Statistics released last week by the U.S. National Oceanic and Atmospheric Administration show that September 2005 was the warmest September since 1880, when record-keeping began.
In the U.S. average temperatures for July through September were the fourth warmest on record. All 48 contiguous states had above-average temperatures during this period.
Temperatures in Florida, Massachusetts, New Jersey, New York, Rhode Island, and Vermont set new records during these months. In the Northeast the period was the warmest in at least 111 years, NOAA reported.
The weather changes predicted by Diffenbaugh and his colleagues could do more than make summers more uncomfortable.
There may be more floods a century from now, and summer heat waves may be much hotter and last much longer.
The scientists warn of "catastrophic losses of property and human life," as well as "exotic diseases," "species extinction," and "dramatic ecological, economic, and sociological impacts."
Some meteorologists, however, aren't ready to conclude that the world's weather is changing.
Joe Bastardi, a meteorologist with the commercial weather service AccuWeather, said recent unusually warm summers don't necessarily indicate permanent changes.
Bastardi questioned whether studies blaming global warming for unusual eventslike an increase in the frequency of hurricaneshave fully considered data from earlier periods.
For example, he said, a severe drought created the so-called Dust Bowl conditions in the U.S. Midwest during the 1930s, long before global warming was being blamed for extreme weather events.
"There are always droughts and floods," Bastardi said. "Someone's always wet, and someone's always dry."
Bastardi said he doesn't dismiss theories about global warming, but he wants to see more evidence.
"It may be happening, but there's always been climate change," he said.
Diffenbaugh acknowledged that more studies are needed before firm conclusions can be drawn.
"The climate system varies," he said. "We have periods when the entire Earth was covered in ice, and times when there was no ice at all. What we're finding, however, is that greenhouse gases enhance that variability."
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