Earth is reflecting less sunlight, and absorbing more heat, than it did several decades ago. Global warming is advancing faster than climate models predicted, with observed temperatures exceeding projections in 2023 and 2024. These trends have scientists scrambling to understand why the atmosphere is letting more light in.

A new study, , shows that reducing air pollution has inadvertently diminished the brightness of marine clouds, which are key regulators of global temperature.

Between 2003 and 2022, clouds over the Northeastern Pacific and Atlantic oceans, both sites of rapid surface warming, became nearly 3% less reflective per decade. Researchers attribute approximately 70% of this change to aerosols — and influence both cloud cover and cloud composition.

When research emerged showing that some aerosols are harmful, efforts to limit particulate pollution — specifically targeting the products of fossil fuel combustion — followed. Aerosol levels will likely continue to fall as clean energy replaces oil and gas. To improve the accuracy of global temperature forecasts, scientists need to capture the true relationship between aerosols, clouds, and heat from the sun in climate models.

“This paper is a substantial contribution to the evidence that reductions in particulate air pollutants are contributing to accelerated warming.” said , a principal research scientist at the 91̽��Cooperative Institute for Climate, Ocean and Ecosystem Studies.

Researchers knew that low clouds over the ocean would dissipate as temperatures rose, exposing more surface area to warming sunlight and amplifying its effect. They also knew that particles in the atmosphere insulate Earth both by deflecting light and making the entire cloud more reflective.

The cooling effect from particulate pollution masked warming from greenhouse gases for decades. Accelerated warming was a potential consequence of improving air quality.

“It is clearly a good thing that we have been reducing particle pollution in the atmosphere,” Doherty said. “We don’t want to go back in time and take away the Clean Air Act.”

, the Clean Air Act marked the first of many worldwide efforts to control pollution.

“Our goal is to understand what is driving current climate changes to estimate how much warming we will see in the future,” Doherty added.

The Northeastern Pacific and Atlantic Oceans are warming faster than almost anywhere else on Earth, threatening and the . The researchers analyzed 20 years of satellite data documenting cloud dynamics above these bodies of water to identify the drivers behind the observed reduction in reflectivity.

They found that aerosols influence clouds in two ways. Small particles give water droplets something to cling to, and with a fixed amount of water, more aerosols means more small, shiny droplets in the clouds. By the same logic, reducing aerosols increases cloud droplet size. Large droplets are heavier, and quicker to fall to Earth as precipitation, which decreases the longevity of clouds, or cloud cover.

“When you cut pollution, you’re losing reflectivity and warming the system by allowing more solar radiation, or sunlight, to reach Earth,” said lead author , a 91̽��senior research scientist of atmospheric and climate science.

Updating aerosol formation and cloud droplet size in climate models improved simulations of cloud reflectivity — a critical variable for projecting future temperatures.

“We may be underestimating warming trends because this connection is stronger than we knew,” von Salzen said. “I think this increases the pressure on everyone to rethink climate mitigation and adaptation because warming is progressing faster than expected.”

While these changes to global cloud reflectivity have prompted rapid warming on Earth, scientists are researching the feasibility of interventions that could make the clouds shinier without polluting the air. One such intervention is known as marine cloud brightening, in which ships spray seawater into the air to make low-lying oceanic clouds more reflective and help minimize warming from the sun.

“You could think of it as replacing unhealthy pollutant particles with another type of particle that is not a pollutant — but that still provides a beneficial cooling effect,” said , a 91̽��professor of atmospheric and climate science.

However, before they are implemented, more research is needed to confirm that these methods are safe and without unintended consequences. In the meantime, this study will help scientists better forecast the impacts of climate change at a global scale.

Additional co-authors include; at the University of Toronto; at Imperial College London; , , and at Environment and Climate Change Canada.

This study was funded by the 91̽�� Marine Cloud Brightening Research Program, Environment and Climate Change Canada, the National Oceanic and Atmospheric Administration, an Imperial College Junior Research Fellowship and a Royal Society University Research Fellowship.

