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 鈥� and influence both cloud cover and cloud composition.

When research emerged showing that some aerosols are harmful, efforts to limit particulate pollution 鈥� specifically targeting the products of fossil fuel combustion 鈥� 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.

鈥淭his paper is a substantial contribution to the evidence that reductions in particulate air pollutants are contributing to accelerated warming.鈥� said , a principal research scientist at the UW 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.

鈥淚t is clearly a good thing that we have been reducing particle pollution in the atmosphere,” Doherty said. 鈥淲e don鈥檛 want to go back in time and take away the Clean Air Act.鈥�

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

鈥淥ur goal is to understand what is driving current climate changes to estimate how much warming we will see in the future,鈥� 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.

鈥淲hen you cut pollution, you鈥檙e losing reflectivity and warming the system by allowing more solar radiation, or sunlight, to reach Earth,鈥� said lead author , a UW senior research scientist of atmospheric and climate science.

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

鈥淲e may be underestimating warming trends because this connection is stronger than we knew,鈥� von Salzen said. 鈥淚 think this increases the pressure on everyone to rethink climate mitigation and adaptation because warming is progressing faster than expected.鈥�

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.

鈥淵ou could think of it as replacing unhealthy pollutant particles with another type of particle that is not a pollutant 鈥� but that still provides a beneficial cooling effect,鈥� said , a UW 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 天美影视传媒 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.听

A new study uses satellite data over the Southern Hemisphere to understand the makeup of global clouds since the Industrial Revolution. This research tackles one of the largest uncertainties in today鈥檚 climate models 鈥� the long-term effect of tiny atmospheric particles on climate change.听

Research led by the 天美影视传媒 and the University of Leeds in the United Kingdom uses remote, pristine parts of the Southern Hemisphere as a window into the early-industrial atmosphere.听

The team compared satellite measurements of cloud droplet concentration in the atmosphere over the Northern Hemisphere 鈥� now heavily polluted with today鈥檚 industrial aerosols 鈥� and over the relatively pristine Southern Ocean. They used this to measure how particles from pollution may have affected Earth鈥檚 temperature since 1850.听

The , published the week of July 27 in the Proceedings of the National Academy of Sciences, suggest that early industrial aerosol concentrations and cloud droplet numbers were much higher than many global climate models estimate. This could mean that human-generated atmospheric aerosols, or particulate pollution, is not damping the warming from carbon dioxide as much as some climate models estimate. The study suggests that the cooling effect of pollution is likely to be more moderate.听聽

鈥淥ne of the biggest surprises for us was how high the concentration of cloud droplets is in Southern Ocean clouds,鈥� said co-lead author , a UW doctoral student in atmospheric sciences.听

The Southern Ocean surrounding Antarctica has few aerosol particles from human activity, but the cloud droplet concentration remains high, especially in summer.听

鈥淭he way that the cloud droplet concentration increases in summertime tells us that ocean biology is playing an important role in setting cloud brightness in unpolluted oceans, now and in the past,鈥� McCoy said.

鈥淲e see high cloud-droplet concentrations in satellite and aircraft observations, but not in climate models,鈥� she added. 鈥淭his suggests that there are gaps in the model representation of aerosol-cloud interactions and aerosol-production mechanisms in pristine environments.鈥�

Climate models represent the global warming effect of greenhouse gases as well as the cooling effects of atmospheric aerosols. The tiny particles that make up these aerosols are produced by human-made sources such as emissions from cars and industry, as well as natural sources, such as phytoplankton and sea spray.听

These particles can influence sunlight and heat flow within the atmosphere as well as interact with clouds. One way the particles affect clouds is by increasing the cloud droplet concentration, causing the clouds to reflect more sunlight back to space.

However, little is known about how aerosol concentrations have changed over the industrial era. This restricts the climate models鈥� ability to accurately estimate the long-term effects of aerosols on global temperatures.听

鈥淟imitations in our ability to measure aerosols in the early-industrial atmosphere have made it hard to reduce uncertainties in how much warming there will be in the 21st century,鈥� said co-lead author , who completed the work at the University of Leeds and is now an assistant professor at the University of Wyoming.

Daniel McCoy, a former UW doctoral student and Isabel鈥檚 brother, authored a previous UW study showing that biological activity in the Southern Ocean influences clouds more than expected.听

鈥淚ce cores provide carbon dioxide concentrations from millennia in the past, but aerosols don鈥檛 hang around in the same way,鈥� he said. 鈥淥ne way that we can try to look back in time is to examine a part of the atmosphere that we haven鈥檛 polluted yet.听

鈥淚sabel proposed this work based on what she saw flying out of Tasmania as part of the UW team participating in the Southern Ocean Clouds, Radiation, Aerosol Transport Experimental Study (SOCRATES). These observations were supported by other recent field data and this grew our confidence in what we had been seeing from space.鈥澛�

Measurements over the Southern Ocean and other remote locations can ultimately help improve global climate models.

鈥淎s we continue to observe pristine environments through satellite, aircraft, and ground platforms, we can improve the representation of the complex mechanisms controlling cloud brightness in climate models and increase the accuracy of our climate projections,鈥� Isabel McCoy said.

Other co-authors are Robert Wood at the UW; Leighton Regayre, Daniel P. Grosvenor, Paul Field, and Ken Carslaw at the University of Leeds; Duncan Watson-Parris at the University of Oxford; Jane P. Mulcahy at the U.K. Met Office; Yongxiang Hu at NASA Langley Research Center; Frida Bender at Stockholm University; and Hamish Gordon at Carnegie-Mellon University.

Research funders include the National Science Foundation, NASA, the U.K. Natural Environment Research Council and the European Union鈥檚 Horizon 2020 program.

For more information, contact Isabel McCoy at imccoy@uw.edu or Daniel McCoy at dmccoy4@uwyo.edu.

This post has been adapted from a University of Leeds .