More Frequent Heat Waves Linked to Global Warming

Lucy Stickland, vacationing from England, cools off in the public fountain at Washington Square Park in New York.
Lucy Stickland, vacationing from England, cools off in the public fountain at Washington Square Park in New York. (By Robert Caplin -- Bloomberg News)
By Juliet Eilperin
Washington Post Staff Writer
Friday, August 4, 2006

Heat waves like those that have scorched Europe and the United States in recent weeks are becoming more frequent because of global warming, say scientists who have studied decades of weather records and computer models of past, present and future climate.

While it is impossible to attribute any one weather event to climate change, several recent studies suggest that human-generated emissions of heat-trapping gases have produced both higher overall temperatures and greater weather variability, which raise the odds of longer, more intense heat waves.

Last week, Paul Della-Marta, a researcher at Switzerland's Federal Office of Meteorology and Climatology, presented findings at an international conference on climate science in Gwatt, Switzerland, showing that since 1880 the duration of heat waves in Western Europe has doubled and the number of unusually hot days in the region has nearly tripled.

In a separate 2004 study, researchers at Britain's Hadley Centre for Climate Prediction and Research produced computer models showing that greenhouse gas emissions had doubled the likelihood of events like the lethal 2003 European heat wave, and that by 2040 it is likely such heat waves will take place there every other year.

And researchers at the National Climatic Data Center in Asheville, N.C., reported this week that nighttime summer temperatures across the country have been unusually high for the past eight years, a record streak.

"It's just incredible, when you look at this thing," said Richard Heim, a research meteorologist at the center. He added that only the Dust Bowl period of the mid-1930s rivaled recent summers for sustained heat levels.

Drew Shindell, an atmospheric physicist at NASA's Goddard Institute for Space Studies who attended Della-Marta's presentation, said the European findings are especially significant because they draw on long-term surface temperature records.

"The European records, being so long, make a convincing case that we're already seeing changes" in the climate, Shindell said. "This is not like 'Centuries from now the ice sheets will melt.' This is 'In a few decades it will be dramatically different.' To me, that's alarming."

Kevin E. Trenberth, chief of the climate-analysis branch of the National Center for Atmospheric Research in Colorado, said, "There are very good reasons to believe that the current U.S. heat wave is at least partly caused by global warming."

Trenberth pointed to a study published in March by the Journal of Geophysical Research that showed that for more than 70 percent of the land researchers had surveyed worldwide, the number of warm nights each year had increased and the number of cold nights had declined, between 1951 and 2003. The researchers, led by Hadley Centre scientist L.V. Alexander, concluded, "This implies a positive shift in the distribution of daily minimum temperature throughout the globe."

Several researchers said it is hard to draw conclusions about the relationship between severe heat waves and climate change because heat waves occur less often than other weather events and arise from specific weather conditions. The current heat wave, said National Weather Service meteorologist Dennis Feltgen, stems from "a large persistent area of high pressure in the upper atmosphere" that has drifted from the West to the East Coast.

Nevertheless, most experts said it is important to pay attention to the high temperatures that have blanketed the United States and Europe over the past few years. Last month the National Oceanic and Atmospheric Administration reported that the first six months of 2006 are the hottest on record in the United States, and last month ranks as England's hottest July since recordkeeping began in 1659.

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