Why Aren't Clouds as Bright as We Thought?

A new study is helping scientists clarify how clouds can affect climate, while also dimming the prospects for some proposed geoengineering ideas.

Jeramy Dedrick and Lynn Russell of UC San Diego’s Scripps Institution of Oceanography led a team of researchers who used observations from the tropical South Atlantic Ocean to quantify the ways that aerosols affect cloud brightness.  Aerosols are natural particles such as dust or sea salt or sometimes human-produced pollutants. Such particles provide the structure for clouds that form every day around the world.

One way aerosols affect clouds is the role they play in activating droplets within the updrafts that form clouds. The name for it is the Twomey effect. That effect plays a role in how much clouds can cool Earth’s surface since the brightness of clouds bounces a certain amount of solar radiation back into space.  Dedrick and Russell’s team found through analysis that the maximum potential for that to happen is about 30 percent less than some climate models predicted.

Russell said catching this was a matter of using observations to account for some missing physics.

“Since global models cover the entire earth, they don’t include all of the detailed distributions of updrafts and aerosol particles that are needed, so this work shows how observations can be used to make model estimates more accurate,” said Russell, a climate scientist at Scripps Oceanography.

Climate researchers who improve understanding of cloud dynamics by simulating them in computer models can use new information from this analysis – which includes physics that occurs on scales too small to represent in global models – to constrain what is in the realm of possible scenarios. The task will be further aided by ongoing regional studies, such as EPCAPE, a field project installed on Scripps Pier and other locations around La Jolla, Calif. in 2023. As with EPCAPE, the U.S. Department of Energy Atmospheric Radiation Measurement facility supported the work.

This clarity will also help researchers understand how feasible artificial efforts to control planetary climate can be.  One solution popular in science circles is to enhance the brightness of clouds through human intervention. 

Russell said the data suggest that the strategy might not be as effective as thought because what happens in real clouds is not the same as what happens in models of clouds.

Dedrick pursued this question as part of his PhD in the Climate Sciences Curricular Group at Scripps Oceanography.  

“It was incredibly exciting to tackle this research question because it brought together everything I had worked on during my PhD,” Dedrick said. “This study not only tied together the complexities of aerosol-cloud interactions but also shed light on their broader implications for climate, which has been a central motivation in my research.”

Study authors include Christian Pelayo and Dan Lubin from Scripps Oceanography and Johannes Mülmenstädt from the Pacific Northwest National Laboratory and Mark Miller from Rutgers University.