for National Geographic News
Hurricanes bring winds and slashing rains that flood streets, flatten homes, and leave survivors struggling to pick up the pieces. But has global warming given the storms an added punch, making the aftereffects more dreadful?
According to hurricane historian Jay Barnes of Pine Knoll Shores, North Carolina, ocean heat is the key ingredient for hurricane formation. More heat could "generate more storms and more intense hurricanes," he said.
Numerous studies in recent years have found no evidence that the number of hurricanes and their northwest Pacific Ocean cousins, typhoons, is increasing because of the rise in global temperatures.
But a new study in the journal Nature found that hurricanes and typhoons have become stronger and longer-lasting over the past 30 years. These upswings correlate with a rise in sea surface temperatures.
The duration and strength of hurricanes have increased by about 50 percent over the last three decades, according to study author Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology in Cambridge.
Emanuel's finding defies existing models for measuring storm strength. Current models suggest that the intensity of hurricanes and typhoons should increase by 5 percent for every 1ºC (1.8ºF) rise in sea surface temperature.
"We've had half a degree [Celsius] of warming, so that should have led to a 2.5 percent increase [in intensity], which is probably not detectable," Emanuel said. "What we've seen is somewhat bigger than that, and we don't really know why."
One possibility, Emanuel said, is that ocean temperatures may be increasing more quickly than atmospheric temperatures.
"When that happens we've shown theoretically you get an increase in the intensity of hurricanes," he said.
Anatomy of a Hurricane
According to Barnes, who has authored several books on U.S. hurricane history, the physics of hurricanes are complex and full of variables. "But the sun beating down on Earth is the primary thing that gets it going," he said.
Barnes explains in his book North Carolina's Hurricane History that the summer heat warms the ocean's surface and spurs evaporation. As heat and moisture rise into the atmosphere, billowing clouds, scattered showers, and thunderstorms form.
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