Earth’s atmosphere isn’t something we can take for granted.
A Thin Layer of Protection
Most of us probably don’t think much about Earth’s atmosphere, let alone how much humans are affecting it. After all, it’s just there.
Gazing into the sky during the day, it’s tough to get a handle on what’s happening up there. Our atmosphere seems tantalizingly close and yet mysteriously distant. The life-sustaining air we breathe envelops our planet like a pale-blue security blanket, clinging to us by the force of gravity. We see birds, planes, an ever-changing patchwork of clouds and, in some places, air pollution. Farther out, our Moon glows down on us and a blazing Sun hangs in the sky. From our Earth-bound perspective, it’s hard to tell where our atmosphere ends and space begins. (Our atmosphere is like a multi-layered cake.)
Then darkness falls, and through the murky blackness, a portal opens to the heavens, punctuated only by the light of the Moon, stars and cosmos. The descent of night makes sizing up our atmosphere an even more baffling proposition.
It’s only when we view Earth from the unique vantage point of space that the true nature of our atmosphere becomes apparent. From Earth orbit, we gain a new window into our planet. Beneath us, the very edge of the atmosphere — known as Earth’s “limb” — appears as a glowing halo of colors; a luminescent layer cake that gradually fades into the blackness of space. And suddenly our atmosphere, which seemed so vast and mysterious from the ground, appears shockingly thin, even fragile.
So thought retired NASA astronaut Scott Kelly. As he neared the end of a one-year stay aboard the International Space Station in February 2016, he told CNN, “When you look at the … atmosphere on the limb of the Earth, I wouldn’t say it looks unhealthy, but it definitely looks very, very fragile and just kind of like this thin film, so it looks like something that we definitely need to take care of.” Other NASA astronauts have made similar remarks.
Indeed, Earth’s atmosphere isn’t something we can take for granted. Without it, life as we know it wouldn’t exist. Not only does it contain the oxygen we need to live, but it also protects us from harmful ultraviolet solar radiation. It creates the pressure without which liquid water couldn’t exist on our planet’s surface. And it warms our planet and keeps temperatures habitable for our living Earth.
In fact, Earth’s atmosphere is very thin, with a mass only about one-millionth that of the planet itself. Further, about 80 percent of the atmosphere is contained within its lowest layer, the troposphere, which is, on average, just 12 kilometers (7.5 miles) thick.
While there’s no exact boundary line between the atmosphere and space, the accepted standard is about 100 kilometers (62 miles) above Earth’s surface. If you drove that distance on the ground, you might see a change in scenery. But travel that distance straight up, and you’ll quickly find yourself in an environment inhospitable to life. At about 8 kilometers (5 miles) altitude, there’s insufficient oxygen in the air to sustain human life. At around 19 kilometers (12 miles) altitude, your blood boils unless you’re in a pressurized environment.
So is Earth’s atmosphere big or small? Is it fragile or robust? Stable or volatile? And how much are humans affecting it, really?
The answer, it seems, is all of the above, and we’re affecting it a lot. We asked several NASA atmospheric scientists to weigh in on the matter.
A ‘Radical’ Chemical That Helps Keep Our Atmosphere Stable
Before we can determine how fragile or stable Earth’s atmosphere is, we first have to define what those terms mean. So says Kevin Bowman of NASA’s Jet Propulsion Laboratory in Pasadena, California, principal investigator for the Tropospheric Emission Spectrometer (TES) instrument on NASA’s Aura satellite. TES operated from 2004 to early 2018.
“The chemistry of Earth’s atmosphere is remarkably stable, providing a relatively safe place for animals and plants to thrive,” said Bowman. “However, even small changes to the quality of the air that we breath can have profound impacts on our health. Understanding that stability, the ways it could be impacted by humans and how it interacts with the broader Earth system are key research tasks in atmospheric chemistry.”
Bowman said one key to that stability is the hydroxyl radical (OH), a chemical that plays a central role in the ability of Earth’s atmosphere to cleanse itself of pollutants. One of the most reactive gases in our atmosphere, OH is like a global detergent that helps keep things in balance by removing pollutants from the lower atmosphere. It’s the main check on concentrations of carbon monoxide, sulfur dioxide, hydrogen sulfide, methane and higher hydrocarbons.
