Scientists question link between hurricanes and climate change

Global warming may have made Hurricane Gustav slightly stronger and wetter, some scientists said today, but few were rushing to make a specific connection between climate change and stronger storms.

The Atlantic is seeing an increase in storms rated among the strongest. In the past four years, Hurricanes Gustav and Katrina, and six other storms have reached Category four or higher with sustained winds of at least 131 mph, according to research at America’s Georgia Tech.

Six scientists said this showed some effect of global warming, but they differed on the size of the effect.

“We are just seeing a lot more Categories 4 and 5 globally than we have ever seen,” said Judith Curry, chairman of Earth and Atmospheric Sciences at Georgia Tech. “The years 2004, 2005 and 2007 are quite high. We’re just seeing more and more.”

Measurements of the energy pumped into the air from the warm waters - essentially fuel for hurricanes – has increased dramatically since the mid 1990s, mostly in the strongest of hurricanes, according to a soon-to-be published paper in the journal Geochemistry, Geophysics, Geosystems by Kevin Trenberth, climate analysis chief at National Centre for Atmospheric Research in Boulder, Colorado.

However, the same scientists caution it is impossible to blame global warming for any single weather event and that some form of Gustav (and other hurricanes) would have likely still formed and turned deadly without man-made climate change.

Yet the fingerprint of global warming on the strongest storms is becoming clearer with new research, scientists said, including Gustav, which reached Category four status on Saturday before weakening.

“The strongest storms are expected to be stronger,” said Gabriel Vecchi, a research oceanographer for a National Oceanic and Atmospheric Administration lab in Princeton, New Jersey. “And since Gustav is a very strong storm, you’d expect Gustav to have had an effect from human-induced global warming.”

He said he could not tell how much effect it had, which made him "uncomfortable as a scientist". Mr Trenberth calculated, in an earlier journal article, that major storms like Katrina and Gustav probably have increased their rainfall by about 6 to 8% because of global warming.

Warmer water makes the surface air warmer, which means it could contain more moisture. That means more hot moist air rises up the hurricane, serving as both fuel for the storm and extra rainfall coming back down, said Peter Webster, professor of atmospheric sciences at Georgia Tech.

For several years scientists have traded papers and criticisms about the effect global warming has already had – if any – on hurricanes. Some scientists, such as Christopher Landsea at the National Hurricane Centre, have faulted the quality of storm numbers and the length of time used for historical study used by others to connect to hurricanes to global warming.

“Yes, climate change is impacting hurricanes,” Mr Landsea said. But the effect on storm intensity now is “very small,” something that can’t be noticed in a storm so big, he said.

Hugh Willoughby, a former government hurricane research director and now professor of meteorology at Florida International University in Miami, is not quite as convinced. However he said a consensus seems to be forming on a global warming effect on just the strongest of hurricanes. But he said he thought others were exaggerating the effects.

Hurricane activity cycles – where about every 25 years a lot of storms form followed by another quarter-century of fewer hurricanes – plays a bigger role than global warming, he said.

“We have a real effect due to climate change. But the dominant effect in my mind is just bad luck.”

He added that Gustav probably has little or no climate change effect to it because it looks just like similar storms from decades and even centuries ago.