Winds of change: storms are getting stormier, though no one knows why.© PunchstockThere are fewer hurricanes today than there were a decade ago, but they are stronger and more destructive.
That's the conclusion of a paper in this week's Science1, which looks at satellite images of storms from 1970 to 2004. But it is not known whether the changes are associated with global warming says the research team, led by Peter Webster of the Georgia Institute of Technology in Atlanta.
With the exception of the North Atlantic, the number of storms and stormy days has recently decreased in all oceans warm enough to form hurricanes, or typhoons as they are called in the Pacific. Globally, the number of stormy days per year reached a peak of 870 days in 1995; it has since decreased by 25% to around 600.
“There must be a complex natural mechanism at play, which we don't yet fully understand.”
Judy Curry
Georgia Tech
The number of storms that reached the highest categories 4 and 5, which have wind speeds of 200 kilometres per hour or more, has almost doubled over the past 35 years, the team found. In the 1970s, an average of ten category-4 and -5 storms occurred each year. The average number of such extreme storms has since risen to 18, although only a fraction of these eventually hit land.
These results agree with recent evidence that hurricanes have become significantly more destructive over the past decade.
The financial loss that a storm causes is roughly proportional to the cube of its wind speed, and Kerry Emanuel, a hurricane researcher at the Massachusetts Institute of Technology in Cambridge, Massachusetts, has used this to calculate a 'power dissipation index'. In a study published last month in Nature2, he showed how this index has sharply increased since 1990.
"They have looked at the problem through a different lens," says Emanuel. "I think these studies nicely complement each other."
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Sea surface temperatures are the dominant variable in hurricane formation. Webster says that a 0.5 ºC rise between 1970 and 2004 in sea surface temperatures is most likely to be an effect of global warming.
But scientists are puzzled as to why, globally, warmer oceans do not lead to uniformly changed hurricane patterns. The shift towards stronger storms has been much less pronounced in the North Atlantic, for example, than in other ocean basins.
"There must be a complex natural mechanism at play that we don't yet fully understand," says Judy Curry, a hurricane researcher at Georgia Tech and co-author of the Science study. "Different ocean basins seem to display different patterns of decadal natural variability. But with so few long-term cycles to look at, it is hard to pin down the differences."
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The relationship between global warming and hurricane behaviour is hotly debated (see 'Hurricane seasons warm up'). Some scientists argue that links must exist; others say warming influences must be tiny when compared with natural swings.
In either case, says Christopher Landsea, a meteorologist with the US National Hurricane Center in Miami, the dramatic increase in economic losses from storms is mostly due to societal changes in hurricane-prone regions.
Georgia Tech
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