Scientists have numerous questions about OH. They want to know how stable it is, how quickly it cleanses these chemicals from the atmosphere, and how the atmosphere’s cleansing capacity has changed in the past and may change in the future. They also want to know how climate change may affect OH’s stability. For example, continued increases in methane — a potent greenhouse gas — will consume OH, resulting in deteriorated air quality.
To predict changes in OH’s capacity to cleanse the atmosphere, scientists rely on atmospheric models based on data from satellites, aircraft and ground measurements. “Studies of ancient climates suggest these models are underestimating the sensitivity of OH to climate change,” said Bowman. “As a result, our atmosphere might be more variable than we thought, and OH could end up changing much more rapidly than predicted, with detrimental effects on Earth’s surface air quality, the concentration of greenhouse gases and ozone.”
Bowman said quantifying OH has always been challenging for scientists since it can’t be measured directly. In the past, scientists derived estimates of OH by tracking quantities of another trace gas, methyl chloroform, which was widely used in the 1950s as an industrial solvent and was created by bomb blasts during that era. Methyl chloroform only reacts with OH, which slowly destroys it. But methyl chloroform was eventually replaced by other solvents, and over time, its concentrations in the atmosphere have decreased enough that it is no longer useful for estimating OH.
Observations from instruments like TES give researchers an alternate approach to estimate OH through atmospheric computer models that produce “chemical weather forecasts.” TES measurements of a number of other chemical elements influenced by OH, such as ozone, carbon monoxide and nitrogen dioxide, have enabled scientists to better represent OH in these models. To date, studies based on TES data show there’s more OH in the northern hemisphere than in the southern hemisphere — consistent with methyl chloroform concentrations — and that OH is sensitive to changes in emissions, especially in the tropics.
Bowman discussed some of the many other science advances TES has made possible. Its biggest contributions have been in advancing our understanding of ozone in the troposphere. TES data, together with data from other instruments aboard Aura, have significantly improved our understanding of how ozone affects human health, climate and other parts of the Earth system. A 2015 TES study showed how ozone produced in Asia was transported around the world, increasing ozone emissions on the U.S. West Coast, even as U.S. ozone emissions were declining. TES data also helped quantify how ozone in the upper troposphere serves as a greenhouse gas, warming the atmosphere. This information was used to test climate model predictions of ozone’s greenhouse effect, quantifying how regional changes in pollutants that create ozone have altered climate. TES measurements have also improved our understanding of global air quality by documenting increases in tropospheric ozone levels in many regions of the world, such as Asia.
“We’re beginning to see a redistribution in the emissions of pollutants that form ozone,” Bowman said. “They’re shifting geographically toward the equator, making ozone a more potent greenhouse gas.”
Bowman said TES has also given us a window into Earth’s water cycle by measuring so-called “heavy” water molecules, a naturally occurring variant of water that contains more neutrons than normal water molecules and provide clues to how the water evaporated and fell as precipitation in the past. This, in turn, helps scientists understand what controls the amount of water vapor in the atmosphere. A study using these data showed how the Amazon initiates its own rainy season.
In addition, TES provided new information about ammonia, a precursor to harmful aerosols; and new measurements of carbon-containing gases such as methane and carbonyl sulfide, giving scientists new insights into the carbon cycle.
“TES was a pioneer,” Bowman said. “It collected a whole new set of measurements using new techniques that are now being used by a new generation of instruments.”
What’s in the Air?
By volume, the dry air in Earth’s atmosphere is about 78.09 percent nitrogen, 20.95 percent oxygen, and 0.93 percent argon.
A brew of trace gases accounts for the other 0.03 percent, including the greenhouse gases carbon dioxide, methane, nitrous oxide and ozone. Yet while these greenhouse gases make up just a tiny percentage of our atmosphere, they play major roles in trapping Earth’s radiant heat and keeping it from escaping into space, thereby warming our planet and contributing to Earth’s greenhouse effect.
The largest greenhouse gas by volume is actually the one most people tend to overlook: water vapor, whose concentration varies significantly depending on temperature. As the temperature of the atmosphere increases, the amount of humidity in the atmosphere also goes up, further heating our planet in a vicious cycle.
