London, January 29 (ANI): Scientists have suggested that a puzzling drop in the amount of water vapour in the Earth's atmosphere may be responsible for a slowdown in average global temperatures.
Although the decade spanning 2000 to 2009 ranks as the warmest on record, average temperatures largely levelled off following two decades of rapid increases.
Researchers have previously eyed everything from the Sun and oceans to random variability in order to explain the pause, which sceptics have claimed shows that climate models are unreliable.
Now, according to a report by Nature News, a team led by researchers at the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado, report that a mysterious 10 percent drop in water vapour in the stratosphere - the atmospheric layer that sits 10-50 kilometres above Earth's surface - since 2000 could have offset the expected warming due to greenhouse gases by roughly 25 percent.
Their model suggests that an increase in stratospheric water vapour might have boosted earlier warming by about 30 percent in the 1980s and 1990s.
The effect on temperature is dominated by water vapour in the lower part of the stratosphere, which absorbs and radiates heat in much the same way as water molecules and other greenhouse gases do in the lower atmosphere.
The drop in water vapour doesn't explain the entire decrease in the rate of warming, but it could contribute to it, according to Susan Solomon, first author of the study and a NOAA scientist who co-chaired the physical-science working group of the Intergovernmental Panel on Climate Change as part of its 2007 assessment.
It remains unclear what is driving the changes in stratospheric water vapour.
Average temperatures at the coldest point in the stratosphere - about 16 kilometres above the tropics - have fallen by about 1 degree Celsius in the past decade, according to Bill Randel, who heads the atmospheric chemistry division at the National Center for Atmospheric Research in Boulder.
Colder temperatures freeze out water vapour that might otherwise have entered the stratosphere.
But, according to Randel, "We don't really understand why that 1-degree temperature change occurred."
"I think it's exciting that this (transition) is happening, because we are going to learn a lot," said Judith Lean, a solar physicist at the Naval Research Laboratory in Washington DC. (ANI)