16 August 2005John Roach
Is this the result of global warming—the rise in Earth's temperature fueled by increased greenhouse gases in the atmosphere?
"You can't point your finger and say, This is caused by global warming," said Kevin Trenberth, a climate scientist with the National Center for Atmospheric Research in Boulder, Colorado.
Summer weather is just naturally hot, he said. High-pressure systems, which lead to stifling-hot days, are a typical weather pattern this time of year. But this summer feels hotter in a lot of places:
• New Yorkers cranked up their air conditioners over the weekend to seek relief from a heat wave, setting an all-time record for energy consumption, Con Edison, a regional power company, reported Monday.
• During July's heat wave in the western U.S., Big Bear Lake, California, which sits 6,790 feet (2,070 meters) above sea level, set an all-time record high of 94ºF (34.4ºC).
• Denver, Colorado, tied its all-time high of 105ºF (40.6ºC) on July 20.
• Las Vegas, Nevada, tied a 1942 record of 117ºF (47.2ºC) on July 19.
• And much of southern Europe is in the grips of a heat wave that is exacerbating widespread drought and fueling a spate of forest fires.
It's Hot, But …
This summer's heat is "not all that unusual" and not linked to global warming, said Jim Laver, director of the U.S. National Oceanic and Atmospheric Administration's Climate Prediction Center in Camp Spring, Maryland.
"The way we like to explain it is, the climate varies from year to year, even within the summer season, and the variability is a lot larger than any long-term trend," he said.
But Trenberth said global warming likely underlies the heat. "One way to say it is, It's summer weather with a clear touch of global warming thrown in," he said.
According to the United Nation's Intergovernmental Panel on Climate Change, the average global temperature increased about 1ºF (0.6ºC) during the 20th century.
Over the next hundred years, Earth's average temperature is expected to rise an additional 2.5º to 10.5ºF (1.4º to 5.8ºC) in response to a doubling of the amount of carbon dioxide in the atmosphere.
The World Wide Fund for Nature, an environmental nonprofit, released a report August 11 showing that summer temperatures in 16 of Europe's capital cities have warmed sharply in the past 30 years.
London, England, experienced the greatest rise in average maximum summer temperature—more than 3.6ºF (2ºC) in the last 30 years. The increase in average summer mean temperature was highest in Madrid, Spain—up by 3.9 ºF (2.2ºC).
"Our report is yet another illustration of something that has become very clear from many studies examining Europe and other parts of the world—overall the temperature is rising," said spokesperson Martin Hiller.
According to the report, as average temperatures continue to increase, so too will the likelihood of more frequent and intense heat waves, droughts, and rainstorms.
Matter of Perception
Jim Laver, the NOAA climate scientist, said that anytime a summer heat wave rolls through or a hurricane hits, humans naturally want to know if global warming has something to do with it.
"It's normal to bring up those questions," he said. "And we try to explain [the answers] by the best science we have available." For now, a direct link between global warming and short-term weather events is impossible to prove, he said.
Trenberth countered that it is also impossible to prove there is not a link. "And given the widespread influence of global climate change, it is therefore likely that there is indeed an influence," he said.
But North Americans, especially in the eastern U.S., may be reluctant to accept a link between the weather and global warming, Trenberth said. Recent summers there have mostly been wetter, not warmer.
Wetter weather, which some scientists say is also a signal of global warming, tends to cool the regional climate, because the sun's energy goes toward evaporating water instead of increasing temperature.
Europeans, by contrast, tend to see a clear link between the weather and global warming. "They've seen increases in temperature, they've seen examples of heat waves, and there's a strong perception it's getting warmer," Trenberth said.
http://news.nationalgeographic.com/news/2005/08/0816_050816_heat_wave.html
