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.
A. E. Dessler, M. R. Schoeberl, T. Wang, S. M. Davis, and K. H. Rosenlof. Stratospheric water vapor feedback. PNAS, September 30, 2013 DOI: 10.1073/pnas.1310344110