March 25, 2009 – Original Source: Reuters. By David Fogarty, Climate Change Correspondent, Asia
SINGAPORE (Reuters) – Global warming is more than a third to blame for a major drop in rainfall that includes a decade-long drought in Australia and a lengthy dry spell in the United States, a scientist said on Wednesday.
Peter Baines of Melbourne University in Australia analyzed global rainfall observations, sea surface temperature data as well as a reconstruction of how the atmosphere has behaved over the past 50 years to reveal rainfall winners and losers.
What he found was an underlying trend where rainfall over the past 15 years or so has been steadily decreasing, with global warming 37 percent responsible for the drop.
“The 37 percent is probably going to increase if global warming continues,” Baines told Reuters from Perth in Western Australia, where he presented his findings at a major climate change conference.
Baines’ analysis revealed four regions where rainfall has been declining. The affected areas were the continental United States, southeastern Australia, a large region of equatorial Africa and the Altiplano in South America.
But there were two areas in the tropics where rainfall has been increasing — northwestern Australia and the Amazon Basin.
“This is all part of a global pattern where the rainfall is generally increasing in the equatorial tropics and decreasing in the sub-tropics in mid-latitudes,” Baines said.
“This is a little bit like the pattern that the (computer) models predict for global warming but this is coming out of the rainfall observations of the past 30 years,” added Baines, of Melbourne University’s civil and environmental engineering department.
The rainfall trend was also accompanied by a trend in global sea surface temperatures (SST), he said, adding he used temperature data going back to 1910.
Sea surface temperatures have been rising as the atmosphere warms.
“If you take the SST data and analyze that over a long period you can break that up into a variety of components, such a global warming component,” he said.
He also looked at the influence on rainfall of major ocean circulation patterns that have a major impact on the world’s weather such as the Atlantic conveyor belt that brings warm temperatures to northern Europe.
Two Pacific circulation patterns, including the Pacific Decadal Oscillation, were also studied for their influence on rainfall.
The key in the analysis was to strip out the influence of the El Nino ocean-climate pattern which causes drought in Southeast Asia and Australia and floods in Chile and Peru.
Baines, who also works for the Department of Earth Sciences at the University of Bristol in England, said the Atlantic conveyor belt was 27 percent to blame for the decreased rainfall, while the two Pacific ocean circulation patterns were 30 percent responsible.