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Are hurricanes actually getting worse?



Whenever there's a really bad storm, environmentalists on the left and apocalypse enthusiasts on the right act as though it's self-evident that hurricanes and other storms are more frequent and intense. Environmentalists cite global warming, while apocalypse enthusiasts blame sinners for incurring God's wrath. But I never see any statistics showing the hurricane situation is actually, quantifiably getting worse. Does anyone have solid facts on this? —Geoffrey Card

Let me declare my bias, Geoff. Do I personally think the wacky weather lately is a sign of climate change? Hell yes. Do we have ironclad scientific proof that hurricanes, the most spectacular weather disaster, are getting worse? No.

At first glance hurricane frequency seems to be on the upswing. Notwithstanding year-to-year fluctuations, the trend from 1878-2008 shows the average annual total of hurricanes increasing from seven to twelve. Aha, you say, proof of global warming! However, closer analysis suggests the rise can mostly be explained by improved weather observation—in the old days storms in remote parts of the globe just weren't reported. When short-lived hurricanes (lasting two days or less) are filtered and missed hurricanes from earlier years are estimated and added in, the long-term average is unchanged.

What has risen sharply, according to some scientists, is severity. The average annual number of category 4 and 5 hurricanes, which together cause nearly half of all hurricane damage, has more than doubled since the early 1970s. That's a big deal because—and here's something few people realize—climate change models suggest the number of hurricanes will, if anything, decline. What will rise is the intensity.

Why? An important factor in hurricane strength is fluctuations in sea surface temperature, or SST. A hurricane is a giant heat engine, drawing its energy from warm ocean water. The warmer the water, the more powerful you'd expect hurricanes fed from it to be.

Sure enough, changes in SST in the North Atlantic during the 20th century track reasonably closely with regional hurricane intensity. From 1950 to the mid-1970s, when ocean waters cooled, possibly because of a buildup in atmospheric pollution, hurricane severity declined. From the mid-70s onward, ocean surface temperatures went back up and hurricanes got worse.

Skeptics, however, contend that what looks like a jump in severity over the past 40 years is the result in part of underestimating storm intensity in the '70s and '80s. This illustrates the larger problem: practically everything we can say about hurricanes is in dispute. A major stumbling block is lousy data.

Observation via aircraft didn't begin until the 1940s. Satellite surveillance started in the 1960s but initially offered only limited info; solid estimates of hurricane wind speeds didn't become available until 1989. Even some doubters acknowledge recent increases in storm severity may be partly attributable to human-caused global warming. But the confidence in such claims is low.

Wait a second, you say. What about that widely publicized report last fall from the insurance company Munich Re saying storm damage claims in North America have quintupled over the past three decades, with a trillion dollars in losses and 30,000 dead? Turns out much of that was due not to climate change but to dopes putting themselves in harm's way, by doing things like building houses in flood zones.

Once you account for population growth and suburban sprawl and whatnot, a good deal of the apparent increase in losses goes away. A 2011 meta-study looked at 22 analyses of loss trends following natural disasters and found that (a) in only eight studies did the researchers conclude there'd been an actual climate-driven increase in losses and (b) any of the eight could have reached the opposite conclusion had certain assumptions and omissions been corrected.

But there's a long list of scholarly studies showing human activity is almost certainly affecting the climate. Hurricane Sandy, by scaring the daylights out of the New York media people who set the national agenda, has at last gotten the climate change conversation off the dime. Can we legitimately blame that disaster on global warming? No, but I'm not going to object if a lot of people do.



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