American Scientist

In late January, Christopher W. Landsea, a hurricane specialist with the National Oceanic and Atmospheric Administration in Miami, publicly announced that he would end his participation in the Intergovernmental Panel on Climate Change (IPCC), a group that comes together periodically under the auspices of the United Nations to report the latest advances in scientific understanding. Landsea's withdrawal was sparked by a press briefing that followed the busy 2004 hurricane season in Florida. That event was promoted by a Harvard University press release headlined "Experts to warn global warming likely to continue spurring more outbreaks of intense hurricane activity." One of these experts was Kevin E. Trenberth of the National Center for Atmospheric Research in Boulder—the person who had invited Landsea to contribute to the IPCC's upcoming fourth assessment report.

Although Trenberth didn't explicitly state that the 2004 Florida hurricanes were caused by global warming, some of the comments he made at the briefing could easily be misconstrued. For example: Referring to the heightened hurricane activity in 2004, Trenberth said, "...in the Atlantic there's no guarantee that this is going to continue, because in the Atlantic there is large, natural decade-to-decade variability in hurricane activity...." So far so good. But he then added: "...now superimposed on that natural variability is also this longer-term trend that we associate with global warming." A listener could easily reach the conclusion that climatologists had at least some evidence of a warming-induced upward trend in hurricane activity.

Although one can find theoretical support for the proposition that hurricanes may indeed get worse—from a consideration of general principles (hurricanes are fueled by the heat from tropical seas, which will likely warm somewhat over the coming decades) and from certain computer models (one 2004 study suggests that hurricane winds might increase by 5 percent or so over roughly the next 80 years)—the historical record does not show any obvious trend.

In an open letter to the climate-science community, Landsea wrote:

...Given Dr. Trenberth's role as the IPCC's Lead Author responsible for preparing the text on hurricanes, his public statements so far outside of current scientific understanding led [to my] concern that it would be very difficult for the IPCC process to proceed objectively.... My view is that when people identify themselves as being associated with the IPCC and then make pronouncements far outside current scientific understandings[,] this will harm the credibility of climate change science and will in the longer term diminish our role in public policy.

Trenberth now says that "it was clear I was not speaking for the IPCC." Yet the moderator for the briefing had introduced Trenberth as "convening lead author of the 2007 Intergovernmental Panel on Climate Change report." And in his opening remarks Trenberth volunteered, "I was a lead author on the 2001 IPCC report for Working Group One, which deals with the science of climate change, and in fact I was involved in developing some of the information that is in that report dealing with hurricanes."

Commenting on Trenberth's controversial statements to the press, Hans von Storch, a climatologist at the Institute for Coastal Research in Geesthacht, Germany, says "It's a demonstration of how highly politicized the IPCC process has become." Von Storch should know, having lately been in the thick of some rather heated climate debates himself. One erupted two years ago, forcing him to resign his position as editor of Climate Research in protest when he was prevented from publishing an editorial critical of an article that had appeared in that journal, one that called the uniqueness of recent global warming into question. (It's clear that here von Storch was acting on principle, not campaigning to defend the notion that the recent warming is extraordinary. Indeed, his own work points in the other direction: Last year von Storch and five coauthors published a paper in Science suggesting that ancient swings in climate might well have been a lot larger than many experts, including those on the IPCC, estimate.)

Although tropical hurricanes are not his specialty, von Storch has studied the evolution of storminess elsewhere. Lars Bärring of Lund University in Sweden and von Storch published a paper last year (in Geophysical Research Letters) that uses measurements of barometric pressure to gauge the degree of storminess in Scandinavia since Napoleonic times. Their conclusion: Although there have been oscillations up and down over the decades, there is no overall trend during this interval. That article is an outgrowth of work that began in the early 1990s, at time when, according to von Storch, European newspapers were full of stories about increasing storm activity, which was described as an early signal of global warming. A more careful analysis by von Storch's group, however, found that the level of storminess in Europe in 1995 was, in fact, similar to the level existing in 1900.

Landsea suspects something similar will happen when Americans look back at the busy hurricane season of 2004: "If I had to make a guess for the next 20 years, I'd say it's going to be a lot like the last 10 years." That is not to say that Landsea discounts any influence of a warming planet. "No one should pooh-pooh the possibility that global warming might do bad things," he says. But he stresses that the increase in hurricane wind strength being suggested on the basis of computer modeling is "pretty tiny." And he points out that the monitoring of hurricane winds today has a coarseness of about 5 miles per hour. So the influence of global warming on hurricanes now, if it exists at all, is in the noise. "Even in 2080," he says, "you might not be able to measure it."—David Schneider