The basic physics is simple enough. A storm gets its potential energy from the ocean, and the warmer the ocean is, the more energy the storm should be able to draw on. In studying the way that tropical storms in turn affect ocean currents, Emanuel developed a measure, or metric, of the power released by a storm over its lifetime. As he played with the data he discovered a surprisingly tight match between the surface temperature of the Atlantic Ocean and the intensity of storms that had brewed atop it. "The thing that really struck me is how beautifully this metric is correlated with sea surface temperature," he observes. What is more, according to his measure, storms in the Atlantic and western North Pacific were 40 to 50 percent more powerful in the last 20 years compared to the previous 20.
The finding was quickly corroborated by another method. Six weeks after Emanuel's report, Peter Webster of the Georgia Institute of Technology and his colleagues reported that, based on satellite measurements of tropical storm intensity, the two most powerful categories of storm had become nearly 60 percent more frequent during the same time period Emanuel had considered. (Overall storm frequency was unchanged.)
Storm clouds have since gathered. The increase that Emanuel and Webster observed is six times the magnitude predicted by Emanuel's own model of hurricane formation, notes Chris Landsea of the National Hurricane Center in Miami. "Everyone agrees the ocean temperatures have been warming," he says, "so either the theory's wrong or the observation's wrong, or they're both wrong." In a recent paper in the journal Nature, Landsea argues that changes in hurricane windspeed measurement techniques vitiate the global warming link. He advocates a storm-by-storm reanalysis of past measurements. Emanuel is less optimistic about that approach. "We have probably milked the existing hurricane data for what we can do with it," he contends. In his view, the problem boils down to resolution: Just as global warming can be masked by shorter-term ups and downs in temperature, so can the link to hurricanes be hard to spot in the short term.
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