U.S. is in a hurricane-friendly era
The last major hurricane to hit New Orleans was Camille, way back in 1969. In the days following Katrina's devastation, old-timers were quoted again and again as saying that this one sure seemed an awful lot worse. And as brutal as 2005 has been already, we're only midway through hurricane season. Does it just seem that way, or are hurricanes really getting worse? Strangely enough, the answer to both questions is yes.
Measured by the dollar value of the damage they caused, 2004 was the worst hurricane season on record, and the 10 most costly hurricanes to hit the U.S. mainland have all occurred since 1992. But only one of those makes the list of the 10 strongest hurricanes in American history. Much of the increased damage, it turns out, is a result of booming coastal development rather than worsening storms.
But that doesn't change the fact that there has been a steep uptick in the number of hurricanes in the Atlantic over the past decade. From 1970 to 1994, there were an average of 8.6 tropical storms, 5 hurricanes, and 1.5 major hurricanes per year in the Atlantic; from 1995 onward, those numbers have jumped to 13.6 storms, 7.8 hurricanes, and 3.8 major hurricanes.
Most scientists agree that the increased frequencies are part of a natural cycle. For all the destructive power of a fully formed hurricane, a nascent tropical cyclone is a fairly fussy thing, requiring just the right conditions in the sea and the air to develop. No one is quite sure why, but every two or three decades, the tropical Atlantic seems to swing back and forth between conditions that favor hurricane developmentwarm surface temperatures that give the storms more energy, for exampleand conditions that tend to impede them, such as strong crosswinds that disrupt the forming cyclones. We've been in a hurricane-friendly period since about 1995, and there's no telling whether we're halfway through it, or only a third.
But while the more frequent Atlantic hurricanes seem to be a natural phenomenon, scientists are finding evidence for a much more disturbing possibility: that global warming is making hurricanes more powerful and loading them up with more rain. In a study published in Nature earlier this month, Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology, says he found that the average intensity of hurricanes around the world has increased by 70 to 80 percent over the past 30 years. And that destructive trend, he says, appears to be directly linked to increased sea surface temperatures over the same period, which is one of the clearest signs of global warming.
Still, says Emanuel, there's no reason to think that Katrina is itself a result of global warming. It wasn't after all an unusually strong storm, and in the Atlantic the influence of fluctuating conditions is stronger than any global warming effect. But that's cold comfort at best. With the current upswing in Atlantic hurricanes expected to persist for at least another decade, Emanuel says, "we're in for a rough ride over the next 10 years."