For more information, contact von Salzen at kvsalzen@uw.edu, Doherty at sdoherty@uw.edu or Wood at robwood2@uw.edu.��

Tiny aerosol particles, emitted by everything from tailpipes to trees, float above us reflecting sunlight, seeding clouds and absorbing solar heat. How exactly this happens – and how it might change in the future – is one of the biggest uncertainties in how humans are influencing climate.

91̽�� scientists are part of a NASA field campaign, , or ORACLES, that is flying research planes around clouds off the coast of Namibia to see how smoke and clouds interact.

“The Namibians are being wonderful hosts and are really helping to make this a success,” said deputy principal investigator , a 91̽��professor of atmospheric sciences.

Wood plans to be in the field until late September. , a research scientist with the 91̽��Joint Institute for the Study of the Atmosphere and Ocean, is also among the roughly 100 participating scientists.

Fires burning on African savannas generate smoke that contains aerosol particles. This smoke rises high in the atmosphere and blows west off the coast, then drops down toward the cloud layer. The interaction between air moisture and smoke pollution is complex and not well understood.

“We still have a lot of pretty fundamental questions unanswered, such as whether the smoke and cloud layers are clearly separated, or, alternatively, if smoke particles end up mixing into the cloud deck and changing the clouds’ properties,” said , a graduate student in Wood’s group who is participating in the campaign.

From August 29 through late September, NASA’s ER-2 and P-3 research aircraft will take off roughly every other day from the project’s base in Walvis Bay, Namibia. The planes will fly at altitudes between sea level and about 3 miles elevation with instruments that look up and down to see how smoke and clouds interact. The team plans to return for follow-up measurements in 2017 and 2018.

Observations could help understand, for example, how forest fires burning inland affect the coastal cloud layer in other parts of the world, and how changes in air quality and global warming will act together on regional weather patterns.

The principal investigator is Jens Redemann of NASA’s Ames Research Center. Other partners on the $30 million, five-year NASA project include the Namibia University of Science and Technology and Namibia’s Gobabeb Research & Training Centre.

“This is a fantastic opportunity to interact with and learn from scientists not only from across the United States, but also from Namibia and South Africa,” Diamond said.

###

For more information, contact Wood at robwood@atmos.washington.edu and Diamond at diamond2@uw.edu.

A 91̽�� scientist is co-author on a new paper that tracks down why the ice sheet is darkening. The , led by Columbia University, was published March 3 in The Cryosphere.

“According to the satellites, Greenland is darkening by about 2 percent per decade since 1996,” said second author , a research scientist at the UW’s Joint Institute for the Study of the Atmosphere and Ocean. “That seems really small, but it’s actually climatically significant.”

Wildfires have been recently proposed as the of Greenland’s ice sheet, but historical records of fires during that period could not explain the changing reflectivity since the mid-90s, Doherty said.

The authors find instead that most of the darkening is a side effect of warming.

“The lion’s share of the darkening is driven by feedback to the snowpack optical properties,” Doherty said. The study shows that as the glacier melts, the snow crystals get larger and impurities surface – both processes familiar to anybody who has seen old, spring snow.

With warming each year’s snow melts completely to expose the darker glacier ice underneath, and there are more melt pools, which also darkens the surface.

“You don’t necessarily have to have a ‘dirtier’ snowpack to make it dark,” said lead author , a research professor at Columbia University.

Looking forward, the study concludes that continued warming will cause Greenland to absorb about 8 percent more sunlight by the end of this century, with bigger changes along its western edge.

The research draws on the by Doherty and , a 91̽��professor of atmospheric sciences. The team collected some 1,200 snow samples from Northern landscapes, including Greenland. Their follow-up looked at how impurities tend to accumulate at the surface as the snow melts.

By combining the on-the-ground observations with satellite images from 1981 to 2012 and computer models of glaciers, the new study shows the past two decades of darkening can be explained by larger crystals that reflect less sunlight and exposed impurities at the ice surface.

The research also draws on Warren’s earlier studies of and how affects polar landscapes.

For Greenland today it appears that warming, not deposited air pollution, is the primary culprit.

“Using satellites, models and other information to put some bounds on what is happening to the glacier’s surface and what’s causing it, our conclusion is that it’s not an increase in wildfires that is causing the darkening,” Doherty said.

