Climate Models May Overstate Clouds’ Cooling Power, Research Says


Courtesy New York Times

The computer models that predict climate change may be overestimating the cooling power of clouds, new research suggests. If the findings are borne out by further research, it suggests that making progress against global warming will be even harder.

The new paper, in the journal Science, focuses on what are known as mixed-phase clouds, which are found around the world and contain both cooled water and ice crystals.

The balance of water and ice in clouds affects the impact that carbon dioxide levels have on atmospheric temperatures, a factor known as equilibrium climate sensitivity. A higher sensitivity would mean that carbon dioxide levels would cause more warming than previously thought.

Using data from instruments aboard the Calipso satellite, which monitors clouds and particles suspended in the atmosphere, the researchers determined that mixed-phase clouds contain more water and less ice than expected.

Water droplets reflect more solar radiation back into the sky than ice crystals do. As the atmosphere warms, clouds tend to have more water and less ice in them, and the more watery clouds prevent solar radiation from reaching the earth. Warming is slowed.

Other recent studies have suggested that climate models may not be accurately assessing the balance of water and ice in clouds in some circumstances.

The new paper suggests the effects of a flaw in the model could be serious: Based on its analysis of one model of climate change, the cloud error could mean an additional 1.3 degrees Celsius of warming than expected.

The new paper, then, if proved correct, would narrow the range left for the climate panel’s goal to 0.7 degree, and the Paris target to just 0.2 degree Celsius.

“Unfortunately, it means staying below 2 degrees is going to be even harder,” said Trude Storelvmo, an atmospheric scientist and another author of the paper, and an associate professor in the Yale department of geology and geophysics. “We have to emit even less CO2 to stay below those limits.”

Other climate researchers said that although the new study is valuable, it should be seen in the broader context of other areas of uncertainty that climate scientists are working through.

Gavin A. Schmidt, the director of NASA’s Goddard Institute for Space Studies, said that the study was in line with other recent papers that find potential issues with models.

“Generally speaking, these lead to numbers that are on the higher end,” he said, adding that “headlines that scream ‘Scientists say sensitivity higher than thought!’ will not be justified.”

Instead, he said, “this is one extra ingredient that needs to go into the hopper.”

Kevin Trenberth, distinguished senior scientist at the climate and global dynamics laboratory of the National Centre for Atmospheric Research, said the new paper “is fine as a first step, but it is not the last step, and much more is needed to establish how clouds change as the climate changes.”

His work, he added, suggests that the centre’s model “has too strong a climate sensitivity rather than too small because of clouds.”

Having less room to manoeuvre in reaching climate goals could lead people to think fighting climate change is futile, Professor Storelvmo said. But that is not the intent of the researchers.

“The hope,” she added, “is that instead of giving up, people get the sense of urgency in this, and in terms of emissions cuts, this leads to more action and less talk — instead of despair, that this triggers action.”