Low-level cloud loss amplifies global warming, simulations suggest Lisa Lock Scientific Editor Robert Egan Senior Editor Low-level clouds over Earth's oceans play a prominent role in keeping our planet cool by reflecting sunlight away from the surface. But their response to climate change has been hard to model. Now, researchers at Caltech and Google have uncovered important insights into how clouds might respond to warming sea-surface temperatures and rising CO2 levels using a large dataset of simulations developed by the group.

"One of the largest open questions in climate prediction is how low clouds will respond to global warming," says Zhaoyi Shen, lead research scientist at Caltech's Ronald and Maxine Linde Center for Global Environmental Science and a co-author of a paper outlining the team's findings published July 24 in Science Advances. "Our results show potentially large, rapid adjustments of low clouds to high CO2 concentrations, which suggests Earth's climate might be more sensitive to high CO2 levels than some climate models currently project." The team also found that the thinning of low clouds—uniform layers or large, lumpy expanses below 6,000 feet (1,800 meters) that cover massive portions of subtropical seas—amplifies global warming through a feedback loop: Rising sea-surface temperatures lead to fewer clouds, meaning less reflected sunlight and a warmer planet. "This supports the growing body of evidence from the last few years," says Tapio Schneider, the Theodore Y.

Wu Professor of Environmental Science and Engineering at Caltech and co-author of the paper; Schneider is also a principal scientist at Google. "We can now confidently rule out the idea that this effect is zero or that it somehow dampens global warming." Resolving a stubborn cloud problem While virtually all global climate models show that Earth is getting warmer, they differ widely in predictions of the exact long-term temperature rise triggered by sustained increases in atmospheric CO2. A large part of the challenge in reaching a scientific consensus has been the inability to resolve the fine-scale atmospheric turbulence that drives low-level cloud formation and dissipation.

By combining a modeling framework for simulations developed by Shen with the power of Google's computing resources, the research team used simulated large-scale weather data from a global climate model developed by the National Oceanic and Atmospheric Administration to drive thousands of high-resolution large-eddy simulations. The simulations used atmospheric and surface-level conditions from 500 randomly selected locations across the tropical Pacific Ocean, taken during four different months to represent seasonal changes. Each location-season combination was then used to drive large-eddy simulations for four climate change scenarios: a 4°C sea-surface temperature increase from baseline; a quadrupling of atmospheric CO2 alone; a 4°C warming with doubled CO2; and a 4°C warming with quadrupled CO2.

Clouds react to CO2 itself "We found that clouds respond directly to CO2 changes, a fact well understood in physics but perhaps a surprise to many," Schneider says.