Climate Models Overestimate the Ability of Clouds to Dampen the Effects of Climate Change

April 14, 2016 | Joanne Kennell

Clouds blue sky, field
Photo credit: theaucitron/flickr (CC BY-SA 2.0)

Clouds won’t save us from a warming climate.

Certain types of clouds, because of their “brightness” — a critical factor that determines how effective clouds are at bouncing sunlight back into space — are thought to reduce the effects of climate change. And this brightness depends on what the clouds are made of.

However, according to new research, climate models are aggressively making clouds brighter as the planet warms, which means we be overestimating their reflective powers in moderating the future climate.

In other words, we have been underestimating the potential effects of climate change, such as warming temperatures and rising sea-levels.

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High in the atmosphere where the air temperature is between 0 and -35 degrees Celsius (32 and -31 degrees Fahrenheit), clouds form a mixture of ice crystals and supercooled water droplets — droplets that remain a liquid even though their temperatures are below freezing. These types of droplets are smaller and more numerous than ice crystals, and they are more effective at reflecting sunlight.

Because liquid clouds reflect more sunlight back to space than ice clouds, this “cloud phase feedback” acts as a brake on global warming in climate models.

As our planet continues to warm, these mixed clouds will get wetter (less ice crystals, more liquid droplets), and thus brighter — reflecting more solar energy. And this cloud feedback should, in some part, counteract the warming. But the important question is: By how much?

A new study, conducted by researchers at Lawrence Livermore National Laboratory (LLNL) and Yale University (YU) and published in the journal Science, has shown that current climate models contain too much ice that is susceptible to becoming liquid with warming, making the cloud feedback unrealistically strong.

The researchers used data from NASA’s CALIPSO satellite, which carries a laser-based instrument that can measure the levels of liquid water in clouds. CALIPSO confirmed that these high-altitude clouds are indeed wetter than had been assumed.

So, using a state-of-the-art climate model, the researchers modified the amounts of liquid and ice in the clouds to what is actually observed in nature. This correction led to a weaker cloud phase feedback and greater warming in response to carbon dioxide (CO2).

“We found that the climate sensitivity increased from 4 degrees C [39 degrees F] in the default model to 5-5.3 degrees C [41-41.5 degrees F] in versions that were modified to bring liquid and ice amounts into closer agreement with observations,” said Yale researcher Ivy Tan, lead author of the paper, in a press release.

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The results add to a growing body of evidence that the cloud feedback at mid to high latitudes in climate models is overestimated. Several recent studies have also concluded that other important cloud feedbacks are likely to exacerbate warming, rather than dampen it.

“They are making a very interesting argument,” said Steven Sherwood at the University of New South Wales in Sydney, Australia, to New Scientist. In 2014, Sherwood and his colleagues suggested that the drying out of low-level tropical clouds may punch another hole in our climate shield.

“What they have found is likely to be independent of the low-level issue, which is pretty concerning,” Sherwood continued. “There are a couple of issues, however, that need sorting out before this new result is accepted.”

This warming could alter the geographical distribution of clouds, which also affects the feedback, so the results need to be checked in other models. “We are trying to draw more attention to cloud phase feedback,” Tan said to New Scientist. “We hope that the community as a whole will look at this again. It is a very pressing and urgent matter.”

“The evidence is piling up against an overall stabilizing cloud feedback,” concluded LLNL co-author Mark Zelinka, in the release. “Clouds do not seem to want to do us any favors when it comes to limiting global warming.”

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