Analysis: Rising hurricane activity is focusing scientific opinion on global warming and on whether any link exists between the two, writes Miguel Bustillo
Is the rash of powerful Atlantic storms in recent years an ominous sign of global warming?
Although most mainstream hurricane scientists continue to be skeptical, linking global warming and heightened storm activity, the growing intensity of hurricanes - coupled with the continuing frequency of large storms - is leading some to rethink long-held views.
Most hurricane scientists continue to maintain that linking global warming to more frequent severe storms, such as Hurricane Katrina, is premature at best. Though warmer sea surface temperatures, caused by climate change, theoretically could boost the frequency and potency of hurricanes, they assert that the 150-year record of Atlantic storms shows ample precedent for the spate of recent events.
However, a paper published last month in the journal Nature by meteorologist Kerry Emanuel of Massachusetts Institute of Technology is part of an emerging body of research that has begun to challenge the prevailing view.
It concluded that the destructive power of hurricanes and tropical cyclones, the name for the same storms in the Pacific Ocean, had increased 50 per cent over the last half-century, and that a rise in surface temperatures linked to global warming was at least partly responsible.
"I was one of those skeptics myself, a year ago," Mr Emanuel said on Monday.
But, after examining aircraft monitoring data on tropical cyclones in the Pacific Ocean as well as hurricanes in the Atlantic, he said: "I was startled to see this upward trend" in the duration as well as the top wind speeds of the storms. "People are beginning to seriously wonder whether there is a [global warming] signal there. I think you are going to see a lot more of a focus on this in coming years."
Hurricane activity in the Atlantic has been higher than normal in nine out of the last 11 years, according to the National Oceanic and Atmospheric Administration (NOAA).
This month the NOAA raised its already high hurricane forecast for this year from 18 to 21 tropical storms, including up to 11 that would become hurricanes and five to seven that would reach major-hurricane status.
That could make 2005 one of the most violent hurricane seasons ever recorded. A typical storm year in the Atlantic results in six hurricanes.
But the agency believes that the increase in hurricanes is most likely the result of a natural confluence of cyclical ocean and atmospheric conditions that tend to produce heightened tropical storms every 20 to 30 years.
If global warming is playing any role in the hurricanes, it is a minor one, the federal agency maintains.
Computer models have shown for years that rising sea-surface temperatures, resulting from global warming, could create more ideal conditions for hurricanes.
Yet before Mr Emanuel's research there were few indications that actual hurricanes had become stronger or more frequent, despite well-documented increases in surface values.
Moreover, skeptical hurricane scientists were quick to point out that worldwide weather records are inadequate for a thorough examination of such trends.
They say that would require an analysis of storm activity going back hundreds if not thousands of years.
"There is absolutely no empirical evidence. The people who have a bias in favour of the argument that humans are making the globe warmer will push any data that suggests that humans are making hurricanes worse, but it just isn't so," said William Gray, a Colorado State University meteorologist who is considered one of the fathers of modern tropical cyclone science and who questions Mr Emanuel's conclusions.
"A lot of my colleagues who have been around a long time are very skeptical of this idea that global warming is leading to more frequent or intense storms. In the Atlantic, there has been a change recently, sure . . . I can't say there is no human signal there, but it's minute."
Nonetheless, some scientists have maintained that the rise in mean global temperatures over the last 50 years - a well-documented trend widely linked to human activities such as the burning of fossil fuels - will inevitably have an effect on storms, if it has not already done so.
"It's the ocean temperatures and sea surface temperatures that provide the fuel for hurricanes," said Kevin Trenberth, a weather scientist at the National Centre for Atmospheric Research, who recently published a paper in the journal Science contending that climate change was causing hurricanes to produce more rain, thereby becoming more dangerous.
Such views remain highly controversial among veteran hurricane scientists.
Last year, in fact, a leading hurricane expert with the NOAA, Chris Landsea, withdrew from the Intergovernmental Panel on Climate Change (IPCC) - an international scientific group that periodically sums up the consensus on global warming.
Mr Landsea said in a letter to scientific colleagues that he resigned because he strongly disagreed with public statements made by Mr Trenberth, who was also part of the IPCC, suggesting that last year's Atlantic hurricanes were linked to global warming.