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Linking global warming to super storms

By Ben Sandilands
THE OBSERVER, CANBERRA
Sunday, Sep 11, 2005, Page 9

The ghosts of Galveston returned to haunt hurricane researchers this week, on the 105th anniversary of the Texan city's destruction by the only "super storm" to have rivalled Katrina's drowning of New Orleans.

Even in the research files of the Commonwealth Scientific and Industrial Research Organization's (CSIRO) Climate Change Risk and Impact Division in Melbourne, there are images of a lost city of immense beauty captured before the giant storm of Sept. 8, 1900 that turned it into piles of corpses and smashed timber.

Galveston never reclaimed is early glory, with nearby Houston taking over its role as a center of commerce and culture. Its streets were littered with between 6,000 and 8,000 dead, its ornate and fanciful 19th century architecture broken apart and blown into the Gulf of Mexico to become flotsam.

Also in the archives are records of Cyclone Mahina, of March 1899 that killed 400 people in Bathurst Bay, Queensland, three quarters of them aboard pearling luggers, and another 100 Aboriginal people living on the mainland.

It was the deadliest and most violent storm ever recorded in Australia, generating 10m high waves and 300kph winds, and outstripping the better known Cyclone Tracy disaster of Christmas Eve, 1974, which destroyed the northern port city of Darwin and killed 65 people.

Hurricanes -- or cyclones or typhoons as they are variously known -- start as low pressure cells that suck up the latent heat energy of dense moist air lying over areas of sea surface temperature of at least 26oC.

But because many of the oceanic storm generating areas now have unprecedentedly high surface temperatures, in the case of the Gulf of Mexico, over 30oC, the principal researcher at the division, Penny Whetton, has spent a week fielding media questions as to whether Katrina was caused by global warming.

"The short answer is we can't prove a link," Whetton says.

But she can explain why a link may well be established in the future.

"The data or signals from tropical storms is very hard to decipher," she says. "We have learned that they are at their most frequent north of the equator they are generally least active south of the equator, and vice versa."

"They also vary in frequency and intensity over very long cycles, of around 20-50 years depending on how you define them."

Whetton says there are four tests of climate change being a factor in violent tropical storms.

"They are: whether these storms are more intense, whether there is a change in their frequency, whether they are beginning to track further north or south of the equator, and whether they are starting to appear in new locations within the narrow latitudinal bands they generally occupy -- roughly 20 degrees north to 20 degrees south yet remaining clear of the actual equator," she says.

She says there is no clear signal of a change in frequency of hurricanes, either in terms of occurring more or less frequently, after taking into account normal cyclical variations.

There is no evidence of cyclones or hurricanes tracking further north or south into higher latitudes.

And with one exception, there has been no evidence of these storms developing over areas where the sea surface temperature has always been warm enough for their development, but prevailing higher level winds tear the hurricane apart before it can turn into the giant circular masses of rotating storm clouds so apparent in weather satellite images of their progress.

That exception was Brazil's first ever hurricane in March last year, which formed over a part of the South Atlantic where there are vast areas of warm water, but giant tropical storms had been unknown.

However the signal or data concerning increased intensity of cyclonic storms looked much stronger.

"There is diminishing room for doubting that in terms of wind velocities, depths of central low pressure in the eye of the hurricane and the discharge of destructive energy these storms are reaching new levels of intensity," Whetton remarks. "It is also a reasonable hypothesis that the elevation of sea surface temperatures in some way contributes to this."

Whetton says it was also possible that global warming was inhibiting the frequency of severe tropical storms, by creating more turbulence or shear in the upper atmosphere.

"We won't know that with certainty for a few decades at least," she says. "But these storms need a still upper atmosphere for the columns of rising moisture to grow into a rotating vortex of storm clouds."

The researchers in the Climate Change Risk and Impact Division believe that the realities of global warming will be confirmed much sooner by mechanisms other than extremely violent tropical storms.

These may already be detectable in declining rainfall on a global basis, offsets by increasingly severe localized flooding "events," and serious continent wide heatwaves, and the retreat of frost, snow and ice zones and the melting of areas of permafrost, liberating vast amounts of currently frozen carbon dioxide and methane greenhouse gases.

"It is much easier to measure these effects than the relationship between cyclones and warming sea surfaces," Whetton says.

The CSIRO's work is mainly aimed at preparing Australia for the consequences of global warming.

But for the time being, the images of Galveston, and the appalling scenes from New Orleans, cast a pall over Australia and the South Pacific, where the next cyclone season is only a few months away.
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