Published online 3 September 2008 | Nature | doi:10.1038/news.2008.1079

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Global warming blamed for growth in storm intensity.

Cyclone Nargis caused devastation earlier this year. Stocktrek Images, Inc. / Alamy

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.

“It'll be pretty hard now for anyone to claim that cyclone activity has not increased.” Judith Curry

Georgia Institute of Technology, Atlanta

One of the most contentious issues in the climate-change debate has been whether the strength, number and duration of tropical cyclones will increase in a warmer world. Basic physics and modelling studies do suggest that tropical storms will become more intense, because warmer oceans provide more energy that can be converted into cyclone wind. But others believe that atmospheric changes might have an inhibiting role. Increasing shearing winds - another predicted consequence of global warming - are thought to suppress the cyclonic rotation of the storms, for example.

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.

Feel the heat

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.

"It'll be pretty hard now for anyone to claim that cyclone activity has not increased," says Judith Curry, an atmospheric researcher at the Georgia Institute of Technology in Atlanta, who was not involved in the study.

Strongest storms matter most

Three years ago, Curry and her team calculated that category 4 and 5 storms have almost doubled in number and proportion since 19702. The study, published two weeks after Hurricane Katrina struck, was later criticized for using a mixture of data taken by various worldwide projects that used different protocols. The new analysis is instead based on a single set of wind-speed data inferred from infrared satellite imagery.

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."

The US National Weather Service's Climate Prediction Center predicts that 14–18 named storms and 3–6 major hurricanes will form this season. An average season has 11 named storms and 2 major hurricanes.

"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."