by Catherine Brahic, environment reporter, New Scientist, February 19, 2009

Hothouse, snowball, slushball, greenhouse -- all terms that have been used to describe the various climatic state of our planet. Here's a new one: Sauna Earth.

On a wet planet like ours, warmer temperatures naturally lead to more humid conditions. And because water vapour is a powerful greenhouse gas, a more humid Earth should also be a warmer one. This is what's known as the water vapour feedback (it's what's thought to contribute to the faster-than-average warming in the Arctic).

Models and theory show it should happen, but observational data to back up the theory has been scarce. No longer.

In an article published in Science today (DOI: 10.1126/science.1171264), Andrew Dessler of Texas A&M University and Steven Sherwood of the University of New South Wales in Australia say there is now enough evidence on the table to say that for every degree of warming caused by greenhouse gas emissions, water vapour probably adds another degree.

"This process is always running, so some of the warming we have experienced over the last century is due to the water vapor feedback," says Dessler. "And models include this process, so the predictions of several degrees C warming over the next 100 years also includes warming by this feedback."

They base their statement on studies of how atmospheric water vapour changes (pdf) with various natural cycles that warm and cool the planet -- for instance, the cyclical change in temperatures caused by seasons and El Nino.

These natural cycles of warming and cooling are very different to long-term global warming - they are not caused by human emissions of greenhouse gases. But Dessler and Sherwood say that anthropogenic climate change will have the same effect: it will warm the planet and cause water to evaporate.

By now, some of you will have heard a little voice in your head saying something along the lines of "isn't this what climate sceptics have said all along? That water vapour is as bad if not worse than carbon dioxide? Doesn't this prove that humans are not the primary or even the main driver of climate change?"

It may seem confusing, but the answer is an emphatic "no." The warming caused by water vapour is a feedback: there wouldn't be more of it in the atmosphere if the planet weren't already warming because of something else -- in this case, industrial greenhouse gas emissions.

Link to article: http://www.newscientist.com/blogs/shortsharpscience/2009/02/sauna-doubles-the-earths-tempe.html