Extreme weather events on the rise

The United States experienced some of the most extreme weather events in its history this spring, including deadly outbreaks of tornadoes, near-record flooding, drought and wildfires.

Damages from these disasters have already passed $32 billion, and the hurricane season, is projected to be above average, according to the National Oceanic and Atmospheric Administration. Government scientists said the frequency of extreme weather has increased over the past two decades, in part as a result of global warming caused by the accumulation of carbon dioxide in the atmosphere. But they were careful not to blame this year’s rash of deadly events on human-caused global warming, saying that in some ways weather patterns were returning to those seen at the beginning of the last century.

“Looking at long-term patterns since 1980, indeed, extreme climatological and meteorological events have increased,” said Thomas R Karl, director of NOAA’s National Climatic Data Center. “But in the early part of the 20th century, there was also a tendency for more extreme events followed by a quiet couple of decades.”

Presenting a new NOAA report on 2011 extreme weather, Karl said that extremes of precipitation have increased as the planet warms and more water evaporates from the oceans. He also said models suggest that as carbon dioxide builds up in the atmosphere and heats the planet, droughts will increase in frequency and intensity.

Some other climate scientists are more categorical about the human contribution to extreme climate events. Kevin Trenberth, distinguished senior scientist at the National Center for Atmospheric Research, a university-sponsored institute in Boulder, Colo., said that when the greenhouse effect caused by burning fossil fuels is added to the natural variability of climate, weather disasters can be expected to occur more frequently. “Global warming is contributing to an increased incidence of extreme weather because the environment in which all storms form, has changed from human activities,” Trenberth said. “Records are not just broken; they are smashed.”

So far this year, there have been nearly 1,400 preliminary tornado reports nationwide; those reports will likely be whittled to about 900 confirmed tornadoes, the second-highest annual total recorded in modern times. The record is 1,011 confirmed tornadoes in 2008. The year also is on track to be one of the deadliest, with 536 fatalities so far from tornadoes, placing 2011 in sixth in US history and the deadliest since 1936, NOAA reported.

John M Broder
Nature News

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