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.
"The trend lines showing so much hot weather in recent years suggests some concern, even if we can't say definitively this is a signal of climate change," said Daniel C. Esty, a professor of environmental law and policy at Yale University.
Scientists and public health officials said they are particularly worried about an increase in summer nighttime temperatures because people tend to recover from excessive heat exposure at night. Joel D. Scheraga, national program director for the U.S. Global Change Research Program of the Environmental Protection Agency, has delivered presentations indicating that with increasing temperatures and population growth, deaths from extreme heat or cold could as much as triple in major American cities from 1993 to 2050.
Scheraga said the EPA chart was not a clear prediction, because federal, state and local officials are working to better protect citizens from the dangers of extreme heat and cold. Nearly 100,000 people have downloaded the EPA's "Excessive Heat Events Guidebook" since it was posted online six weeks ago.
"These are avoidable deaths. There's an opportunity to save lives," Scheraga said. "With climate change, with warming and an intense hydrological cycle, the water cycle, we do in fact expect more extremes, more flooding and more heat waves."
Since mid-July, 179 Americans, most of them Californians, have died in the current heat wave; more than 52,000 died during the 2003 episode in Europe, where air conditioning is less common.
A group of Swiss researchers including Mark A. Liniger, a senior researcher at the Federal Office of Meteorology and Climatology, wrote in a 2004 paper in the journal Nature that if the increased temperature variability continued, "it would represent a serious challenge to adaptive response strategies designed to cope with climate change."
Some climate experts and industry lobbyists, however, question the correlation between global warming and heat waves. Konstantin Vinnikov, a senior research scientist at the University of Maryland at College Park, said he expected climate change to have only a minor effect on future scorchers.
"These are events that have happened in the past and will happen in the future. Climate trends related to climate change cannot change it too much," Vinnikov said.
And Bracewell & Giuliani LLP lobbyist Frank V. Maisano, who represents coal-fired power plants, sent an e-mail to reporters this week noting that more than half of the days with temperatures at or above 100 degrees Fahrenheit in the Washington-Baltimore region occurred between 1874 and 1934.