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 temperaturemore than 3.6ºF (2ºC) in the last 30 years. The increase in average summer mean temperature was highest in Madrid, Spainup 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 worldoverall 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.
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