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"A better understanding of the stratospheric water vapor feedback could help explain some of the spread among predictions of future climate change from different models," says Sean Davis. (Credit: Jens-Petter Salvesen/Flickr, Bevan Font by Vernon Adams)

climate change

How water vapor skews climate predictions

In the stratosphere, changes in water vapor contribute to warmer temperatures and likely play a key role in the evolution of Earth’s climate, say researchers.

The scientists found that increased surface temperatures, such as from the addition of carbon dioxide to the atmosphere, lead to increased humidity in the stratosphere. Because stratospheric water vapor is a greenhouse gas, this leads to additional warming, they say. This cycle is frequently called a climate feedback.

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“We find that this stratospheric water vapor feedback is probably responsible for 5-10 percent of the total warming you get from adding carbon dioxide to the climate,” explains Andrew Dessler, an atmospheric sciences professor at Texas A&M University.

“While it’s not really surprising that this process is going on, we were surprised at how important the process is for our climate system.”

Climate models already include this process, but unevenly. Some models predict large increases in stratospheric humidity, while others don’t, say the researchers, whose findings appear in the Proceedings of the National Academy of Sciences.

“It’s clear to us that, if models want to make accurate predictions of climate change, they should get stratospheric water vapor right,” says study coauthor Sean Davis, a research scientist at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado at Boulder.

He adds, “A better understanding of the stratospheric water vapor feedback could help explain some of the spread among predictions of future climate change from different models,” referring to the projections made by the recently released 5th Assessment report of the Intergovernmental Panel on Climate Change (IPCC) last week.

Several years ago, Dessler was the first to observationally calculate the strength of the cloud feedback, showing that clouds play a key role in climate change.

The researchers used water vapor measurements from the Microwave Limb Sounder on board NASA’s Aura satellite. The team also used simulations from NASA’s Goddard Earth Observing System Chemistry Climate Model.

Additional researchers from the National Oceanic and Atmospheric Administration (NOAA) and the Science and Technology Corp. also contributed to the findings, which the National Science Foundation partially funded.

Source: Texas A&M University

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