 By LEE BOWMAN Scripps Howard News Service June 16, 2005
A perspective article published Thursday by Kevin Trenberth of the National Center for Atmospheric Research, in Boulder, Colo., argues that a warmer, moister climate over the Atlantic is almost certain to make future hurricanes more intense and perhaps more frequent.
Writing in the journal Science, Trenberth continues an argument that garnered considerable media attention _ and drew attacks from many experts _ during last fall's intense battering of Florida and the Gulf Coast. Most hurricane experts insist that there's no clear link between an increase in tropical storms since 1995 and any long-term change in global temperatures, which scientists think have been rising gradually for the past century. "There is no reasonable scientific way any such interpretation of this recent upward shift in Atlantic hurricane activity can be made," said Colorado State University tropical-storm researcher William Gray, who predicted the recent surge in storms and expects them to continue. Gray's culprit isn't greenhouse gases and global warming, but a long-term change in the salinity and deep-ocean currents of the North Atlantic that results in warmer surface temperatures and makes hurricanes more likely to form. Trenberth contends in his paper that statistical models used in most hurricane forecasting simply don't capture the impacts of global warming. "Trends in human-induced environmental changes are now evident in hurricane regions," Trenberth wrote. "These changes are expected to affect hurricane intensity and rainfall, but the effect on hurricane numbers remains unclear. The key scientific question is how hurricanes are changing." Trenberth points to several computer simulations done for the National Oceanic and Atmospheric Administration in recent years that show hurricanes gaining intensity with an 80-year buildup of carbon dioxide in the atmosphere. He notes that sea-surface temperatures in the North Atlantic over the last decade have been the warmest on record, and water vapor over oceans worldwide has risen by about 2 percent since 1988. Both conditions supply more potential energy for the showers and thunderstorms that fuel hurricanes, he said. However, Trenberth concedes that no one knows for sure whether global warming will enhance or impair wind-circulation patterns in the tropics that can either support or discourage hurricane formation. For instance, cold-water "La Nina" events in the Pacific set up trade-wind patterns that make for fewer hurricanes. Nor, he writes, is there any solid evidence that a warmer world will include more weather patterns that steer hurricanes landward. It was a high-pressure system that parked off the East Coast last fall and kept pushing storms into the Caribbean and Florida. In a letter to key lawmakers last fall attacking the global warming/hurricane link, a group of climatologists led by James O'Brien of Florida State University made a case for fewer severe tropical storms in a warmer world. Most climate-change experts agree that more pronounced warming will occur in polar regions. And it is the difference in temperatures between tropics and poles that sets up circulation patterns to guide storms. "Warmer polar regions would reduce this gradient and thus lessen the overall intensity or frequency or both of storms _ not just tropical storms, but mid-latitude winter storms as well," the climate scientists wrote. Studies of long-term climate change seem to bear this out. "In the past, warmer periods have seen a decline in the number and severity of storms," they said. Trenberth argues there's uncertainty on that point, too. "There is no sound theoretical basis for drawing any conclusion about how (human-induced) climate change affects hurricane numbers or tracks, and thus how many hit land," he wrote.
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