Burgess COMMENTARY

Peter Burgess

No Silver Lining: Climate Models Overestimating Clouds’ Earth-Cooling Abilities

Clouds may hold less ice than we thought, leading to much more warming than predicted.

At any given time, around two-thirds of the Earth’s surface is blanketed by clouds, many of which act as a sort of air conditioner for the planet as their mix of water and ice crystals reflects radiation back into space.

Now, new research finds that climate models predicting how much warming we’ll see in the wake of rising greenhouse gas emissions have overestimated the amount of ice in those clouds—meaning we could be in for even higher temperatures than we thought.

How much higher? As much as 1.3 degrees Celsius higher (2.3 degrees Fahrenheit), according to the new study, published in the journal Science.

Yale University graduate student Ivy Tan and associate geophysics professor Trude Storelvmo looked at data from NASA’s CALIPSO satellite, which uses LIDAR and infrared imagery to monitor the makeup of clouds, aerosols, and other tiny particles in the atmosphere.

Mixed-phase clouds—which mostly show up in the atmosphere in the form of stratus or cumulus clouds—are comprised of a combination of ice crystals and tiny water droplets. Because water droplets reflect more solar radiation than ice crystals do, the higher the ratio of water droplets to ice crystals, the greater the clouds’ cooling effect.

But Tan discovered that these mixed-phase clouds contain more water and less ice than researchers previously thought.

“It was thought that mixed-phase clouds were mostly made up of ice, and that when global warming increased, that ice would turn to water droplets and the clouds would have more impact on cooling the Earth,” she said. “But in our findings, the clouds are already in a weakened state, with less capacity to negate warming increases.”

In one example, Tan said that if we doubled the amount of carbon dioxide in the Earth’s atmosphere compared with preindustrial levels, previous climate models had estimated we would see temperatures increase anywhere from 2 degrees Celsius to 4.6 degrees. But factoring in these clouds’ weakened ability to cool the Earth, we might be tacking on an extra 1.3 degrees of unexpected warming.

The Intergovernmental Panel on Climate Change has warned that an increase in global temperatures above 2 degrees Celsius could have catastrophic impacts around the world, including rising sea levels, more frequent and extreme weather occurrences, severe droughts, and forest fires. At the climate talks in Paris last year, 196 nations agreed to reduce greenhouse gas emissions in an effort to limit temperatures from exceeding 1.5 degrees Celsius.

If the new findings on clouds hold up, the world could be looking at a razor-thin margin of error—0.2 degrees Celsius—for reaching its target.

“It’s going to be a lot more difficult for governments to reach those goals,” Tan said.

She added that the 1.3-degree figure isn’t exact, as more research in the area of cloud microphysics is needed. “We can’t go into every cloud and continually take measurements, so we have been relying on these global measurements, and they have their limitations.”

Some of those limitations include the CALIPSO satellite’s inability to penetrate and analyze certain types of clouds, and relying on empirical observations of clouds that can lead to different interpretations.

“What we need is more observations and more study into the small-scale processes of these mixed-phase clouds, which we’re finding are really important to determining warming patterns,” Tan said.