Tiny solid or liquid particles known as aerosols, which are produced both naturally and by human activities, are also present in variable amounts, along with human-produced industrial pollutants and natural and human-produced sulfur compounds.
Getting a Handle on Carbon Dioxide
Earth’s atmosphere is resilient to many of the changes humans have imposed on it. But, says atmospheric scientist David Crisp of NASA’s Jet Propulsion Laboratory in Pasadena, California, that doesn’t necessarily mean that our society is.
“The resilience of Earth’s atmosphere has been proven throughout our planet’s climate history,” said Crisp, science team lead for NASA’s Orbiting Carbon Observatory-2 (OCO-2) satellite and its successor instrument, OCO-3, which launched to the International Space Station on May 4. “Humans have increased the abundance of carbon dioxide by 45 percent since the beginning of the Industrial Age. That’s making big changes in our environment, but at the same time, it’s not going to lead to a runaway greenhouse effect or something like that. So, our atmosphere will survive, but, as suggested by UCLA professor and Pulitzer-Prize-winning author Jared Diamond, even the most advanced societies can be more fragile than the atmosphere is.
Changes to our atmosphere associated with reactive gases (gases that undergo chemical reactions) like ozone and ozone-forming chemicals like nitrous oxides, are relatively short-lived. Carbon dioxide is a different animal, however. Once it’s added to the atmosphere, it hangs around, for a long time: between 300 to 1,000 years. Thus, as humans change the atmosphere by emitting carbon dioxide, those changes will endure on the timescale of many human lives.
Earth’s atmosphere is associated with many types of cycles, such as the carbon cycle and the water cycle. Crisp says that while our atmosphere is very stable, those cycles aren’t.
“Humanity’s ability to thrive depends on these other planetary cycles and processes working the way they now do,” he said. “Thanks to detailed observations of our planet from space, we’ve seen some changes over the last 30 years that are quite alarming: changes in precipitation patterns, in where and how plants grow, in sea and land ice, in entire ecosystems like tropical rain forests. These changes should attract our attention.
“One could say that because the atmosphere is so thin, the activity of 7.7 billion humans can actually make significant changes to the entire system,” he added. “The composition of Earth’s atmosphere has most certainly been altered. Half of the increase in atmospheric carbon dioxide concentrations in the last 300 years has occurred since 1980, and one quarter of it since 2000. Methane concentrations have increased 2.5 times since the start of the Industrial Age, with almost all of that occurring since 1980. So changes are coming faster, and they’re becoming more significant.”
The concentration of carbon dioxide in Earth’s atmosphere is currently at nearly 412 parts per million (ppm) and rising. This represents a 47 percent increase since the beginning of the Industrial Age, when the concentration was near 280 ppm, and an 11 percent increase since 2000, when it was near 370 ppm. Crisp points out that scientists know the increases in carbon dioxide are caused primarily by human activities because carbon produced by burning fossil fuels has a different ratio of heavy-to-light carbon atoms, so it leaves a distinct “fingerprint” that instruments can measure. A relative decline in the amount of heavy carbon-13 isotopes in the atmosphere points to fossil fuel sources. Burning fossil fuels also depletes oxygen and lowers the ratio of oxygen to nitrogen in the atmosphere.
OCO-2, launched in July 2014, gathers global measurements of atmospheric carbon dioxide with the resolution, precision and coverage needed to understand how this important greenhouse gas — the principal human-produced driver of climate change — moves through the Earth system at regional scales, and how it changes over time. From its vantage point in space, OCO-2 makes roughly 100,000 measurements of atmospheric carbon dioxide every day.
Crisp says OCO-2 has already provided new insights into the processes emitting carbon dioxide to the atmosphere and those that are absorbing it.
“For as long as we can remember, we’ve talked about Earth’s tropical rainforests as the ‘lungs’ of our planet,” he said. “Most scientists considered them to be the principal absorber and storage place of carbon dioxide in the Earth system, with Earth’s northern boreal forests playing a secondary role. But that’s not what’s being borne out by our data. We’re seeing that Earth’s tropical regions are a net source of carbon dioxide to the atmosphere, at least since 2009. This changes our understanding of things.”
