Just over two years ago, Hurricane Katrina exploded into a Category 5 storm over the Gulf of Mexico, entering the record books as the fourth strongest hurricane ever recorded in the Atlantic region (a record that has since dropped to sixth). At landfall the storm didn't just cause unprecedented damage; it also touched off perhaps the most contentious meteorological argument of the decade: Is global warming responsible for worsening the already deadly cyclonic storms of the tropics? What had once been a minor offshoot of the climate change debate suddenly became, post Katrina, its central focus. Discover magazine proclaimed hurricanes and global warming the top science story of the year; and soon the movie poster for Al Gore's An Inconvenient Truth featured an image of a hurricane being belched forth from a smokestack. The truth was that when it came to climate change, Katrina was simultaneously the best of evidence and the worst of evidence. Scientists have long known that hurricanes draw their power from warm tropical seas. More recently, they've also determined that global warming is raising temperatures at the surface of the planet's oceans. In this context hurricane experts would expect the average storm to grow more intense and potentially more destructive; and indeed, clustered around Katrina's landfall two papers came out in top scientific journals suggesting this intensification had already begun in dramatic fashion.
But scientists also insist that while a changing climate should affect weather in the aggregate, no single event can be directly caused by it. Katrina will thus always remain the icon that wasn't: Suggestive, perhaps, but impossible to directly pin on global warming. And in fact, the hurricane-climate relationship grows murkier still. In response to claims that hurricanes had dramatically worsened, some storm specialists countered that many intense hurricanes were probably under-classified back when we lacked our present capacity to measure their true strength. The alleged trend toward stronger storms, they suggested, was probably a mere artifact of changing measurement systems.
And so in the weeks and months following Katrina, the media and public demanded definitive answers about hurricanes and climate change that scientists, as a group, were simply unable to provide. Science never confers absolute certainty, and especially not on a subject as novel and complex as this one. The intense hurricanes of recent years -- including last week's Category 5 Hurricane Dean -- may or may not be a smoking gun for global warming. But the debate over them following Katrina teaches something very different: How (and how not) to navigate knotty science policy debates in which the underlying information is hardly definitive, and yet nevertheless of great consequence.
Even as scientists argued vigorously -- and sometimes nastily -- over hurricanes and global warming, they also saw their results pulled into a media and political maelstrom. From left and from right, advocates demanded -- and often depicted -- a degree of scientific certainty that simply wasn't attainable. For environmentalists, hurricanes became a new way of framing global warming as a Pandora's Box that would unleash a raft of scary consequences. Industry groups and many conservatives, in turn, seemed equally certain -- but in the opposite direction. Consider Washington Post columnist Charles Krauthammer's absurdly assured 2005 assertion: "There is no relationship between global warming and the frequency and intensity of Atlantic hurricanes. Period."
Since Katrina, meanwhile, the passage of time has done relatively little to dispel pervasive scientific uncertainty on the hurricane-climate front. Recently scientists have shifted from arguing over global trends in hurricane intensity to disputing whether Atlantic storm numbers have increased. And they're just beginning to examine possible changes to other hurricane attributes: storm size, season length, regional distributions, areas of impact. Powerful though they are, hurricanes are also exceedingly delicate systems, unable to develop without a complicated set of atmospheric and oceanic conditions in place. Global warming will certainly change them, but no one can yet say precisely what the change will be. One thing seems clear: The answers won't come neatly packaged, and won't arrive precisely when policymakers and society have need of them.
And yet there was and is a path out of this science policy mess -- one that the scientists themselves ultimately uncovered. In mid-2006, researchers on both sides of the hurricane-climate debate endorsed a statement that reframed the issue by stressing the staggering vulnerability of U.S. coastlines to hurricane destruction. This vulnerability is very much human-induced, but not due to global warming: The chief culprit is our mass movement of persons and property into coastal areas -- a trend encouraged, among other factors, by unwise insurance policies. This "lemming-like march to the sea," as the scientists' statement put it, makes us more vulnerable to hurricanes whether or not the storms are independently worsening. Throw in rising sea levels and the prospect of still ill-defined but plainly worrisome changes to the storms themselves, and the policy picture becomes far clearer than the scientific one: Expecting more deadly hurricanes, we should just start protecting ourselves.
The debate over hurricanes and global warming following Katrina, then, ultimately shows the pitfalls of demanding unequivocal answers from a scientific process that, by its very nature, often cannot provide them. Instead, we should respect the tentative nature of scientific knowledge -- which still incurs a moral imperative to use the best available information, as soon as possible, to make the best decisions. After all, if uncertainty were a bar to political action there wouldn't ever be any, on global warming, hurricanes, or anything else. Science fights can be as paralyzing as they are intriguing. Sometimes, you just have to act.
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