Stratospheric water vapor concentrations decreased by about 10% after the year 2000. Here, we show that this acted to slow the rate of increase in global surface temperature over 2000 to 2009 by about 25% compared to that which would have occurred due only to carbon dioxide and other greenhouse gases. More limited data suggest that stratospheric water vapor probably increased between 1980 and 2000, which would have enhanced the decadal rate of surface warming during the 1990s by about 30% compared to estimates neglecting this change. These findings show that stratospheric water vapor represents an important driver of decadal global surface climate change.
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The Guardian in London
Jan 30, 2010
Scientists have underestimated the role of water vapour in determining global temperatures, according to a study that could fuel further attacks on the science of climate change.
They 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, led by one of the world’s top climate scientists, suggests that almost a 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 the global temperature rise.
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 US National Oceanic and Atmospheric Administration’s new research, led by Susan Solomon, who co-chaired the 2007 IPCC report on the science of global warming, is published today in the journal Science.
Solomon 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 around the world who are trying very hard to understand and to explain to people openly and honestly what has happened over the last decade,” she said.
The study analysed water vapour in the stratosphere, about 16 kilometres up, where it acts as a potent greenhouse gas and traps heat.
Satellite measurements were used to show water vapour levels in the stratosphere have dropped about 10 per cent since 2000. When the scientists fed this change into a climate model, they found it could have reduced, by about 25 per cent over the past 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 per cent drop in water vapour, 10 miles (16 kilometres) 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.
• The US Administration formally embraced the Copenhagen Accord on global warming on Thursday, a day after President Barack Obama urged a fractious Congress to get to work on comprehensive legislation to stem the nation’s emissions.
US climate envoy Todd Stern gave notice to the United Nations that the country will aim for a 17 per cent cut in emissions of carbon dioxide and other gases blamed for global warming by 2020, from 2005 levels. The move, which confirmed the goal set by the White House late last year, was conditional on other countries also submitting their pollution-cutting targets to the accord, Stern said.
Source: Reuters & Guardian