The chilling effect of air pollution

(Press-News.org) 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, published Nov. 5 in Nature Communications, 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 — fine particles that float through the atmosphere and influence both cloud cover and cloud composition.

Research showing that some aerosols are harmful drove efforts to limit particulate pollution that specifically targeted products of fossil fuel combustion. 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 Sarah Doherty, 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.

“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.” 

Passed in 1963, 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 fishery stocks and the health of marine ecosystems. 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 Knut von Salzen, 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 — 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 Robert Wood, 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; Luke Fraser-Leach at the University of Toronto; Edward Gryspeerdt at Imperial College London; Ayodeji Akingunola, Jason N. S. Cole, Ruth A. R. Digby and Michael Sigmond at Environment and Climate Change Canada

This study was funded by the University of Washington 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. 

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