Climate scientists have in the past been cautious about suggesting that extreme weather can be linked to climate change. But a new report from the Potsdam Institute for Climate Impact Research (PIK) argues just that.
Previous studies have shown that the weather is fluctuating more broadly than thirty years ago.
In 2011 alone, the US was hit by 14 extreme weather events, while Japan registered record rainfalls and the Yangtze river basin in China suffered a record drought.
In 2010, Western Russia experienced the hottest summer in centuries, while in Pakistan and Australia record-breaking amounts of rain fell. 2003 saw Europe´s hottest summer in at least 500 years.
And, says the Potsdam team, this isn't an accident, with extreme rainfall and heat waves clearly linked with human-caused global warming. Less clear is the link between warming and storms, despite the observed increase in the intensity of they say.
"The question is whether these weather extremes are coincidental or a result of climate change," says Dim Coumou.
"Global warming can generally not be proven to cause individual extreme events – but in the sum of events the link to climate change becomes clear."
The scientists based their analysis on basic physics, statistical analysis and computer simulations.
Elementary physical principles already suggest that a warming of the atmosphere leads to more extremes - for example, warm air can hold more moisture until it rains out.
Secondly, clear statistical trends can be found in temperature and precipitation data; and, thirdly, detailed computer simulations also confirm a link between warming and both temperature and precipitation.
With warmer ocean temperatures, tropical storms – called typhoons or hurricanes, depending on the region – should increase in intensity but not in number. In the past decade, there have been several record-breaking storms, such as hurricane Wilma in 2004.
However, it's not clear that global warming is the reason. The increase in intensity of tropical storms in the North Atlantic between 1980 and 2005, for example, could be caused not just by surface warming but by a cooling of the upper atmosphere.
Overall, says the team, cold extremes decrease with global warming, but this doesn't make up for the increase in extreme heat.
"Single weather extremes are often related to regional processes, like a blocking high pressure system or natural phenomena like El Niño," says Stefan Rahmstorf, chair of the Earth System Analysis department at PIK.
"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."