CLIMATE CHANGE: A new study will fuel the debate on whether warmer ocean surfaces can be directly linked to more intense hurricanes, writes Bryn Nelson
Warmer ocean surfaces can be directly linked to a trend toward more intense hurricanes since 1970, according to a new study likely to fuel the debate over global warming and increasingly severe storms.
A statistical analysis by researchers at Georgia Institute of Technology in Atlanta found that the growing might of satellite-observed hurricanes in six ocean basins is strongly correlated with rising sea surface temperatures - not with other potential influences such as humidity, air circulation patterns or wind shear.
"It is an important study that increases our confidence that there is a link between global warming and intense tropical storms," said climatologist James Hansen of NASA's Goddard Institute for Space Studies in Manhattan.
Hansen, who was not involved in the research, said the result was logical "because an increase of ocean surface temperature provides more 'fuel' to drive strong storms". That fuel is the "latent heat" in water vapour, he said, "the same fuel that powers thunderstorms".
A higher sea surface temperature means more water can evaporate from the ocean and offer heat to a storm.
Hansen said global warming has also driven up ocean temperatures at intermediate depths, reducing the ability of this cooler water stirred up by a storm to check the hurricane's strength.
Another variable known as wind shear, in which high- and low-altitude winds move in opposing directions, can sap a hurricane's power by tearing the funnel's top from its bottom.
Although the new research found that wind shear can be an important factor on a seasonal basis, "there is no global trend in wind shear that you can associate with an increased intensity in category four and five hurricanes," said study co-author Judith Curry, chairwoman of the school of earth and atmospheric sciences at Georgia Institute of Technology.
She said the same was true for air circulation patterns and specific humidity, which relates to the amount of heat-laden moisture in the air.
Other researchers, however, doubt the global warming-hurricane link, and questioned the conclusions of the study, released on Thursday afternoon in an online version of the journal Science.
Chris Landsea, science and operations officer at a national hurricane centre in Miami, said the results conflict with those predicted by climate models, although he and others have argued in the past that real-world observations should take precedence. Nevertheless, he characterised the satellite data used to determine the intensity of storms as flawed.
In the Atlantic Ocean, he said, the data indicated an increase in intense storms, but only as part of a natural cycle instead of as a growing trend. The jump elsewhere around the world was probably an artificial one due to the unreliable database and the much enhanced ability to observe hurricanes.
Curry conceded that some hurricane data from the north Indian Ocean in the 1970s "is rather dodgy". However the study's conclusions were the same even when the data from that ocean basin were removed from the overall analysis.
She said researchers will need to better understand normal activity cycles in the oceans and warming trends "or no one's going to be able to make any sensible forecasts".