Measurements of atmospheric carbon dioxide in the tropics are consistently higher than anything around them, and scientists don’t know why, Crisp said. OCO-2 and the Japan Aerospace Exploration Agency’s Greenhouse gases Observing SATellite (GOSAT) are tracking plant growth in the tropics by observing solar-induced fluorescence (SIF) from chlorophyll in plants. SIF is an indicator of the rate at which plants convert light from the Sun and carbon dioxide from the atmosphere into chemical energy.
“We’re finding that plant respiration is outstripping their ability to absorb carbon dioxide,” he said. “This is happening throughout the tropics, and almost all of the time. When we first launched OCO-2, our first two years of on-orbit operations occurred during a strong El Niño event, which had a strong impact on global carbon dioxide emissions. Now we have more than five years of data, and we see that the tropics are always a source (of carbon dioxide), in every season. In fact, the only time we see significant absorption of carbon dioxide in the tropics is in Africa during June, July and August. So that’s half the story.
“The other half is also quite interesting,” he added. “We’re seeing northern mid- and high-latitude rainforests becoming better and better absorbers for carbon dioxide over time. One possible explanation for this is that the growing season is getting longer. Things that didn’t used to grow well at high latitudes are growing better and things that were growing well there before are growing longer. We’re seeing that in our data set. We see that South America’s high southern latitudes — the so-called cone of South America — are also strong absorbers for carbon. We don’t know if it was always this way and our previous understandings were incomplete or wrong, or if climate change has increased the intensity of the growing season. So we’ve established a new baseline, and it appears to be somewhat of a paradigm shift. Our space-based measurements are beginning to change our understanding of how the carbon cycle works and are providing new tools to allow us to monitor changes in the future in response to climate change.”
Crisp says OCO-2, OCO-3 and other new satellites are giving us new tools to understand how, where and how much carbon dioxide human activities are emitting into the atmosphere and how those emissions are interacting with Earth’s natural cycles. “We’re getting a sharper picture of those processes,” he said.
Impacts from agricultural activities also seem to be changing, he says. During summer in the U.S. upper Midwest, scientists are seeing an intense absorption of carbon dioxide associated with agricultural activities. The same thing is being observed in Eastern and Southern Asia. The strong absorption of carbon dioxide across China is erasing all but a thin strip of fossil fuel emissions along the coast, with Central China now functioning as a net absorber of carbon dioxide during the growing season. Thanks to the development of big, sophisticated computer models combined with wind and other measurements, we’re able to quantify these changes for the first time.
In response to the rapid changes observed in carbon dioxide concentrations and their potential impact on our climate, 33 of the world’s space agencies, including participants from the United States, Europe, Japan and China, are now working together to develop a global greenhouse gas monitoring system that could be implemented as soon as the late 2020s, Crisp added. The system would include a series of spacecraft making coordinated measurements to monitor these changes. Key components of the system would include the OCO-2 and OCO-3 missions, Japan’s GOSAT and GOSAT-2, and Europe’s Copernicus missions. The system would be complemented by ground-based and aerial research.
Crisp said he and his fellow team members are eagerly poring over the first science data from OCO-3. The new instrument, installed on the exterior of the space station, will extend and enhance the OCO-2 data set by collecting the first dawn-to-dusk observations of variations in carbon dioxide from space over tropical and mid-latitude regions, giving scientists a better view of emission and absorption processes. This is made possible by the space station’s unique orbit, which carries OCO-3 over locations on the ground at slightly different times each orbit.
The Copernicus CO2 Mission, scheduled for launch around 2025, will be the first operational carbon dioxide monitoring satellite constellation. Crisp, who’s a member of its Mission Advisory Group, said the constellation will include multiple satellites with wide viewing swaths that will be able to map Earth’s entire surface at weekly intervals. While its basic measurement technique evolved from the GOSAT and OCO-2 missions, there’s a key difference: the earlier satellites are sampling systems focused on improving understanding of Earth’s natural carbon cycle, while Copernicus will be an imaging system focused on monitoring human-produced emissions. In fact, it will have the ability to estimate the emissions of every large power plant in every city around the world.
Crisp says as time goes on the objective is to build an operational system that will monitor all aspects of Earth’s environment. Pioneering satellites like OCO-2, OCO-3, GOSAT and GOSAT-2 are adding greenhouse gas measurements to the data on temperature, water vapor, cloud cover, air quality and other atmospheric properties that have been collected for decades.
