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As Hurricane Sandy, perhaps the biggest storm in the history of the United States, draws closer to the Eastern Seaboard, there has been an inevitable outburst of the obvious question: has climate change caused it? The answer is no, or yes, or better yet “you’re asking the wrong question”.
Imagine you have a big fairground Wheel of Fortune thing, with the numbers one to 10 on it and an arrow at the bottom which shows the winning number. If you spin it 100 times, you’d expect to see the number 10 come up roughly 10 times. Then you put a small weight on the number 10, and spin it again. Say it lands on the 10. Did the weight cause the 10 to come up? What does that mean? It might have landed on a 10 anyway.
But if you spin it another 100 times, and find that instead of getting the 10 about 10 times, you see it 50 times, you can say with confidence that the added weight has caused the statistical change. It’s meaningless to ask, of each individual spin, what “caused” it – the different factors, like how hard you spun it, how well the axle is greased, etc, all contribute. But over the longer term, you can observe the changes in probability, and see the effect that the weight is having on average.
That is, roughly, analogous to what is expected to happen with tropical storms and hurricanes as the world gets warmer. Warmer seas mean more energy in the cyclone systems that form tropical storms, which, it is hypothesised, means higher wind speeds; warmer air can carry more moisture, meaning, theoretically, more rainfall. Apparently the Earth’s atmosphere is roughly four per cent more humid than it was in 1970. More wind, more rain: you can see why climate scientists have long proposed that global warming will increase the frequency and intensity of hurricanes.
But, of course, it’s not as simple as that. For a start, there are enough unknown factors in the complex and chaotic climate system for it to surprise us entirely. But more importantly, because hurricanes are relatively few in number, it’s difficult to sort out signal from noise: to return to our wheel-of-fortune analogy, if you only spin the thing once a year, you’ll have to wait a long time before you can tell whether the 10s really are more common or if it’s just randomness. If Hurricane Sandy really is the biggest storm of its kind to hit the States, you might think that adds weight to the climate change hypothesis, but on its own it tells us nothing; statistical outliers happen sometimes. A 2005 review of the literature found that there was “weak” evidence of an increase in hurricane intensity over the last 50 years; it also found a “near doubling” in the “power dissipation” of storms (a measure determined by a combination of duration and wind speed). But the authors say that may be due to other factors.
More importantly, the effect of global warming on how damaging these storms are is dwarfed by the effect of preparedness, and of population growth and increased wealth in the high-risk areas. The above-mentioned review of the literature says that “By 2050, for every additional dollar in damage that the IPCC expects to result from the effects of global warming on tropical cyclones, we should expect between $22 and $60 of increase in damage due to population growth and wealth.” There are excellent reasons to try to mitigate the effects of climate change. But while the evidence for a link between climate change and hurricanes remains weak, it seems that policymakers in hurricane regions should worry more about keeping the levees in good repair, and arranging good storm-shelters and evacuation procedures, than trying to stop people from driving SUVs.