A new UW study found an unintended consequence of reducing air pollution is that clouds reflect less sunlight, accelerating climate warming.
Well-intended efforts to reduce air pollution are inadvertently warming the Earth, according to University of Washington scientists. Their recent research illustrates how pollution particles have masked greenhouse gas warming and raises the possibility of adding natural particles to clouds to help cool the climate.
The years 2023 and 2024 set records for global climate warming. Low-lying clouds over the ocean, including those off Washington’s coast, reflect solar radiation back into space, mitigating the warming effect. But as humans have reduced air pollution over time, marine clouds have become less bright.
Scientists at the UW assessed cloud reflectivity from 2003 through 2022 over the North Atlantic and Northeast Pacific oceans. They published a study in November in Nature Communications that showed these clouds became almost 3% less reflective per decade and attributed about 70% of the decrease to reductions in aerosols.
“This is essentially uncovering warming that’s in the system, but which was hidden by air pollution,” said lead author Knut von Salzen, an atmospheric and climate scientist at UW.
His team used satellite data and climate models to assess the driving forces behind cloud reflectivity decreasing and predict future trends. They found aerosols have a larger role than previously reported.
“There’s something missing in the models,” von Salzen said. “Somehow, models need to be improved.”
Scientists have known that clouds need particles — say, from mineral dust or sea salt — to be reflective.
“You need a little bit of that in the air to form cloud droplets,” von Salzen said.
Locally, von Salzen said, low reflective clouds off the Pacific Coast have historically mixed with pollutants such as sulfur dioxide from power plants in China.
“They make it all the way across the Pacific and are affecting clouds in our region, hundreds or thousands of miles away,” he said.
As countries like China curb emissions, scientists wonder if other particles, such as sea spray, could be used to cool the climate. It’s possible this technology could someday be deployed in the North Pacific Ocean.
Next, von Salzen’s team is hoping to improve climate models to better predict global temperature and account for aerosol-cloud interactions.
“We really want to make sure that we understand … where the warming is coming from,” von Salzen said. “And then I guess it’s up to society to make a decision (on) what they want to do about it.”
Would adding natural particles help or hurt?
Von Salzen’s co-author Sarah Doherty is the director of a UW program testing a technique called marine cloud brightening. The geoengineering experiment artificially adds particles to marine clouds to help them become more reflective.
“Instead of doing it with pollution particles, you would do it with particles that are native to the natural environment,” Doherty said.
The technology adds small sea salt particles to clouds, so they can help the cloud form many small droplets, which are more reflective than large droplets, she said.
“Basically, you’re taking sea water and you’re spraying it out of something that looks a lot like a snow making machine at a ski area,” Doherty said. “But instead of producing snowflakes, you’re basically producing tiny, tiny, tiny saline droplets.”
While her team has no concrete plans to use the technology locally, she said someday, possibly, people could see these sea water cannons far offshore in the North Pacific Ocean.
“You wouldn’t want to be doing this right next to the San Juan Islands, because you’d be adding salt to the atmosphere and then salt to land,” Doherty said.
A local take
Bob Mitchell is an engineering geology professor at Western Washington University. He’s also a member of the Skagit Climate Science Consortium, a scientist-led nonprofit.
Mitchell’s initial reaction to work on marine cloud brightening was “that’s pretty cool.”
“It’s a cool gee whiz,” he said. “But there’s many layers of issues you’d have to consider to implement it.”
For example, he wondered about the natural system byproducts of adding sea salt particles to clouds.
“How would that change our own precipitation patterns, depositions and other side effects, so to speak?” Mitchell said.
UW’s Doherty thinks of the technology as an emergency band aid, rather than a “solution” to climate warming. Her team is testing the basic science behind cloud brightening, including what climate impacts artificially adding particles to clouds could have.
As a member of the Whatcom County Climate Impact Advisory Committee, Mitchell helps make recommendations for the county’s Climate Action Plan and is often assessing how the region can become more resilient to climate change.
“As a scientist, I’m always thinking about these geoengineering projects and the caveats,” he said.
As an engineer and environmentalist, he also questions how the technology would be implemented.
“How would you scale this up to actually make a difference, and what would be the cost and infrastructure and energy production required to do that?” Mitchell said. “There’s a lot of politics involved … who’s going to control this, and what would be the regulations?”
Actually deploying marine cloud brightening technology will be a multinational decision process, Doherty said. The goal would be to use it minimally, to reduce unintended changes to the local climate system, and ensure emissions reductions happen as well.
“Using these things is a long way out,” Doherty said. “You clearly wouldn’t want to do it if you’re going to produce harmful impacts.”
In 2024, the UW program’s tests of its sea salt spraying device in Alameda, California, were shut down by the city council, even as scientists said it posed no risk.
“We’re on a pathway of having significantly greater climate impacts ahead,” Doherty said. “When we’re in a much more difficult situation in terms of climate impacts, I think people are just going to say: ‘we need to use these things.’”
While Mitchell is still uncertain about the technology, he maintains a belief in science and engineering to help humans mitigate climate impacts.
“I have faith that we’re going to come up with solutions to live in these circumstances,” he said. “Maybe I’m naive in that regard, but I’ve spent my whole life in it.”
— Anna Marie Yanny
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• “PNW Climate Week kicks off with climate solutions,” July 17, 2025
• “Pollution cleaned up, Anacortes shoreline preps for development,” Jan. 31, 2023