“We know our atmosphere is changing and that these changes may affect our civilization,” he said. “We now have the tools to monitor our atmosphere very carefully so that we can give policymakers the best information available. If you’ve invested in a carbon reduction strategy, such as converting from coal to natural gas or transitioning from fossil fuels to renewables, wouldn’t you like to know that it worked? You can only manage what you can measure.”
Tracking the Ongoing Recovery of Earth’s Ozone Hole
Earth’s atmosphere is largely able to cleanse itself of pollutants, but there are a few things that humans have produced that are much more long-lived when emitted into the atmosphere, degrading its quality and creating harmful environmental effects.
One such family of chemical compounds is chlorofluorocarbons (CFCs), whose contribution to depleting ozone in Earth’s upper atmosphere has led to large springtime decreases in ozone around Earth’s polar regions, especially over Antarctica, a phenomenon known as the ozone hole that was first reported in 1985. But, as NASA atmospheric scientist Nathaniel Livesey explains, today, thanks to the phase-out of CFCs, Earth’s ozone hole is in recovery. He says the turnaround provides a great example of what humans can do when they work together to solve a global atmospheric problem.
“Humans produced a lot of CFCs from the 1950s through the early 1990s that were useful for a variety of purposes and widely adopted around the world,” said Livesey, principal investigator for the Microwave Limb Sounder (MLS) instrument on NASA’s Aura satellite at NASA’s Jet Propulsion Laboratory in Pasadena, California. The CFCs were added to the atmosphere at the parts per billion level. “But CFCs were also very effective at depleting stratospheric ozone, which protects us from harmful solar ultraviolet radiation, and their use created a hole in Earth’s stratospheric ozone layer. Luckily, we were able to identify the problem in time and come to a worldwide agreement, the Montreal Protocol, which phased out their use.”
Under the Montreal Protocol, which was finalized in 1987, and its 2016 amendment, a multi-phased plan was implemented involving the use of hydrochlorofluorocarbons (HCFCs), which aren’t as damaging to the environment as CFCs and could be used in the same equipment, since their chemical structure is very similar to CFCs. But HCFCs also contribute to ozone layer destruction, albeit at a smaller rate than CFCs did, as well as to global warming, so their use is also being gradually phased out over the next decade.
While the Montreal Protocol is a great success story, Livesey cautions that tackling Earth’s carbon dioxide and methane emission problems will be more difficult to address.
“People everywhere used CFCs but, in actuality, there were only about four companies in the world that actually produced them,” he said. “With carbon dioxide, the problem is much more complex. All of us produce carbon dioxide. And there are way more coal-burning power plants than there ever were CFC plants. Methane emissions resulting from human activities are also a major contributor. So it’s very hard to point to one thing to fix the problem like we could with CFCs.”
Livesey says Earth’s recent temperature increases simply cannot be explained without accounting for human emissions of carbon dioxide, which builds up over time and has a long life once emitted into the atmosphere. To those who claim that carbon dioxide, methane and other greenhouse gases don’t have a significant impact on global warming, he offers a simple scientific experiment.
“Take a gallon of water and put a drop of food coloring in it. You’re going to immediately notice a change,” he said. “The same is true for adding trace greenhouse gases to the atmosphere. It doesn’t take very much of an increase before their presence literally changes the color of the atmosphere as observed by infrared satellite instruments.”
Livesey says the MLS instrument has contributed to our understanding of atmospheric ozone. For example, it’s been instrumental in verifying the recovery of the ozone layer. MLS has also contributed to studies of how much stratospheric ozone descends into the lower atmosphere, contributing to surface pollution. Surface-level ozone pollution has a detrimental impact on plant growth, resulting in billions of dollars in estimated crop losses.
“NASA is mandated to study the upper atmosphere, and the word ozone appears in that mandate,” Livesay says. “It’s also in the U.S. Clean Air Act. NASA has spearheaded numerous ozone research campaigns and has contributed to many of the big-name atmospheric ozone models. We’ve done a lot of work on satellite measurements of air quality. And the A Train constellation of atmospheric research satellites, of which Aura/MLS is one component, has been a huge benefit to the atmospheric science community.” (Learn more about NASA’s role in studying Earth’s atmosphere.)
