Thursday, September 4, 2008

Hurricanes are getting fiercer

Nature: As this year's Atlantic hurricane season becomes ever more violent, scientists have come up with the firmest evidence so far that global warming will significantly increase the intensity of the most extreme storms worldwide. The maximum wind speeds of the strongest tropical cyclones have increased significantly since 1981, according to research published in Nature this week1. And the upward trend, thought to be driven by rising ocean temperatures, is unlikely to stop at any time soon.

In May 2008, Cyclone Nargis killed more than 100,000 people in southern Myanmar. New Orleans, where Hurricane Katrina wrought havoc in 2005, was luckily spared another flood disaster this week as Hurricane Gustav had weakened by the time it hit the coast of Louisiana.

….James Elsner, a climatologist at Florida State University in Tallahassee, and his colleagues have now found that the strongest tropical storms are getting stronger, with the most notable increases in the North Atlantic and northern Indian oceans. Very strong storms, Elsner says, can more easily overcome any inhibiting effects of shearing winds than weaker storms, and go on to reach their maximum possible strength.

The team statistically analysed satellite-derived data of cyclone wind speeds. Although there was hardly any increase in the average number or intensity of all storms, the team found a significant shift in distribution towards stronger storms that wreak the greatest havoc. This meant that, overall, there were more storms with a maximum wind speed exceeding 210 kilometres per hour (category 4 and 5 storms on the Saffir–Simpson scale).

Rising ocean temperatures are thought to be the main cause of the observed shift. The team calculates that a 1 ºC increase in sea-surface temperatures would result in a 31% increase in the global frequency of category 4 and 5 storms per year: from 13 of those storms to 17. Since 1970, the tropical oceans have warmed on average by around 0.5 ºC. Computer models suggest they may warm by a further 2 ºC by 2100.

…The results, says Peter Webster, a hurricane expert also at the Georgia Institute of Technology, add urgency to the need to find ways of improving forecasting and warning systems, particularly for poorer countries. "A warning lead time of two days may be long enough here, but it is clearly not long enough in Myanmar or Bangladesh," he says. "Communicating more accurate forecasts to people in coastal areas more quickly can reduce the death toll enormously."

…"People should now stop saying 'who cares, storm activity is just a few per cent up'," says Curry. "It's the strongest storms that matter most."

Tropical cyclone Nargis making landfall, NASA, Wikimedia Commons

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