研究：全球变暖让云层减少

With rising global carbon emissions, the planet will heat up and cloud cover will dissipate, according to a new study.

The concentration of heat-trapping carbon dioxide in the atmosphere has climbed 40 percent over the last century. And, the new study reports, in response to the release of CO2 emissions, from the burning of fossil fuels in power plants, cars and buildings, the Earth will continue to warm to dangerous levels.

Steven Sherwood, a climate scientist at Australia's Centre of Excellence for Climate System Science and lead author of the report, says the prediction of a 4-degree celsius warming is based on the role of water vapor in cloud formation.

“What we see in the observations is that when air picks up water vapor from the ocean surface and rises up, it often only rises a few kilometers before it begins its descent back to the surface," Sherwood said. "Otherwise it might go up 10 or 15 kilometers. And those shorter trajectories turn out to be crucial to giving us a higher climate sensitivity because of what they do to pull water vapor away from the surface and cause clouds to dissipate as the climate warms up.”
 
Under this scenario, in which clouds do not form, the Earth would absorb more sunlight.

“The question for many years has been ,what is going to happen to the amount of low clouds?" Sherwood said. "Does it decrease when the Earth warms up or does it stay the same or maybe even increase?  And what we found is that it should be decreasing because of this mixing process, which pulls water vapor away from the layers where these clouds form and causes there to be fewer of them in the warmer atmosphere.”

Climate models that show a slight global temperature response to carbon dioxide do not take the lower altitude water vapor process into account, according to Sherwood. Instead their simulations assume that all water vapor rises to 15 kilometers and forms clouds.

“Estimates of the climate sensitivity that have been lower are founded on models of the atmosphere that are not consistent with observations,” he said.

When the processes in climate models are corrected to match the real world observations, the simulations produce cycles that take water vapor to a wider range of heights in the atmosphere. Sherwood says the study is yet another warning to curb emissions or deal with the impacts of climate change.