IT SEEMED obvious. Hurricanes form over the oceans when sea surface temperatures exceed about 26 掳C. So as the oceans warm, there should be more hurricanes, and we should be battening down the hatches from Tampa to Taipei.
Now things are not so clear. That 26 掳C rule assumes other things are equal 鈥� and they never are. This week a major modelling study forecast that warming of the north Atlantic could make hurricanes scarcer 鈥� while the worst ones might have stronger winds and produce more rain.
Thomas Knutson and colleagues from NASA鈥檚 Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, have previously produced a remarkably accurate year-by-year 鈥渉indcast鈥� of hurricane numbers over the past 30 years. So their prediction of an 18 per cent decline in the annual hurricane count by late this century commands attention.
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What鈥檚 more, Knutson鈥檚 team have a clear idea about why their numbers come out the way they do. What matters in hurricane formation, they say, is not so much the sea temperature itself as the temperature difference between the sea and the top of the troposphere, the atmosphere鈥檚 weather zone. Knutson says the upper troposphere is set to warm even more than the Atlantic surface in the coming decades, and that this will tend to suppress hurricane formation.
The recent increase in north Atlantic cyclones is largely a result of unusual warming in Atlantic waters compared to other oceans and the upper atmosphere, he says. But this won鈥檛 last.
Knutson鈥檚 prediction comes from one of the first modelling studies detailed enough to get a handle on hurricanes. But the model has its limitations. It still can鈥檛 reproduce the big hurricanes, which makes his prediction that the most intense storms could worsen especially speculative. It also comes amidst growing doubts over local forecasts based on global warming predictions. 鈥淐limate models do a lousy job in the tropics,鈥� says Kevin Trenberth at the US National Center for Atmospheric Research in Boulder, Colorado. He doubts Knutson鈥檚 forecast, because known flaws in climate models lead to an underestimate of the frequency of hurricanes.
He also warns the prediction is specific for the north Atlantic. If, as predicted, the tropical Pacific slips into a near-permanent El Ni帽o state, then hurricanes will become more likely in every ocean except the Atlantic 鈥� so hold onto your hats in Hanoi.
