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Past decade’s extreme weather is manmade, new study suggests

By Nina Chestney (Reuters) - March 27, 2012

Extreme weather events over the past decade have increased and were “very likely” caused by manmade global warming, a study in the journal Nature Climate Change said on Sunday.

Scientists at Germany’s Potsdam Institute for Climate Research used physics, statistical analysis and computer simulations to link extreme rainfall and heat waves to global warming. The link between warming and storms was less clear.

“It is very likely that several of the unprecedented extremes of the past decade would not have occurred without anthropogenic global warming,” said the study.

The past decade was probably the warmest globally for at least a millennium. Last year was the eleventh hottest on record, the World Meteorological Organisation said on Friday.

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Extreme weather events were devastating in their impacts and affected nearly all regions of the globe.

They included severe floods and record hot summers in Europe; a record number of tropical storms and hurricanes in the Atlantic in 2005; the hottest Russian summer since 1500 in 2010 and the worst flooding in Pakistan’s history.

Last year alone, the United States suffered 14 weather events which caused losses of over $1 billion each.

The high amount of extremes is not normal, the study said.

Even between March 13 and 19 this year, historical heat records were exceeded in more than 1,000 places in North America.

For some types of extreme weather, there are physical reasons why they would increase in a warming climate. For example, if average temperature rises, then so will the number of heat records if all else remains equal, the study said.

Natural weather patterns like El Nino or La Nina can also cause highs in global temperature or increased precipitation which leads to floods.

“Single weather extremes are often related to regional processes, like a blocking high pressure system or natural phenomena like El Nino,” said Stefan Rahmstorf, co-author of the study and chair of the institute’s earth system analysis department.

“These are complex processes that we are investigating further. But now these processes unfold against the background of climatic warming. That can turn an extreme event into a record-breaking event.”

Recent years have seen an exceptionally large number of record-breaking and destructive heatwaves in many parts of the world and research suggests that many or even most of these would not have happened without global warming.

Currently, nearly twice as many record hot days as record cold days are being observed both in the United States and Australia, the length of summer heatwaves in western Europe has almost doubled and the frequency of hot days has almost tripled over the period from 1880 to 2005.

Extremely hot summers are now observed in about 10 percent of the global land area, compared with only about 0.1-0.2 percent for the period 1951 to 1980, the study said.

The link between storms and hurricanes and global warming is less conclusive but at least some of recent rainfall extremes can be attributed to human influences on the climate, it added.