Scientists have underestimated the role that water vapour plays in determining global temperatures, according to a study that could fuel further attacks on the science of climate change.

The research, led by one of the world’s top climate scientists, suggests that almost one-third of the global warming recorded in the 1990s was due to an increase in water vapour in the high atmosphere, not human emissions of greenhouse gases. A decline in water vapour after 2000 could explain a recent slowdown in global temperature rise, the scientists add.

The experts say their research does not undermine the scientific consensus that emissions of greenhouse gases from human activity drive global warming, but they call for “closer examination” of the way climate computer models consider water vapour.

The research comes at a difficult time for climate scientists, after an embarrassing mistake in the 2007 report of the Intergovernmental Panel on Climate Change (IPCC), which included false claims that Himalayan glaciers could melt away by 2035. There has also been criticism over the way climate scientists at the University of East Anglia apparently tried to prevent the release of data requested under Freedom of Information laws.

The new research, led by Susan Solomon, at the U.S. National Oceanic and Atmospheric Administration, who co-chaired the 2007 IPCC report on the science of global warming, is published today in the journal Science.

She would not comment on the mistake in the IPCC report — which was published in a separate section on likely impacts — or on calls for Rajendra Pachauri, the IPCC chairman, to step down.

“What I will say, is that this [new study] shows there are climate scientists round the world who are trying very hard to understand and to explain to people openly and honestly what has happened over the last decade.” The study analysed water vapour in the stratosphere, about 10 miles up, where it acts as a potent greenhouse gas and traps heat at the Earth’s surface.

Satellite measurements were used to show that water vapour levels in the stratosphere have dropped about 10% since 2000. When the scientists fed this change into a climate model, they found it could have reduced, by about 25% over the last decade, the amount of warming expected to be caused by carbon dioxide and other greenhouse gases.

They conclude: “The decline in stratospheric water vapour after 2000 should be expected to have significantly contributed to the flattening of the global warming trend in the last decade.” Solomon said: “We call this the 10, 10, 10 problem. A 10% drop in water vapour, 10 miles up has had an effect on global warming over the last 10 years.” Until now, scientists have struggled to explain the temperature slowdown since 2000, a problem climate sceptics have exploited.

She said it was not clear if the water vapour decrease reflects a natural shift, or was a consequence of a warming world. If the latter is true, then more warming could see more falls in water vapour, acting as a brake on future temperature rises.

Copyright: Guardian News & Media 2010