More Frequent Heat Waves Linked to Global Warming
U.S. and European Researchers Call Long Hot Spells Likely
By Juliet Eilperin
Washington Post Staff Writer
Friday, August 4, 2006; A03
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
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
"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
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
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
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.