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2011 weather extremes down to global warming and La Niña

Climate change substantially increased the odds for many — but not all — extreme weather events in 2011.

Global warming made the 2011 Texan spring and summer heatwave 20 times more likely to happen than a similar event would have been 50 years ago. Likewise, the mild weather last November in the United Kingdom was almost certainly influenced by global warming, according to an analysis by a group of scientists who study how climate change might affect weather extremes worldwide.

However, an analysis of the atmospheric and hydrological conditions that favoured the devastating monsoon floods last summer in Thailand found no fingerprint of climate change in that event.

These are the main findings reported today by the the group for Attribution of Climate-related Events (ACE) — a loose coalition formed last year of climate researchers in the United States, Canada and Britain. The group is working towards an international fast-analysis system for assessing the impact of climate change on weird weather on an annual basis. The results of its first attribution exercise — which also includes the 2011 drought in Eastern Africa and the exceptionally cold 2010–11 winter in the United Kingdom — appear in the July issue of the Bulletin of the American Meteorological Society.

“Every weather event that happens now takes place in the context of a changing global environment,” says Kathryn Sullivan, deputy administrator of the US National and Oceanic Administration (NOAA), which took part in the exercise.

But assessing the extent to which weather and climate-related events, however dramatic, can be traced to either man-made or natural climate change is difficult. Scientists first need to produce thousands of simulations of a specific type of ‘freak’ weather in climate models with and without elevated greenhouse gases, and then compare the results. But as the spatial resolution of even the most sophisticated climate models is limited, some kinds of extreme weather — tornadoes, for instance — fall through the grid.

2011 will be remembered as a year of extreme weather events, even though it was the coolest year on record since 2008, according to NOAA’s 2011 State of the Climate Report, released today. The report, compiled by 378 scientists from 48 countries, highlights two ‘back-to-back’ La Niñas, which cooled the globe but also contributed to some of the extreme weather last year.

Image: The lead character of the 2011 climate story was a double dip La Niña, which chilled the Pacific at the start and end of the year. Many of the 2011 seasonal climate patterns around the world were consistent with common side effects of La Niña. Credit: NOAA Climate Portal


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