Peter Webster of the Georgia Institute of Technology and his colleagues analyzed hurricane data collected between 1970 and 2004 around the world. Studying the number, duration and the intensity of the storms, the researchers determined that the number of days with tropical cyclone activity on an annual basis has been declining since a peak in 1995. At the same time, the strength of the storms demonstrate an upswing. "In the 1970s, there was an average of about 10 Category 4 and 5 storms hurricanes per year globally," Webster notes. "Since 1990, the number of Category 4 and 5 hurricanes has almost doubled, averaging 18 per year globally."
Because tropical storms draw energy from ocean water to gain strength, it has been hypothesized that global warming--and the warmer waters associated with it--could lead to stronger hurricanes. Thus the scientists also compared hurricane data to sea surface temperature (SST) measurements around the world. "Our work is consistent with the concept that there is a relationship between increasing sea surface temperature and hurricane intensity," Webster says. "However, it is not a simple relationship. In fact, it's difficult to explain why the total number of hurricanes and their longevity has decreased during the last decade, when sea surface temperatures have risen the most." In their report, the authors conclude that before the observed trend can be attributed to global warming both a longer global data record and a better understanding of how hurricanes fit into general atmospheric and oceanic circulation trends is required. "Without this understanding," Webster remarks, "a forecast of the number and intensity of tropical storms in a future warmer world would be merely statistical extrapolation."