THIS time last year, hurricane Katrina was bearing down on New Orleans. The tragedy that followed thrust the link between global warming and hurricanes into the public eye. This year, however, the hurricane season has begun tamely. So is global warming off the hook?
Not at all, say climate scientists. The first and obvious point is that weather is fickle. 鈥淕lobal warming gives you a baseline that everything is riding on, but it doesn鈥檛 mean that every year is going to be the same,鈥� says Kevin Trenberth of the National Center for Atmospheric Research (NCAR) in Boulder, Colorado.
Many factors influence hurricane formation and development. While warmer sea-surface temperatures are expected to increase the evaporation of moisture that fuels storms, other factors have the opposite effect: shear winds can disrupt the formation of a hurricane, and strong winds and cloud can temporarily cool surface waters in the eastern Atlantic, where hurricanes often form. Strong winds have kept sea-surface temperatures below average this year, which helps explain the relative calm so far.
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Yet by past standards, 鈥渃alm鈥� may not be the right word. 鈥淲e鈥檙e right about average in terms of numbers of named storms,鈥� says Greg Holland of NCAR. With September approaching, the season is only now getting into full swing. 鈥淔rom here on in, the numbers start to climb rapidly,鈥� he says.
And let鈥檚 not forget the storms in other parts of the world. While the beginning of the Atlantic hurricane season has been fairly uneventful, a string of typhoons in the Pacific has been pounding China and Japan, causing hundreds of deaths. Typhoon Saomai struck China early this month with winds exceeding 260 kilometres per hour, making it the strongest typhoon to make landfall in China since records began. 鈥淲hen you have a lot more action out in the Pacific, which is what is going on, it tends to have an adverse effect on Atlantic storms,鈥� Trenberth says. The two ocean basins are linked by global wind and water temperature patterns such as El Ni帽o. This year, El Ni帽o conditions are starting to emerge, and these encourage cyclone formation in the Pacific and disrupt storms in the Atlantic.
Whether global warming will bring stronger and more frequent hurricanes over the long term is still a matter of debate, but the weight of evidence appears to be shifting toward a real, if modest, effect. Some predictions point to a 5 to 10 per cent increase in storm intensity, while others say this forecast is too conservative and predict a greater increase in both the intensity and the number of major hurricanes in the decades to come.
Hurricanes 鈥� awesomely destructive, and they may be getting worse. Keep up with the latest in our continually updated special report.
