As residents of New Orleans prepare to return home and breathe a sigh of relief that Hurricane Gustav was less damaging than feared, new research published today in Nature [subscription] suggests that the strongest tropical cyclones will pick up speed in the coming decades.
Weighing in on the long-running and at times very stormy debate over whether and how warmer seas will affect the intensity and frequency of hurricanes is a team led by climatologist James Elsner of Florida State University.
Using a 25-year archive of satellite data, Elsner and colleagues derive wind speeds for tropical cyclones over the globe. They find that the maximum wind speeds reached by the strongest tropical cyclones increased from 1981-2006 in most ocean basins, with the greatest changes in the North Atlantic and Northern Indian Oceans.
There was no trend in the intensity of cyclones occuring over the South Pacific, however, and the upward trend observed over a couple of ocean regions was not statistically significant. The researchers also found no increase in either the frequency or average intensity of tropical cyclones over the globe.
The approach taken by Elsner and colleagues – looking at whether the most severe cyclones will hit a higher speed limit during their lifetime – is both novel and socially relevant, simply because the most severe storms do the most damage if they make landfall. Once tropical cyclones reach speeds of over 74mph, they are officially classified as hurricanes.
The real bone of contention within the scientific community has been whether hurricanes will become more intense and more frequent as a result of human-induced climate change. Elsner and colleagues steer well clear of linking the trend to global warming though – they can’t attribute cause as their study doesn’t investigate other factors such as cyclone origin and duration, proximity to land, El Niño conditions and solar activity.
However they note that the theoretical maximum intensity of a cyclone is directly related to the sea surface temperature (SST) below, all else being equal. From that assumption, they project that a 1°C rise in SST would increase the global frequency of strong cyclones from 13 to 17 cyclones (31%) per year. This is consistent with the heat engine theory of cyclone intensity, which says that a warmer ocean ultimately provides more energy to fuel the formation of tropical storms over the sea.
Another study, published in Geophysical Research Letters, also suggests that the north Indian Ocean is in for a rough ride. Scientists led by V. Brahmananda Rao at the Center for Weather Forecast and Climate Studies, in São Paulo, Brazil, say that the region, which is just beginning to recover from the impacts of Cyclone Nargis, is likely to see an increase in the frequency and intensity of summer tropical cyclones over this century.
My colleague Quirin has a news story on the Elsner paper over on Nature News.
IMAGE: NASA satellite image of Hurricane Gustav on September 1, 2008