That distinction matters, she said, because it means that regulating particulate emissions from diesels or reducing wildfire smoke would not save Greenland’s ice. Instead, the darkening is being driven by increased melting, which is caused by climate warming.

“There’s a potential for the Greenland ice sheet to contribute significantly to sea-level rise in the next 50 to 100 years,” Doherty said. When calculating future sea-level rise, she added, scientists need to be able to model all the processes that are significant.

“We now have a better understanding of what’s causing the darkening, and we know that it’s significant enough that we need to include it in our models.”

Other co-authors are Xavier Fettweis at Belgium’s University of Liege, Patrick Alexander at NASA’s Goddard Institute of Space Studies, Jeyavinoth Jeyaratnam at the City College of New York, and Julienne Stroeve at the University of Colorado. The research was funded by the National Science Foundation and NASA.

###

For more information, contact Doherty at 206-543-6674 or sdoherty@uw.edu.

See also a Columbia University .

The results, published in the , show that North American snow away from cities is similar to Arctic snow in many places, with more pollution in the U.S. Great Plains. They also show that agricultural practices, not just smokestacks and tailpipes, may have a big impact on snow purity.

During their almost 10,000-mile trek across North American snowfields, the researchers were particularly interested in the Bakken oil fields of northwest North Dakota.

“With all this oil exploration, diesel trucks and new oil wells, people wondered: Is there a huge amount of air pollution making the snowpack darker?” said lead author , a research scientist at the UW’s .

What they found was that these activities do appear to be adding extra soot to the snow, but perhaps just as important is the dirt. Disturbance from clearing oil pads, new housing sites and all the extra truck traffic on unpaved roads means dirtier snow. But even away from the oil fields, soil is disturbed by agriculture.

“Our work suggests that land use and farming practices might matter as much as diesel emissions in many parts of the Great Plains,” Doherty said.

Doherty was part of a team of 91̽��atmospheric scientists who spent the winter of 2013 driving across northwestern U.S. states and some Canadian provinces to get a firsthand look at the continent’s snow.

The project involved collecting hundreds of snow samples from 67 sites away from any cities or major roads. The trip took the researchers from Seattle to North Dakota to Churchill, Manitoba. Every few days they melted and filtered the snow in their motel rooms, then back at their 91̽��lab they shone light through a filter to see how much light was blocked, and did chemical analyses to determine what particles were responsible.

Their main focus was , a very light-absorbing particle emitted by burning diesel, coal or wood. Many countries have regulated black carbon because of its effects on air quality and human health, but more recently climate scientists also have become interested because the tiny particles darken the snow and hasten melting.

The cleanest samples they collected were from northern Canada, with overall levels of black carbon, or soot, similar to that of Arctic snowpack. The Pacific Northwest and Rocky Mountain states had levels slightly higher. The Great Plains readings were more variable and sometimes two to three or more times higher than in other parts of the country, typically 15 to 70 nanograms of soot per gram of snow.

Doherty previously worked with co-author , a 91̽��emeritus professor of atmospheric sciences, on a 2006-2010 he led of snow in the Arctic. Warren and Doherty also worked with Chinese collaborators in 2010 s of snow in northern China, all using the same techniques so the combined results can provide a first-ever global map of snow cleanliness.

Results from China showed rates of pollution tens to hundreds of times greater than in North America, with the highest rate in northeast China of 1,220 nanograms of soot per gram of snow, likely because of industrial activity and other emissions in the Beijing area. But dirt and desert dust also were prevalent in central North China snow.

“For a lot of the central U.S. and north China Great Plains the snow is not very deep. In the U.S., almost the whole area is agricultural fields and in China there is a lot of animal grazing,” Doherty said. “When the wind blows the dirt gets lofted, maybe just 10 feet off the ground, and gets mixed in with the snow.” North Dakota locals refer to the mixture as “snirt.”

The new paper documents how much light is blocked, and at which wavelengths, by filtered snow samples. Other co-authors and snow collectors were research professor and graduate students and , all in 91̽��atmospheric sciences.

A companion by Dang and Hegg involved a chemical analysis of the North American samples to pinpoint exactly which compounds are contained in the snow.