In addition to ozone, MLS tracks water vapor, numerous trace gasses and mid-atmospheric temperatures.
Regarding water vapor, Livesey says scientists still don’t fully understand the processes that control humidity in the stratosphere. For example, in 2000, measurements showed the amount of stratospheric water vapor decreased by about 10 percent, which slowed the rate of global surface temperature increases by about 25 percent. But scientists are still not completely sure why it decreased. “Since stratospheric water vapor is a greenhouse gas, we want to be able to predict its future evolution well,” he said. “We don’t yet fully understand the interplay of the various processes involved and how they will evolve in a warming climate. MLS data are contributing to atmospheric models that are assisting in this area of research.”
One of the biggest surprises from MLS data has been its observations of a phenomenon that allows pollution from strong forest fires to penetrate into the stratosphere. Fires are a significant contributor to stratospheric aerosols and thus have the potential to affect surface warming. “MLS has allowed us to track this pollution around the globe. We wouldn’t have guessed before that a single forest fire could do that,” Livesey said.”
As the MLS data record approaches 15 years, Livesey says he’s hopeful MLS will continue to provide important science for several more years. The instrument continues to work well and the biggest limitation on its life is the amount of fuel on the Aura spacecraft, which should run out in about 2025, although the team is considering adopting a less fuel-intensive orbit maintenance strategy that could add several more years of operations.
Fresh Insights on Air Quality, Ozone and Climate
While satellite data have revolutionized how we view Earth and its atmosphere, people don’t need to travel to space to understand that our Blue Planet really isn’t that big and our atmosphere not very thick. In fact, says atmospheric scientist Bryan Duncan of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, all you have to do is, in the words of Oscar Hammerstein, climb every mountain.
“From the perspective of air pollution and human health, there’s only a small amount of Earth’s atmosphere that we can actually breathe or live in,” Duncan said. “Take Mount Everest, for example. It’s only habitable for humans for the first few, lowest kilometers.”
As project scientist for NASA’s Aura satellite, Duncan has seen how much humans are affecting Earth’s atmosphere and the quality of the air we breathe. Aura, launched in 2004 as a key component of NASA’s Earth Observing System, studies Earth’s ozone layer, air quality and climate. The mission has been instrumental in expanding our understanding of the composition, chemistry and dynamics of Earth’s atmosphere.
“Humans have been modifying our environment for thousands of years,” Duncan said. “Earth monitoring satellites have clearly shown that humans are changing our atmosphere, even over the 15 years Aura has been in orbit. We clearly see major changes in air pollution around the world. For example, we’ve observed that air pollution in the United States and Europe has been reduced, demonstrating the effectiveness of policy initiatives such as the Clean Air Act and other environmental regulations, but in other places, such as India, it’s getting worse. We’ve also seen improvements in regions like China.” (Trends for air pollutants around the world, many of which are based upon Aura data, may be seen at http://airquality.gsfc.nasa.gov.)
Duncan says Aura has benefitted science and society in two primary ways. First, it has allowed us to better understand and observe the atmospheric chemistry and dynamics that determine Earth’s protective ozone layer. Aura data have allowed us to not only monitor ozone, but also to observe the chemicals involved in ozone’s formation and destruction. This information is helping scientists understand why the ozone layer is varying over time, including how human-produced ozone-destroying chemicals thinned the ozone layer and caused the “ozone hole” over Antarctica.
Second, it has provided some of the first long-term observations of air pollutants around the world. These include chemicals such as sulfur dioxide, formaldehyde and nitrogen dioxide, which is primarily produced by burning fossil fuels in vehicles and power plants and contributes to surface-level ozone. “Aura has really given us a window into some of the most important pollutants and how they’re changing over time,” he said.
Aura launched with four instruments, two of which are still in operation: the Ozone Monitoring Instrument (OMI), the first satellite instrument to give us a long-term record of air pollution at high spatial resolution; and the Microwave Limb Sounder, which measures naturally occurring microwaves at the edge of the atmosphere to track stratospheric atmospheric gases, temperature, pressure and cloud ice. Aura’s other two instruments were the Tropospheric Emission Spectrometer and the High-Resolution Dynamics Limb Sounder. The Aura spacecraft is healthy and is expected to continue operating until it runs out of fuel in 2025.
