Study: As Oceans Warm, Cyclones Gain Strength

Tropical cyclones have been getting stronger over the past several decades, according to a new report in the journal Nature. This finding supports a theory that storms will get stronger as the surface of the ocean heats up because of global warming.

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Right now, four big storms are stretched out from the southern United States to the Eastern Atlantic - Gustav, Hanna, Ike and Josephine.

And you might be wondering whether global warming has anything to do with this. Scientists dont see a link between climate change and the total number of storms.

But as NPRs Richard Harris reports, theres new evidence that the strongest storms are getting even stronger.

RICHARD HARRIS: Hurricane scientists have a lively disagreement about the role of global warming and storms. There have been studies showing that storms in certain regions, like the Atlantic Ocean, have been getting more intense.

But if the cause is global warming, the trend should be global. And James Elsner at Florida State University says thats been tough to show.

Professor JAMES ELSNER (Geography, Florida State University): I think the evidence in the Atlantic, kind of being the canary and the coal mine, has been shown before. But I dont think anyone has looked at it globally and been consistent in showing a trend.

HARRIS: One problem is theres no long and reliable record of big storms from all around the world. Elsner and his colleagues did find a record that stretches back to 1981, at least. They figured out how to infer top wind speeds by looking at satellite images of storms.

In fact, they looked at every big storm between 1981 and 2006. And they now report in Nature magazine that the highest winds in the strongest storms have been getting more intense over time.

Prof. ELSNER: We looked at six of the ocean basins, the six big basins where this tropical cycle had formed, and we saw that in five of the six basins, it was a fairly consistent signal across the whole group.

HARRIS: This makes sense to Elsner. Storms like this get their energy from warm ocean waters and the ocean surface has been getting gradually warmer.

Prof. ELSNER: With all else being equal, it stands to reason with warmer waters, wed have stronger storms.

HARRIS: And climate forecast project that the sea surface temperature will continue to climb as a result of human-induced global warming.

Prof. ELSNER: We should expect to see the strongest storms getting stronger.

HARRIS: Thats a point of contention, though. Some hurricane scientists say wind patterns could also change over time and those could end up breaking up big storms, counteracting the effects of warmer seas.

So Roger Pielke Jr. at the University of Colorado says this new paper in Nature wont settle the debate over climate change and hurricanes.

Professor ROGER PIELKE JR. (Environmental Studies, University of Colorado): Dont put too much weight on the latest and greatest study, as good as it is, because it adds to a very large and growing body of knowledge that fails yet to paint a clear picture.

HARRIS: One problem is hurricanes seem to have a natural cycle. Our Atlantic Coast, for example, experienced a major low throughout the 1970s, 80s and 90s. So its tricky to say how much of the trend since then is a result of climate change and how much is natural cycle. But Pielke says that doesnt really matter in some ways.

Prof. PIELKE JR.: If youre in the Louisiana or in the Gulf Coast or East Florida, the East Coast of the U.S., the exact nature and intensity of storms is much less important than the reality that you will be hit, may not be this year, may not be next year, but you will be hit. So, being well prepared makes sense regardless of the trends that scientists ultimately find.

HARRIS: And by the way, Hurricane Gustav did end up with maximum winds that at one point topped 150 miles an hour. That makes it another extreme storm by James Elsners reckoning.

Hannas winds are building even now, as it continues to move across the warm waters of the tropical Atlantic.

Richard Harris, NPR News.

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