that may have contributed to a bad haze outbreak in eastern China the next winter. (NASA Earth Observatory graph by Joshua Stevens, based on data from the National Snow and Ice Data Center.)