Long before it ends, the 2005 Atlantic hurricane season has already rewritten its chapter in America's record books.
No season had generated as many tropical storms and hurricanes as early as this one. Now, Rita is forecast to plow into the Texas coast near Galveston. If it hits, with maximum winds of more than 165 miles an hour, it would be the second catastrophic-level storm to pile into the Gulf Coast in three weeks. Rita and Katrina are among the most powerful Atlantic storms on record.
It's not by chance. Around the world, powerful hurricanes - rated Category 4 or 5 - have become more frequent compared with 30 years ago. Coastal communities can expect more of the same, researchers say, for a variety of reasons that may eventually include global warming.
Two studies by researchers in the past two months, using slightly different approaches, have reported a noticeable increase in storm strength and in the share of strong storms a season experiences.
One group finds that over the past 35 years, the number of Category 4 and 5 storms has nearly doubled worldwide. In the 1970s, roughly 10 of these catastrophic-level storms occurred each season. Since 1990, the number is up to 18 a season, according to Peter Webster, a Georgia Institute of Technology atmospheric scientist who led the study team.
Moreover, these powerful storms made up an increasing share of all storms over time, rising from 20 percent in the 1970s to 35 percent in the '90s. The largest increases in the number of intense hurricanes occurred in the Pacific and Indian Oceans.
The work follows on the heels of a study published in August noting a significant increase in the power of storms since the mid-1970s. This power index shows a near doubling of storms' destructive potential during the past 30 to 40 years, according to Kerry Emanuel, who specialized in tropical meteorology at the Massachusetts Institute of Technology in Cambridge, Mass. The change reflects a combination of more-powerful storms and storms retaining their peak intensity longer than in the past.
"The 'why' is still somewhat mysterious," he says. "A lot of it is duration. They're clearly lasting longer at higher intensity than they used to."
Both studies note that these trends correlate well with rising ocean temperatures in the globe's hurricane-generating regions. Tropical storms typically gain strength over warm waters.
Both groups are wary of attributing the trends to global warming, although other researchers have said that some portion of the increase in ocean temperatures is probably due to human-influenced climate change.
For one thing, "we need a longer record of hurricane statistics, and we need to understand more about the role hurricanes play in regulating the heat balance and circulation in the atmosphere and oceans," Dr. Webster notes. "If we can understand why the world sees about 85 named storms a year and not, for example, 200 or 25, then we might be able to say that what we're seeing is consistent with what we'd expect in a global warming scenario."
Moreover, for the Atlantic, Dr. Emanuel notes that through the first half of this century, natural long-term cycles in the region's climate and ocean conditions will be more influential in determining storm intensity and duration than global warming. And the levels of damage over this period will be determined more by human factors than by climate change's effect on the strength of a storm at landfall.
Those human factors include the number of people moving to storm-prone coasts and the types of zoning rules and building codes that coastal communities adopt.
Over the second half of the century, however, especially as sea levels rise, "global warming may indeed have a discernible influence on hurricane damage," Emanuel adds.