“A lot of the focus in climate models has been on black carbon, because it’s a pollutant and it’s very dark,” Doherty said. “But the snow is darkened by other things as well, like organics, and also by dust and soil that can get in the snowpack.”

In fact, they found that in the Great Plains states up to half of light absorption is due to organic matter, or “brown carbon” from burning fossil fuels and from soil that mixes in with falling snow.

The deposits affect both global and local climates. Pollution on the Himalayan glaciers, for instance, is raising concerns that it will speed melt rates and harm water supplies. For U.S. farmers, changes in the snow’s reflectivity could affect when the spring melt will occur and when meltwater will drain out.

Whether the pollution the researchers found in North Dakota is enough to change snow melt timing will have to be answered by region-specific climate models, Doherty said.

“But first the models have to do a more accurate job of representing the amount of dirt that’s in the snowpack,” she added.

The work was funded by the Environmental Protection Agency and the China Scholarship Fund.

###

For more information, contact Doherty at 206-543-6674 or sdoherty@uw.edu and Dang at chengd1@uw.edu. High-res version of the images are available .

The new study concludes that , the soot particles in smoke and smog, contributes about twice as much to global warming as previously estimated, even by the 2007 Intergovernmental Panel on Climate Change.

“We were surprised at its potential contribution to climate,” said , a 91̽�� atmospheric scientist and one of four coordinating lead authors.

The silver lining may be that controlling these emissions can deliver more immediate climate benefits than trying to control carbon dioxide, she said.

The paper was made freely available online today (Jan. 15) in the .

Some previous research had hinted that models were underestimating black-carbon emissions, Doherty said, from such things as open burning of forests, crops and grasslands, and from energy-related emissions in Southeast Asia and East Asia.

Black carbon’s role in climate is complex. Dark particles in the air work to shade the Earth’s surface while warming the atmosphere. Black carbon that settles on the surface of snow and ice darkens the surface to absorb more sunlight and increase melting. Finally, soot particles influence cloud formation in ways that can have either a cooling or warming impact.

The report surveyed past studies and included new research to quantify the sources of black carbon and better understand its overall effect on the climate.

Doherty was executive director of the in 2009 when policy groups were seeking better information on the benefits of reducing black-carbon emissions. The research team undertook a comprehensive assessment, funded by IGAC and the U.S. National Oceanic and Atmospheric Administration.

“Because of a lack of action to reduce carbon dioxide emissions, the policy community is asking what else we can do, particularly to help places like the Arctic that are melting much more quickly than we had anticipated,” Doherty said. “We hope reducing black-carbon emissions buys us some time. But it doesn’t replace cutting back on CO2 emissions.”

While carbon dioxide has a half-life of 100 years, black carbon stays in the atmosphere for only a few days.

The authors investigated various sources of black carbon to see which reductions might have the most short-term cooling impact. Regulating emissions from diesel engines followed by replacing some wood- and coal-burning household stoves, authors find, would have the greatest immediate cooling impact.

“If you’re just thinking about impact on climate, you would want to be strategic about which sources you cut back on,” Doherty said. “We looked at the overall impact because some of these sources also emit associated particles that can have counteracting effects.”

Black carbon contributes to climate change in the mid to high latitudes, including the northern United States, Canada, northern Europe and northern Asia, as well as affecting rainfall patterns of the .

The report incorporates data that Doherty, a member of the UW’s , and co-author , a 91̽��professor of atmospheric sciences, gathered between 2007 and 2009 to measure soot on Arctic snow. Calculating black carbon deposits in the Arctic is difficult, so data are essential for testing and correcting models.

First author , now at the University of Illinois, earned a doctoral degree at the 91̽��in 2000 that combined engineering, chemistry and atmospheric science to measure emissions from burning that have atmospheric importance.

“Mitigating black carbon is good for curbing short-term climate change, but to really solve the long-term climate problem, carbon dioxide emissions must also be reduced,” Bond said in a .

In related research, Doherty, Warren and 91̽��graduate student Cheng Dang will travel next month to Colorado, Wyoming, the Dakotas, Saskatchewan, Manitoba and elsewhere to collect snow samples and investigate black carbon’s effects on North America’s Great Plains.

###

For more information, contact Doherty at 206-543-6674 or sarahd@atmos.washington.edu.