WASHINGTON — A study of a dozen of 2012's wildest weather events found that man-made global warming increased the likelihood of about half of them, including Superstorm Sandy's devastating surge and the blistering U.S. summer heat.
The other half — including a record wet British summer and the U.S. drought last year — simply reflected the random freakiness of weather, researchers with the U.S. National Oceanic and Atmospheric Administration and the British meteorological office concluded in a report issued Thursday.
The scientists conducted thousands of runs of different computer simulations that looked at various factors, such as moisture in the air, atmospheric flow, and sea temperature and level.
The approach represents an evolution in the field. Scientists used to say that individual weather events cannot be attributed to climate change. But recently, researchers have used computer simulations to look at extreme events in a more nuanced way and measure the influence of climate change on their likelihood and magnitude.
This is the second year that NOAA and the British meteorology office have teamed up to look at the greenhouse gas connection to the previous year's unusual events.
"We've got some new evidence that human influence has changed the risk and has changed it enough that we can detect it," study lead author Peter Stott, head of climate monitoring and attribution for the British meteorological office, said at a news conference.
The researchers said climate change had made these 2012 events more likely: U.S. heat waves, Superstorm Sandy flooding, shrinking Arctic sea ice, drought in Europe's Iberian peninsula and extreme rainfall in Australia and New Zealand.
The 78 international researchers, however, found no global warming connection for the U.S. drought, Europe's summer extremes, a cold spell in the Netherlands, drought in eastern Kenya and Somalia, floods in northern China and heavy rain in southwestern Japan.
That doesn't mean that there weren't climate change factors involved, just that researchers couldn't find or prove them, said the authors of the 84-page study, published in the Bulletin of the American Meteorological Society.
All 12 events — chosen in part because of their location and the effect they had on society — would have happened anyway, but their magnitude and likelihood were boosted in some cases by global warming, the researchers said.