Page last updated at 16:42 GMT, Monday, 8 May 2006 17:42 UK

Storm debate swirls ahead of season

By Molly Bentley

Eye of a hurricane (Eumetsat)
The forecast is for another very active season
With the start of the Atlantic hurricane season less than a month away, and the coastal United States and Caribbean braced for another sequence of powerful storms, debate continues to roil over whether human activity is intensifying these weather events.

The subject was taken up at a panel discussion at the recent meeting of the American Meteorological Society (AMS) in Monterey, California, after a paper last year in the journal Nature linked ocean warming to more intense hurricanes.

Other scientists say it is premature to make a link between hurricane activity and global warming.

The debate has been contentious - even nasty - in the time since Kerry Emanuel, a professor of atmospheric sciences at Massachusetts Institute of Technology published his paper.

The AMS moderator cautioned the panellists against personal attacks; but the discussion was civil, albeit vigorous.

It has particular urgency. After a record-breaking hurricane season last year, this year's Atlantic hurricane forecast, from Colorado State University, predicts another very active season and a higher than average likelihood of landfalls, or storms coming ashore, on the US coast and the Caribbean.

Nature's power

Some facts at the conference were not in dispute: sea surface temperature has risen by 0.4C in the tropics and subtropics since 1970, and hurricane activity in recent decades has heightened.

The question is whether the two are causally linked. Dr Emanuel stood by his original finding: hurricane energy is closely tied to ocean temperature.

"It turns out to be an astoundingly high correlation," said Dr Emanuel, who surveyed thousands of hurricanes going back decades.

We agree the potential is there for hurricanes to get worse due to man-made global warming, but the big issue is by how much
Christopher Landsea, Noaa
Hurricane frequency has not changed; but, Dr Emanuel says, their power has doubled in the last 30 years as sea surface temperatures have crept up.

"The fact is, even if you go back to the 19th Century, this particular metric remains very well correlated with ocean temperature," he said.

And Peter Webster, from the Georgia Institute of Technology, and co-author of a paper that appeared last year, said that his team found a "substantial increase" in the intensity of tropical storms for the period over which ocean temperature had increased and that the major storms were lingering longer.

If the studies are correct, they would suggest that the destructive force of Hurricane Katrina, which ravaged New Orleans in 2005, was intensified by human-induced climate change.

Watching trends

However, Christopher Landsea, of the National Oceanic and Atmospheric Administration (Noaa) National Hurricane Center in Florida, says the results are ambiguous.

"We agree the potential is there for hurricanes to get worse due to man-made global warming, but the big issue is by how much," he said.

Computer models suggest that end-of-the-century hurricanes would be worse in intensity by 5%, he said, which translates to 1% stronger today.

Hurricane schematic (BBC)
A hurricane is a spinning vortex of winds swirling round a eye of very low pressure
Warm, moist air is drawn upwards around the eye
Cooler dry air is sucked downwards by the low pressure centre
Banks of thunderstorms surround the edges
"It's so small, you couldn't measure the change," said Dr Landsea. "In other words, it's hard to distinguish between winds whipping by at 200km/hour from those at 202km/hour."

In addition, the unreliability of past hurricane data made it difficult to pull out accurate trends, he said.

The recent advent of global positioning satellites and sophisticated observation methods has allowed scientists to measure hurricane onset and strength more precisely, which means hurricanes from just 20 years ago may not have been recognised for what they were.

By combining old satellite images, scientists had found that many hurricanes were stronger than indicated at the time, said Dr Landsea, and a re-analysis of past data might smooth out an apparent trend toward more powerful storms.

That trend might also be explained by fickleness in Atlantic hurricane conditions. The Atlantic Multidecadal Oscillation (AMO) was a natural cycle, not well understood, between busy and quite hurricane periods, said Dr Landsea.

It includes fluctuations in water temperature and wind shear, a mid- to upper-level wind feature that can disrupt hurricane formation.

"In the 70s, 80s and 90s, when we had a lack of hurricane activity, we saw cooler waters and more shear," said Dr Landsea. "Go back to late 1920s and late 1960s, busy times for Atlantic hurricanes, and the waters were warm and the shear was a lot lower."

Debating 'relevance'

But Dr Emanuel dismissed the idea of natural cycles in sea surface temperatures in the Atlantic and maintained that they are largely due to anthropogenic - human-induced - forcing.

"There is no evidence for natural oscillation in later summer tropical Atlantic sea surface temperatures on these time scales," he said.

Ken Caldeira, from the Carnegie Institution Department of Global Ecology at Stanford University in California, who did not attend the conference, believes that the warming of the oceans is not sufficient to explain the increase in hurricane intensity inferred in Dr Emanuel's paper.

"But I see that as irrelevant because we know that with added greenhouse gases, sea temperatures will go up and that will provide more fuel for hurricanes," said Dr Caldeira, on the day that Noaa issued its annual greenhouse gas index, showing a continuing, steady rise in heat-trapping gases in the atmosphere.

"The prudent assumption is that the data might reflect what could happen in the future," he said.

The Colorado State University's respected hurricane forecasting team has warned of another very active season ahead - although it says there are likely to be fewer intense hurricanes making landfall than in 2005.

The team's forecast anticipates 17 named storms will form in the Atlantic basin between 1 June and 30 November. Nine of the 17 storms are predicted to become hurricanes, and of those nine, five are expected to develop into intense or major hurricanes (a category 3, 4, or 5 on the Saffir/Simpson Scale) with sustained winds of 178km/h (111mph) or greater.

The 2005 season witnessed 27 named storms, 15 hurricanes and seven intense hurricanes.

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