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Hotter Oceans, Fiercer Storms

Hotter Oceans, Fiercer Storms


Dr. Peter Webster, Georgia Institute of Technology

Read Peter Webster's explanation of what the data worldwide are showing, excerpted from his interview with NOVA scienceNOW. Then take a look at data he and colleagues published in a September 2005 paper in Science entitled "Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming Environment."

Webster: The bottom line of our study is that we found a consistency between the increase of surface temperature in all of the oceans and a change in intensity to more intense storms. Now, if it were natural variability, what one would expect to find is that the variation would be different in each of the basins. I think the first conclusion, perhaps the most important one, is that the characteristic change to more intense hurricanes is a global phenomenon and not a local phenomenon.

Kerry Emanuel went back into the data further than we did, roughly from 1950 onwards, both in the Atlantic Ocean and the North Pacific Ocean, and he found a very, very strong association between intensity of storms and the increase of sea surface temperature, much the same as we did. We also looked at the other ocean basins, the South Pacific and the South Indian Ocean. The Southern Hemisphere accounts for roughly 30 percent of all the storms, so we wanted to make sure we were doing a global study. And these two different studies using some common overlap data but different regions tend to support each other quite well.

Webster 1

Sea Surface Temperature
Since 1970, sea surface temperature (SST) in tropical regions of the world's oceans has been on the rise. While the average rise, roughly 0.5ºC, may seem slight, this extra heat in trillions of tons of water amounts to a vast reserve of energy. And it is well known that hurricanes draw on warm-water energy to gain strength. The graph above charts SST in particular ocean basins during their tropical cyclone seasons.

Webster 2a

Webster 2b

Global Number and Duration of Storms
Has the global rise in sea surface temperature spawned more hurricanes and tropical storms worldwide or increased the total number of storm days? Webster and his colleagues conclude not. They found no long-term upward trend over the past 35 years, only natural ups and downs. In the graphs above and below, the thin lines chart year-by-year variability; the bold lines show the five-year running averages.

Webster 3a

Webster 3b

Regional Number and Duration of Storms
Looking at specific ocean basins, Webster's team again doesn't see long-term upward swings in the annual frequency of hurricanes and total number of storm days. The exception, though, is the North Atlantic, where storm numbers and durations have surged since 1995. Some researchers have speculated that this increase is due to global warming, but Webster doesn't think such a simple attribution is warranted. If global warming and related rising sea surface temperature alone led to more hurricanes and storm days, then one would expect to see an upward trend in other ocean basins.

Webster 4a
Webster 4b

Hurricane Intensity
Hurricane intensity—whether a storm ranks as a Category 1, 2, 3, 4, or 5—is based on wind speed. As the graphs above show, the number of Category 4 and 5 storms has nearly doubled in the past 35 years. This upward trend occurs in all of the ocean basins Webster's team examined. There has not been a change in the maximum wind speed of the most intense storms (black line). Yet overall, hurricanes have indeed grown more intense—and potentially destructive—in recent decades. (Note: The horizontal dashed lines in both graphs show 1970-2004 averages.)

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