Are hurricanes getting worse?

March 8, 2013

Dear Cecil:

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 becoming 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? Are there more (and worse) hurricanes than in the past?

Cecil replies:

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, we don’t.

At first glance hurricane frequency seems to be on the upswing. Notwithstanding year-to-year fluctuations, the trend from 1878 to 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 from the historical results, and missed hurricanes from earlier years are estimated and added in, the long-term average is essentially unchanged.

What has risen sharply, according to some scientists, is hurricane severity. They say 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 of the ones we get.

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.

Or so some claim. Skeptics, however, contend that what looks like a jump in hurricane 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. In contrast to tree rings, sediment layers, and other reliable climate change markers, information about hurricanes sucks.

Observation via aircraft didn’t begin until the 1940s. Satellite surveillance started in the 1960s but initially offered only limited information; solid estimates of hurricane wind speeds didn’t become available until 1989. Result: we lack enough reliable, long-term global hurricane data to draw any firm conclusions.

We do have decent long-term data for North Atlantic hurricanes, but they represent only a fraction of the world total and are subject to a decades-long natural cycle that at the moment is on the rise. All that having been taken into account, 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 recent 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 (that is, not due to there being more victims with more to lose) and (b) any of the eight could have reached the opposite conclusion had certain assumptions and omissions been corrected.

But you know what? I’d just as soon keep mum about that. There’s a long list of scholarly studies showing human activity is almost certainly affecting the climate. So far they’ve had no noticeable impact on the know-nothing element, whose idea of a smoking gun evidently is having the ocean lapping at their doorsteps. 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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References

 

Banerjee, Neela “Hurricane Sandy as 'super storm': Is climate change a factor?” Los Angeles Times 27 October, 2012. Online edition, accessed 10 February, 2013. http://articles.latimes.com/2012/oct/27/nation/la-na-nn-hurricane-sandy-heads-to-northeast-20121027

Bouwer, Laurens M. “Have Disaster Losses Increased due to Anthropogenic Climate Change?” American Meteorological Society (January, 2011): 39-46.

Brooks, H.E. “Severe thunderstorms and climate change” Atmospheric Research 112 (2012).

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Hansen, James et al. “Public Perception of Climate Change and the New Climate Dice” (2011).

Holland, Greg J. “Hurricanes and rising global temperatures” PNAS 109.48 (November 27, 2012): 19513-19514.

Knutson, Thomas R. “Has There Been Any Discernible Human Influence On Atlantic Hurricane Climate?” NOAA/Geophysical Fluid Dynamics Laboratory, Princeton, New Jersey 08542 U.S.A.

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Morris A. Bender, et al. “Modeled Impact of Anthropogenic Warming on the Frequency of Intense Atlantic Hurricanes” Science 327.5964 (22 January, 2010): 454 – 458.

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Mousavi, Mir Emad et al. “Global warming and hurricanes: the potential impact of hurricane intensification and sea level rise on coastal flooding” Climatic Change Published online, 12 January, 2010.

Munich Re. Press release: North America most affected by increase in weather-related natural catastrophes Munich, 17 October 2012. Online edition, accessed 15 February, 2013. http://www.munichre.com/en/media_relations/press_releases/2012/2012_10_17_press_release.aspx

 

Testimony of Christopher B. Field, Director, Department of Global Ecology, Carnegie Institution for Science, Professor, Departments of Biology and Environmental Earth System Science, Stanford University before United States Senate Committee on Environment and Public Works on Update on the Latest Climate Change Science. 1 August 2012.

The Straight Dope: We know about global warming, but what about global dimming? http://www.straightdope.com/columns/read/2703/we-know-about-global-warming-but-what-about-global-dimming

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