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Greenhouse effect could lead to more powerful hurricanes, Emanuel says

The maximum intensity of hurricanes will increase if global warming caused by the greenhouse effect leads to an increase in tropical sea-surface temperatures, according to an MIT researcher.

And while it doesn't look as though the total area on the planet prone to hurricane activity will increase as the climate changes, there may be rearrangements of activity within hurricane-prone areas, according to Professor of Meteorology Kerry Emanuel.

Tropical cyclone frequency could go either way, he said. (Tropical cyclone is the generic name for the storms that people call hurricanes in the Atlantic and eastern North Pacific.)

Professor Emanuel published the first article on this subject in Nature magazine in 1987. He argued that maximum wind speeds would increase about 10 percent for a 2��������� Celsius increase in tropical sea surface temperatures. The question is whether global warming will amount to this much of an increase.

Will global climate change affect hurricane frequency, intensity (the amount of hurricane damage increases roughly as the square of the intensity of the storms, measured by maximum wind speed) and geographic distribution?

At the annual meeting of the Association of American Geographers in Boston on March 28, Professor Emanuel emphasized that much more information is needed on the basic physics of these tropical cyclones to make significant progress in the field. Compared to earthquakes, he said, there are very few scientists working on this problem.

The new field of paleotempestology, which uses geological records to study hurricane activity from prehistoric times, may offer the best hope for determining the nature of the relationship between hurricanes, cyclones and climate.

"Pushing the record of landfalling tropical cyclones way back into prehistory, perhaps even to the last ice age, may be the key to understanding the relationship between cyclone activity and climate," Professor Emanuel said.

While the study of tropical cyclones may not attract droves of researchers, the storms themselves are quirky weather phenomena, real "accidents of nature," he said. Unlike other types of storms, tropical cyclones do not arise spontaneously. They are triggered by a series of unrelated disturbances -- some that lead to hurricanes and some that don't -- so they are almost impossible to predict.

Tropical cyclones affect the tropical Atlantic, including the Caribbean Sea and Gulf of Mexico, the north Pacific and the north and south Indian Ocean. Their development seems to require the establishment of a 50-to-100-mile-wide column of very humid air that extends through the entire depth of the tropical troposphere, which is normally somewhat dry at the middle levels.

Professor Emanuel found that thunderstorms that develop within this pillar do not produce the dry, cold downdrafts driven by partial evaporation of rain that normally accompany these storms. When the evaporation is reduced and downdrafts are inhibited within the humid pillar, it's prime time for a hurricane to develop.

And once it does, as inhabitants of the East Coast know all too well in August and September, it's time to watch the Weather Channel to see if windows need to be boarded up and boats secured.

"In spite of years of study, it remains largely a matter of guesswork as to whether a particular tropical disturbance will become a hurricane," Professor Emanuel said.

The good news is that there is a limit to how ferocious hurricanes can get, and very few storms approach their worst possible intensity. By looking at atmospheric and ocean conditions, researchers can determine this limit but no one understands what keeps hurricanes from reaching their potential intensity.

Professor Emanuel said that there isn't enough information right now to predict how climate change will affect the frequency of tropical cyclones. But he expects that paleotempestology will provide new material to work with in the future.

A version of this article appeared in MIT Tech Talk on April 1, 1998.

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