Much of the global warming predicted for the next century is not from direct warming by carbon dioxide, but due to other changes driven by carbon dioxide known as feedbacks. Now researchers have calculated the effects of a previously poorly quantified feedback caused by water vapor in the stratosphere, which could play a significant role in climate change, report findings detailed this week in the Proceedings of the National Academy of Sciences.

Carbon dioxide is a greenhouse gas that traps heat and helps warm the globe. Doubling carbon dioxide in Earth’s atmosphere by itself would lead to a global average warming of about 1.2 degrees C. However, this warming from carbon dioxide in turn drives feedbacks that increase the eventual warming to 2.0 to 4.5 degrees C.

Carbon dioxide is not the only greenhouse gas in Earth’s atmosphere — water vapor is another. The strongest climate feedback is due to water vapor in the troposphere, the bottom 6 to 9 miles (10 to 15 kilometers) of the atmosphere. Moisture in the troposphere more than doubles the direct warming from carbon dioxide.

Water vapor also exists in the stratosphere, the layer of Earth’s atmosphere right above the troposphere, reaching up to about 30 miles (50 kilometers) high. Climate scientist Andrew Dessler at Texas A&M University reasoned stratospheric water vapor feedback was possible, whereby a warming globe would cause more water to vaporize, increasing water vapor levels in the stratosphere and in turn helping warm the globe.

“People have talked about it, but little actual research had been done on it,” Dessler says. “I think it has been ignored mainly because people didn’t have good ideas about how to get an estimate of its magnitude.”

With the aid of satellite data and a climate model, Dessler and his colleagues estimated the stratospheric water vapor feedback was likely responsible for 5 to 10 percent of the warming predicted from a doubling of atmospheric carbon dioxide levels.

“The most surprising thing is how big the stratospheric water vapor feedback might be,” Dessler says.

Existing climate models already take stratospheric water vapor into account. “However, it’s also fair to say that essentially no one has looked at this process in the models, and there appears to be a reasonably large spread among the models in what they project for the stratosphere,” Dessler says. “It may be that this process explains some spread in climate sensitivity among the models.”

[youtube http://www.youtube.com/watch?v=EDI8DkyqA74]