Did global warming cause hurricane Sandy?

Hurricane Sandy was being talked about a couple of weeks ago, when it was gathering energy out in the Atlantic, well to the south of where it finally crossed the coast. What impressed observers was its size. To put it in Australian terms, it encompassed Hobart, Adelaide, Melbourne, Canberra, Sydney and Brisbane. But apart from its girth it was not seen at the beginning as a major worry, being very slow and likely to drift in towards the coast and then out again.

It became a huge worry because instead of heading out to sea again, as it approached the US coast, it was blocked by a high-pressure system to the north. And it collided with a cold front from the west, bringing early snow. Add in a jet stream from the south, and the fact of a full moon, meaning a high tide and a strong storm surge in the sea, and you have all the ingredients of really massive turbulence and damage.

Measured on the standard scale of storms (ACE — accumulated cyclone energy)  Sandy wasn’t by any means the biggest, and its winds were not the most extreme.  Its effects, however, were arguably the worst experienced, at least by those in Manhattan and New Jersey. At once the cries went out: ‘You see? This is what you get from global warming!’ I knew this would happen, and have been interested in the scientific arguments that might be used to support such a claim.

The high tide effect, important though it is, was not produced by global warming. That much is clear. But it is being argued that the blocking high in the north was the result of recent melting of Arctic sea-ice. That is hard to disprove, and it is at least consistent with model projections for a warmer planet. The fact no such storm has occurred over the past few years, despite the Arctic ocean’s having been ice-free in summer, doesn’t disprove it either. There is no proof either way.

In truth, as I delved more deeply into what experts were saying, we just don’t know whether or not what happened in the Arctic over the northern summer made any contribution at all to Sandy’s effects. Indeed, we don’t know a great deal about climate, but because it is now a mainstream topic, and because there is this guilt factor about (‘It’s all our fault because we burn fossil fuels’) everyone who has an axe to grind has offered his or her opinion. The popular name ‘Frankenstorm’ that has been given to Sandy carries the notion that it is a human-created disaster.

Let me give just one example of the uncertainty: storms in the past. Given that the northeast of the USA has been settled by Europeans for four centuries, there is a good deal of written evidence of the effects of great storms in the past, and there have been several. But the evidence there is hardly to be compared with today’s instrument-based precision. We can say, nevertheless, that there have been great storms in New York, New Jersey and New England in the last few centuries that, on the  evidence, seem to have been of much the same scale as Sandy, if not bigger.

Then there is proxy evidence.One example, drawn from an analysis of lake-bed sediments, was published in Nature ten years ago by Noren and others. They suggest that there is a periodic pattern in great storms, with a roughly 3,000-year cycle. They think the pattern may be caused by ‘long-term changes in the average sign of the Arctic Oscillation’. On their evidence we may be due for the next great increase in such storms. And they say that this factor (whose causes can be guessed at but are so far not known) ‘may account for a significant fraction of Holocene climate variability in North America and Europe’.

‘Ah!’, cry the orthodox. ‘But we’re adding to it, and that will make things worse. Look what happened with hurricane Sandy!’

An agnostic like me will continue to say that we just don’t know. Sandy ‘s scale is consistent with the models, and it is consistent with natural variability, too. The science doesn’t seem to be at all settled here. And while we can’t explain the real cause of storms such as this one, which even closed the New York Stock Exchange for two days, which hasn’t happened since 1886, we will continue to hear cries of doom from those who think that AGW is the wrath of Gaia returning to us.

Perhaps it is. I am an agnostic about all of this, not a root-and-branch opponent. But if more storms are on the way, whether caused by natural variability or by AGW, my response is to look to the strength of our house at the coast and to the likelihood of an usual storm surge there, and do what I can to anticipate and deflect any harmful effects.



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