Global warming Katrina ignites scientific storm over hurricane trends

The raw fuel for tropical storms is a warm sea, so experts have long speculated that global warming, by driving up ocean surface temperatures, is bound to boost hurricanes and their Asian cousins, typhoons. That debate was largely framed on what might happen 30 or 40 years from now or even longer. Today, thanks to the shock of Katrina and a slew of new studies, the focus has shifted abruptly from the middle distance to the here and now. Has the feared link between warming and storms already kicked in? Many scientists urge caution. They say hurricane hyperactivity, as was the case in 2004 and again in 2005, occurs in what appears to be decadal cycles and argue the meteorological data is just too recent or too sketchy to venture a firm conclusion. Others, though, offer long-term evidence for saying Atlantic storms have become progressively worse in the last few decades. And, they note, this increase also tracks a rise in emissions of carbon dioxide (CO2) gas from fossil fuels, which traps solar heat and thus warms the seas. "We have known since 1987 that the intensity of hurricanes is related to surface sea temperature and we know that, over the last 15 to 20 years, surface sea temperatures in these regions have increased by half a degree Centigrade (0.9 F)," the British government's chief scientist, David King, says. "So it is easy to conclude that the increased intensity of hurricanes is associated with global warming." One of the most respected voices in the debate is a Massachusetts Institute of Technology (MIT) professor, Kerry Emanuel, who began to warn of a link between hurricane intensity and global warming way back in 1987. According to Emanuel's calculations, the power of tropical cyclones has roughly doubled since the 1950s. His measurement, called a power dissipation index, is based on storm intensity and duration. He notes a significant rise over the last 30 decades -- when Earth's surface temperatures began to rise significantly -- but especially since 1995, when global mean temperatures began to scale ever-higher annual peaks. "The large upswing in the last decade is unprecedented and probably reflects the effects of global warming," says Emanuel. Peter Webster of the Georgia Institute of Technology and the US National Center for Atmospheric Research calculates that category 4 and 5 hurricanes -- the two most powerful storm ratings -- have nearly doubled in number over the past three decades.




Critics point their guns at what they say is the weakness of historical data. US meteorologist Philip Klotzbach of Colorado State University argues that satellite records prior to 1984 are unreliable. He sees an increase in post-1984 hurricane frequency but says it is largely an illusion, prompted by the simple fact that better measuring tools have become available. That opinion is shared by Chris Landsea of the US National Hurricane Center, who notes that, for instance, in 1975, only two geostationary satellites were available for monitoring hurricanes. Today, the figure is 10, providing a much better view of storms as they gather at sea and a far better imaging capability. With this more accurate view, forecasters today may well assign top-scale wind speeds to a storm that, a few decades ago, they would have been reluctant to categorise as a 4 or 5, he argues. Landsea, whose co-authored paper appeared in July in the journal Science, acknowledges that global warming could be boosting hurricane winds, but only by one or two percent, which is far lower than Emanuel's estimate. For other experts, the question as to whether hurricanes are already worse today compared with yesterday is less urgent than the fate of coastal megacities that are already badly exposed to storms -- and will become terrifyingly vulnerable if the global-warming doomsters prove right. Robert Nicholls, an environmental researcher at Britain's University of Southampton, says New Orleans was a case study in failure -- of building a city below sea level while simultaneously destroying coastal marshlands that would have helped to protect it from Katrina's storm surge. But, notes Nicholl, very few of the more than 1.5 billion people around the world who live in coastal areas are prepared for violent weather events and rising seas. "The impacts (of global warming) take time to occur, but then become significant," Nicholls said at a UNESCO conference in Paris in June. "More tropical storms would clearly be very worrisome." © 2006 AFP