Is the rash of powerful Atlantic storms in recent years a symptom of global warming?
Although most mainstream hurricane scientists are skeptical of any connection between global warming and heightened storm activity, the growing intensity of hurricanes and the frequency of large storms are leading some to rethink long-held views.
Most hurricane scientists 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, scientists say the 150-year record of Atlantic storms shows ample precedent for recent events.
But a paper published last month in the journal Nature by meteorologist Kerry Emanuel of the Massachusetts Institute of Technology is part of an emerging body of research challenging the prevailing view.
It concluded that the destructive power of hurricanes had increased 50% over the last half a 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," Emanuel said Monday.
But after examining data on hurricanes in the Atlantic and typhoons in the Pacific, he said, "I was startled to see this upward trend" in duration and top wind speeds.
"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 of the last 11 years, said the National Oceanic and Atmospheric Administration.
This month, the agency raised its already-high hurricane forecast for this year to 18 to 21 tropical storms, including as many as 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 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 Emanuel's research there were few indications that hurricanes had become stronger or more frequent, despite well-documented increases in surface temperatures.
Moreover, skeptical hurricane scientists were quick to point out that worldwide weather records were too inadequate for a thorough examination of such trends. They said 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 favor 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 sharply questions 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," Gray said. "In the Atlantic, there has been a change recently, sure. But if you go back to the 1930s, you see a lot of storms again. These are natural cycles, not related to changes in global temperature. 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 half a century -- 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 hasn't already.
"It's the ocean temperatures and sea-surface temperatures that provide the fuel for hurricanes," said Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research who recently published a paper in the journal Science contending that climate change could cause hurricanes to produce more rain and thereby become more dangerous.
"It's the big guys, the more intense storms, that have been increasing," Trenberth said. Hurricane scientists have been "unduly influenced by what has been happening in their corner of the world in the Atlantic. But if you look more broadly, at what has been happening in the Indian Ocean and the Pacific Ocean, there is a clear trend."
Such views remain controversial among veteran hurricane scientists.