EXTREME WEATHER: Greenhouse gas emissions from human activity may be predisposing the climate to produce more extreme weather events, such as Hurricane Katrina. Image: Jeff Schmaltz, MODIS Rapid Response Team, NASA/GSFC
In the United States, 2011 was a year when weather seemed to ping-pong between extremes.
A historic drought struck Texas while floods devastated communities along the Missouri and Mississippi rivers and Hurricane Irene swamped the East Coast. Swarms of tornadoes rolled through the center of the country, and record-setting wildfires blazed in the Southwest.
But while 2011's litany of extreme weather was notable, it was "not unique" -- at least not in recent experience, according to a new analysis published yesterday in the journal Nature Climate Change.
The world has experienced an "exceptional number of unprecedented extreme weather events" for the past decade, say co-authors Dim Coumou and Stefan Rahmstorf, researchers at the Potsdam Institute for Climate Impact Research in Germany, who surveyed recent research linking climate change to shifts in the frequency and intensity of extreme weather.
"The evidence is strong that anthropogenic, unprecedented heat and rainfall extremes are here -- and are causing intense human suffering," the two write.
Recent studies suggest that the number of warm nights increased significantly between 1951 and 2003, and twice as many record hot days than record cold days are occurring in the United States and Australia.
More footprints appear in weather patterns
The number of record hot months observed at different points around the globe is three times as high as it would be in a climate that was not changing, the scientists note.
Notable heat waves of the past decade include those that struck Europe in summer 2003 (the warmest summer recorded there in at least 500 years), Greece in summer 2007 and central Russia in 2010 (when July temperatures broke the previous record by 2.5 degrees Celsius).
"Several recent studies indicate that many, possibly most of these heat waves would not have occurred without global warming," the new analysis concludes.
But it's not just heat waves that appear to be altered by climate change. Previous research has linked some recent extreme rainfall events to climate change -- including the precipitation that doused England and Wales in autumn 2000, during the wettest fall season there since recordkeeping began in 1766. The floods damaged more than 10,000 properties and caused losses estimated at £1.3 billion.
When extreme weather hits, journalists and the public often ask whether climate change has caused a particular event -- and are told that scientists cannot say a single weather event was caused by climate change.
"This is often misunderstood by the public to mean that the event is not linked to global warming, even though that may be the case -- we just can't be certain," Coumou and Rahmstorf write. "If a loaded dice rolls a six, we cannot say that this particular outcome was due to the manipulation -- the question is ill-posed."
The more correct analogy is that loaded dice will roll more sixes than if they weren't loaded, thus man appears to be raising the odds for certain types of extreme weather to turn up as the climate warms.
March continues a roll call of extreme U.S. weather
"Attribution is not a 'yes or no' issue as the media might prefer," Coumou and Rahmstorf say. "It is an issue of probability. It is very likely that several of the unprecedented extremes of the past decade would not have occurred without [human-caused] global warming."
The study echoes findings of the Intergovernmental Panel on Climate Change, which released its own report last year examining the effect of climate change on weather extremes.
That report found evidence that climate change is increasing the frequency of drought and heat waves and the intensity of rainstorms, warning that such shifts will require the world's governments to change how they cope with natural disasters (Greenwire, Nov. 18, 2011).