If there is a God, she has a nasty sense of humor when it comes to hurricanes and global warming. How else to explain that 2005 was one of the worst hurricane years on record in North America, but was followed by 2006, one of the most amiable years, hurricanically speaking, in memory? Everyone who jumped to cite Hurricane Katrina as the definitive evidence of how climate change will wreak havoc in years to come was immediately pounced upon by climate skeptics just as eager to use this year's lack of hurricanes as proof that global warming is a hoax perpetrated by capitalism-hating hippie Marxists. (Never mind that 2006 was a horrible year for typhoons.)
Now along comes William Nordhaus, the Sterling Professor of Economics at Yale University, boasting a résumé sparkling with honors and influential positions, with the latest word on the topic, "The Economics of Hurricanes in the United States." (Thanks to Environmental Economics for the link.)
The first point to note is that 2005 was no normal year for hurricanes. It was, in Nordhaus' words, a "quadruple outlier," "involving a record number of North Atlantic tropical cyclones, a large fraction of intense storms, a large fraction of the intense storms making landfall in the United States, and an intense storm hitting the most vulnerable high-value region in the country."
In other words, 2005 was special, with or without the added oomph of global warming.
But Nordhaus also concludes that global warming is adding some oomph. The evidence on storm frequency is inconclusive, but storm intensity, which is closely correlated with rising sea-surface temperatures, has been growing over time.
So can we look forward to a future in which Katrina-like storms smash into the Gulf Coast every year? Nordhaus says no. Here's the money quote: "We would expect hurricanes with impact as high as Katrina once every 86 years without global warming but once every 28 years with global warming."
There's a specificity to that prediction that no doubt has whatever deities on high who might be messing with our weather patterns giggling wildly. But the difference in frequency is not a difference without a distinction. Once every 86 years is once in a lifetime -- an unheard of thing, kind of like the Red Sox winning the World Series. Once every 28 years means I'm likely to witness another such cataclysm in my lifetime. Which is unacceptable.