In addition to Aura, the other two flagship missions in NASA’s Earth Observing System — Terra and Aqua — have also made major contributions to understanding Earth’s atmosphere. “While Terra and Aqua were designed to monitor Earth’s land surface and hydrosphere (all water on, under and above Earth’s surface), respectively, they’ve also contributed to atmospheric science with instruments like the Atmospheric Infrared Sounder (AIRS) on Aqua, the Multi-angle Imaging SpectroRadiometer (MISR) on Terra, and the Moderate Resolution Imaging Spectroradiometers (MODIS) on Aqua and Terra, which monitor particulate matter (the solid and liquid particles suspended in the air) around the world,” Duncan said.
“Our job as scientists is to make these satellite data sets available, help interpret them, and ensure they’re clear enough so that they can be used to create effective and efficient policy,” he concluded. “That way, they can be used to create positive change in society.”
Keeping a Weather Eye on Earth’s Climate Instabilities
“I wouldn’t describe Earth’s atmosphere as fragile so much as I’d say our climate system is unstable,” said atmospheric scientist Eric Fetzer of NASA’s Jet Propulsion Laboratory in Pasadena, California. “Climate is being changed by the addition of greenhouse gases to the atmosphere.”
Fetzer said humanity has pushed climate instability well away from where it has been for many millennia. “We’ve had 8,000 years of pretty much the same climate, and only about a century where things have really started to change,” he says.
Fetzer is project scientist for the Atmospheric Infrared Sounder (AIRS) instrument on NASA’s Aqua satellite. Launched in 2002, AIRS is one of six instruments aboard Aqua. At the time of its launch, AIRS was the most advanced atmospheric sounding system ever deployed in space, and it continues to make major contributions to our understanding of climate. During its 17 years in orbit, AIRS data have also improved operational weather forecasts around the world, and the instrument measures atmospheric temperature, water vapor and a number of trace gases, including carbon dioxide, ammonia, methane and carbon monoxide.
Fetzer highlighted a few of AIRS’ many scientific achievements.
“We found that Earth’s climate system has responded to increasing carbon dioxide concentrations as climate models predicted,” he said. “Atmospheric water vapor is sensitive to the presence of carbon dioxide. The more carbon dioxide, the more the atmosphere warms due to the greenhouse effect. A warmer atmosphere holds more water vapor, which is itself a greenhouse gas. This is how water vapor triples the warming from increasing carbon dioxide and other greenhouse gases.”
AIRS data have detected significant changes in the climate of the Arctic. “We see increases in Arctic water vapor levels,” he said. “The Arctic atmosphere is becoming more moist, adding to its warming, and the ocean is becoming more ice-free. These changes are happening more rapidly than scientists expected. I didn’t anticipate seeing them in a 17-year data record.”
Fetzer said the AIRS team has also observed significant increases in the concentration of atmospheric ammonia in areas like northern India and eastern China due to agricultural activities. This has negatively impacted air quality in these regions.
Fetzer added that while greenhouse gases are arguably the biggest driver of global climate change, other chemicals such as carbon monoxide and ammonia are also changing significantly, and AIRS is tracking those changes.
While a replacement for AIRS was not specifically called out in the latest National Academy of Sciences’ Earth science decadal survey, which provides a roadmap for future Earth science satellite missions, Fetzer’s team is still planning for such a possibility.
“Most NASA instruments are typically one-of-a-kind, and there aren’t usually plans for replacements,” he said. “In the years since AIRS was developed, atmospheric sounding techniques have changed significantly. But while the technologies may be different, the nature of the measurement remains the same.”
In fact, says Fetzer, many of the instruments developed in recent years to measure the atmosphere derive heritage from AIRS. For example, techniques developed for use on AIRS have been applied to such missions as the Cross-track Infrared Sounder (CrIS) instrument on the NASA/NOAA Suomi National Polar-orbiting Partnership (NPP) satellite, NOAA’s Joint Polar Satellite System (JPSS)-1 satellite, and the Infrared Atmospheric Sounding Interferometer (IASI) instruments on the three polar-orbiting MetOp meteorological satellites developed by the European Space Agency and operated by the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT).
“Even if an AIRS replacement is never built, AIRS has firmly cemented its place in atmospheric science history,” he said.