Water in stratosphere plays key role in Earth's climate

Water vapor changes in the stratosphere contribute to warmer temperatures and likely play an important role in the evolution of Earth's climate, says a research team led by a Texas A&M University professor.

Andrew Dessler, a Texas A&M atmospheric sciences professor, and colleagues from the University of Colorado, NOAA (National Oceanic and Atmospheric Administration) and the Science and Technology Corp. have had their findings published in the Proceedings of the National Academy of Sciences.

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

"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," Dessler explained. "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, the researchers say.

"It's clear to us that, if models want to make accurate predictions of climate change, they should get stratospheric water vapor right," said Sean Davis, a research scientist at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado at Boulder and study coauthor. He added, "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. It also used simulations from NASA's Goddard Earth Observing System Chemistry Climate Model. The project was funded by a grant from the National Science Foundation.