Contrary to the widespread view that a warming world will bring more hurricanes, a controversial new study suggests the number of cyclones could actually drop in the North Atlantic.
Hurricanes have become a lightning rod for arguments over what global warming might have in store. Most researchers agree that, since 1950, the number of hurricanes forming over the Atlantic has increased, and that since at least since 1980, they have become fiercer.
Many studies have blamed the increase in Atlantic hurricanes on increasing sea-surface temperatures, which fuel the storms. However, other ocean basins have not seen a similar increase in hurricanes.
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Storm predictions
and colleagues at the US National Oceanic and Atmospheric Administration鈥檚 Geophysical Fluid Dynamics Laboratory in Princeton, US, built a computer model to simulate the formation of North Atlantic hurricanes, based on observed sea-surface temperatures and atmospheric conditions.
Tropical storms and hurricanes formed spontaneously in the model, following much the same tracks as real storms do.
They also found that the model predicted fairly closely the number of North Atlantic hurricanes over the past 25 years 鈥� with 162 predicted storms, as opposed to 143 recorded. The strength of the most severe storms was not predicted, however.
Next they fed the model predictions of how Atlantic waters are likely to warm with climate change.
These were based on an ensemble of climate models in which emissions rise until mid-century, then start to taper off 鈥� an optimistic scenario for future carbon-dioxide emissions. Though emissions drop off, the ocean would continue warming through the end of the century and beyond.
Fewer hurricanes
Surprisingly, the model simulated fewer tropical storms as the ocean warmed, and fewer hurricanes. By the end of this century, it predicted, the Atlantic will see a drop in tropical storms of 27%, and a drop in hurricanes of 18%.
鈥淭his study does not support the idea there is a large increasing trend [in hurricanes] due to greenhouse warming,鈥� says Knutson.
The model also predicted that the fewer hurricanes that do emerge will become slightly stronger on average, but Knutson is cautious about this finding. 鈥淲e don鈥檛 want to take it quantitatively too seriously because it鈥檚 not simulating stronger storms very well,鈥� he says.
Nonetheless, he adds, 鈥渋t gives more support to the idea that in the tropics in general, hurricanes will be slightly more intense and have more rainfall.鈥�
Other researchers are sceptical about drawing any big conclusions from the study, pointing to the fact that the model did not produce any really strong hurricanes, of category four or five, in either the 20th century or in the coming century.
Low resolution
This 鈥渋s a major shortcoming,鈥� says of the National Center for Atmospheric Research in Boulder, Colorado, US.
鈥淚n this study there is a suggestion that the strongest storms increase, but the model never simulates them in the first place,鈥� Trenberth says. 鈥淚t could be grossly in error for the strongest storms, which are the ones that do most damage.鈥�
of the Massachusetts Institute of Technology in Cambridge, US agrees. 鈥淢y only reservation is that the model鈥檚 spatial resolution is still too low to allow for intense hurricanes,鈥� he says.
In a with higher resolution, but simpler models, Emanuel and colleagues also found an increase in hurricanes鈥� intensities in the northern hemisphere.
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