Using a new "power dissipation index" that reflects both the duration of storms and their maximum wind speeds, Kerry Emanuel, an atmospheric researcher at the Massachusetts Institute of Technology, reports that tropical storms' overall intensity has increased by about 50 percent since the mid-1970s.

Although many of the fiercest storms of the past three decades haven't made landfall when they were at peak intensity, "the near-doubling of hurricanes' power during this period should be a matter of some concern, as it's a measure of the (future) destructive potential of these storms," Emanuel said.

His paper, published online by the journal Nature, illustrates that the increases in storm intensity have been mirrored by increases in the average temperatures at the surface of tropical oceans.

And while much of this warming has been attributed to decades-long swings that come and go in the Atlantic and Pacific, Emanuel said his research shows there have been increases in tropical sea surface temperatures worldwide, even outside zones that have been affected by the established patterns.

Moreover, he pointed out in an interview that "the intensity of hurricanes depends both on how much heat can be transferred from the ocean to the atmosphere, which depends on the temperature of the ocean, and on how high air rising in the eyewall can go. This depends on the temperature profile of the atmosphere."

So if climate change continues to warm both seawater and the air above it during the rest of this century, as most scientists expect, "future warming may lead to an upward trend in tropical cyclone destructive potential, and, taking into account an increasing coastal population, (also) lead to a substantial increase in hurricane-related losses in the 21st century,'' Emanuel warned.

The research, sponsored by the National Science Foundation, "is an innovative application of a theoretical concept and has produced a new analysis of hurricanes' strength and destructive potential,'' said Jay Fein, director of the foundation's climate dynamics program.

Emanuel also has a theory that increased tropical cyclone activity would increase the transport of heat through the oceans from the tropics to high latitudes. If this turns out to be true, then this "feedback" could contribute to making things even warmer around the northern Pacific and Atlantic basins.

Recent simulations done by scientists at the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory indicate that the global warming anticipated over the next 80 years would increase average hurricane wind speeds by 5 percent to 10 percent, or by about half a category on the five-category Saffir-Simpson scale of hurricane intensity.

While a number of other climate and hurricane researchers have been debating whether a warming climate could make hurricanes more frequent, Emanuel is one of relatively few who has considered the impact on intensity.

Reliable records of hurricanes, particularly those with measures of intensity, go back only a few decades, and many researchers argue that it's too early to tell if warming temperatures will make a significant difference. Some climate models suggest that changing tropical wind patterns in a warming world will cause hurricanes to break up more easily, making the storms less frequent, even if temperatures that fuel the systems are higher. And there's no evidence that climate change will affect the tracks of hurricanes to make them any more or less likely to hit land, virtually all experts in the field say.

Emanuel agrees that the question of climate control on hurricane frequency is still open and subject to a lot more study.

But hurricane watchers, particularly those with connections to insurers, note that despite the freak pileup of four hurricanes on Florida last year, some of the biggest losses from recent hurricanes - like Hugo in 1989 and Andrew in 1992 - came in seasons where there weren't a large number of storms.

On the Net: www.nature.com.

www.nsf.gov.

www.nhc.noaa.gov