Increasing fury of tropical storms linked to warming sea

Tropical storms have become significantly more intense in both the Atlantic and Pacific oceans during the past 30 years, possibly linked to rising seawater temperatures worldwide, according to an analysis published yesterday.

Using a new "power dissipation index" that reflects 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 although much of the 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 those zones.

Moreover, he pointed out 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 eye wall 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.

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.

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.

Researchers are using new methods to analyze those storms and others going back as far as 1851.

If early storms turn out to be more powerful than originally thought, Emmanuel's findings on global warming's influence on recent tropical storms might not hold up, they said.

This report includes information from The Associated Press.

On the Net: www.nature.com, www.nsf.gov, www.nhc.noaa.gov