Water in Earth's atmosphere may cause climate warming

Water vapour in Earth's stratosphere contributes to warmer temperatures and likely plays an important role in the evolution of climate on our planet, a new study has found.
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 vapour 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 vapour feedback is probably responsible for 5-10 per cent of the total warming you get from adding carbon dioxide to the climate," Andrew Dessler, a Texas A&M university atmospheric sciences professor, said.

"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," said Dessler.
Climate models already include this process, but unevenly. Some models predict large increases in stratospheric humidity, while others don't, researchers said.

"It's clear to us that, if models want to make accurate predictions of climate change, they should get stratospheric water vapour right," said Sean Davis, scientist at the Cooperative Institute for Research in Environmental Sciences at the University of Colorado at Boulder and study coauthor.
"A better understanding of the stratospheric water vapour feedback could help explain some of the spread among predictions of future climate change from different models," Davis said, referring to the projections made by the recently released 5th Assessment report of the Intergovernmental Panel on Climate Change (IPCC).

Dessler was the first to observationally calculate the strength of the cloud feedback, showing that clouds play a key role in climate change.
The study was published in the journal Proceedings of the National Academy of Sciences.