WASHINGTON—Humans are largely to blame for the recent trend toward more powerful hurricanes, a group of 19 American and European scientists declared Monday.
In a paper appearing in the Proceedings of the National Academy of Sciences, the scientists claim to have established a solid chain linking human burning of fossil fuels, global warming, higher ocean temperatures, and the intensity and duration of recent hurricanes such as Katrina and Wilma.
The scientists' key finding was that as sea surface temperatures rise and fall, the maximum wind speed of hurricanes goes up and down in step with them.
"Human-caused changes in greenhouse gases are the main driver" of warmer waters in the tropical Atlantic and northwestern Pacific oceans, where hurricanes and cyclones are born, the paper says.
Its principal author was Benjamin Santer, a senior climate researcher at Lawrence Livermore National Laboratory in Livermore, Calif. Other contributors come from 11 different laboratories in the United States, Germany and England.
Their report is unlikely to end the controversy over the connection between human burning of fossil fuels in cars, buildings and factories, the warming of the world's air and seas, and the surge in category 4 and 5 hurricanes.
"Sea surface temperature is only one among many factors impacting hurricanes," Richard Lindzen, an atmospheric scientist at the Massachusetts Institute of Technology in Cambridge, said in an e-mail message. "Our inability to have forecast the current (mild hurricane) season should provide a measure of humility."
Some skeptics say that other factors—including natural climate variability, wind patterns and volcanic eruptions—cause so much statistical "noise" that they blur the connection between sea surface temperature and the ferocity of hurricanes.
"The linkage, if real, is so noisy that it can't be found in a quarter-century of real hurricane data," said Patrick Michaels, a professor of environmental research at the University of Virginia in Charlottesville.
Christopher Landsea, a government expert at the National Hurricane Center in Miami, also has questioned the link between ocean temperatures and hurricanes.
"Extreme tropical cyclones and overall tropical cyclone activity have globally been flat from 1986 until 2005, despite a sea surface temperature warming of 0.25 C (0.45 degrees Fahrenheit)," Landsea wrote in the July 28 issue of the journal Science.
Some experts who weren't part of the Santer group supported its findings.
"The human-induced buildup of greenhouse gases in the atmosphere appears to be the primary driver of increasing hurricane activity," said Robert Corell, a meteorologist at MIT.
Greg Holland, a meteorologist at the National Center for Atmospheric Research in Boulder, Colo., said, "We still don't understand a lot of the linkages, but the relationship is quite solid."
Holland acknowledged that global warming isn't the sole cause of the escalation in extreme hurricanes. He estimated that natural variability may account for up to 30 percent of the changes in the storms' power but that the other 70 percent is "due to climate change."
The experts attributed this year's relatively mild hurricane season, after the catastrophes of 2005, to random year-to-year changes that can disguise the underlying long-range trend.
"The smaller number of hurricanes this year doesn't mean the trend has gone off," Holland said. "It's just natural variability: low one year, high the next year."
Corell put it this way: "There is natural variability underneath, but two-thirds of the human-induced temperature rise in these (hurricane-spawning) regions is due to the greenhouse-gas warming effect."
Thomas Wigley, a climate expert at the National Center for Atmospheric Research, predicted that ocean temperatures and hurricane violence will continue to increase.
"The changes we expect over the next 100 years are far greater than over the past 100 years," said Wigley, a co-author of the Proceedings of the National Academy of Sciences paper. "Sea surface temperatures in the past are small beans compared to what we're going to see in the future."
(c) 2006, McClatchy-Tribune Information Services.
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