Severe hurricanes increasing

The study, published today in the journal Science, is the second in six weeks to draw this conclusion, but other climatologists dispute the findings and argue that recent severe storms reflect nothing more than normal weather variability.

Katrina’s destructiveness has given a sharp new edge to the ongoing debate over whether the United States should do more to curb greenhouse gas emissions linked to global warming. Domestic and European critics have pointed to Katrina as a reason to take action, while sceptics say climate activists are capitalising on a national disaster to further their own agenda.

According to data gathered by researchers at the School of Earth and Atmospheric Sciences at Georgia Tech and the National Centre for Atmospheric Research, the number of major Category 4 and 5 hurricanes worldwide has nearly doubled over the past 35 years, even though the total number of hurricanes, including weaker ones, has dropped since the 1990s. Katrina was a Category 4 storm when it made landfall.

Georgia Tech atmospheric scientist Judith A Curry — co-author of the study with colleagues Peter J Webster and Hai-Ru Chang, and NCAR’s Greg J Holland — said in an interview that their survey, coupled with computer models and scientists’ understanding of how hurricanes work, has given the researchers a better sense of how rising sea temperatures are linked to more-intense storms.

"There is increasing confidence, as the result of our study, that there’s some level of greenhouse warming in what we’re seeing," Curry said. "Is it the whole story? We don’t know."

Higher ocean temperatures result in more water vapour in the air, which, combined with certain wind patterns, helps power stronger hurricanes, Webster said. Small increases in sea temperature, he added, can "exponentially provide more and more fuel for the hurricanes."

Other studies and computer models also have pointed to an increase in storm intensity: Massachusetts Institute of Technology atmospheric scientist Kerry A Emanuel wrote last month in the journal Nature that the duration and maximum wind speeds of storms in the North Atlantic and North Pacific have increased about 50 percent since the mid-1970s. The storms’ growing violence stemmed in part from higher ocean temperatures, he concluded.

Some researchers, however, question the connection with more severe hurricanes and cyclones. Gerry Bell, the lead seasonal hurricane forecaster at the National Oceanic and Atmospheric Administration, said the rise in strong hurricanes reflects a natural weather pattern spanning several decades. Hurricanes in the Atlantic Ocean were more powerful in the 1950s and ’60s, weakened in the 1970s, ’80s and early ’90s, and have strengthened again since 1995.

"It’s not linked to global warming or anything like that," Bell said. "This is normal climate variability. It’s just that this trend lasts for decades."