Did global warming cause superstorm Sandy?

Let's get this out of the way, right from the start: Climate change may, possibly, have contributed in some small way to the size, duration, and strength of Hurricane Sandy, but it did not cause Hurricane Sandy.

I've seen a lot of claims that climate scientists "shy away" from making claims about the links between weather events and climate change, like the scientists are somehow trying to avoid something unpleasant or embarrassing. The latest I read was in this piece, by columnist Terrance Corcoran. However, there is nothing to "shy away" from. To paraphrase an excellent analogy that comes straight from the IPCC report itself (FAQ, pg 96): Claiming that scientists shy away from direct claims between specific weather events and climate change is like claiming that doctors shy away from direct claims between the age when a specific person died and the average life expectancy in that area.

There are two simple facts that must be dealt with about weather and climate: 1) climate cannot say anything about any single weather event, and 2) a single weather event cannot say anything about climate. These also apply to weather and climate change.

However unfortunate those two facts are — because, hey, it would be really nice to have a solid indication of exactly what's going on — that's just how it is. Climate represents the long-term trends of weather in a particular region. Just because it happens to snow in July somewhere where it has never snowed in July before, doesn't mean that one can point at climate, or even climate change, as the 'culprit'. That snow-in-July event does go into the climate record though, and if it starts to happen more frequently in the years after that, it can — eventually — be considered part of the local climate, and it can be said that the local climate changed.

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To corner weather and climate experts every time some weather event happens, and force them to answer the question "did climate change cause this?" is, at best, naive, and at worst, manipulative. To make matters worse, if the expert answers "no", there seems to be a trend of using it as 'proof' that climate change doesn't exist, no matter what additional information the expert gives.

So, can anything be said about climate change and Hurricane Sandy?

Well, there were a few 'unusual' factors in play for this particular storm: For one, it was a hybrid storm, meaning that it merged with a weather system that was passing over land, and it drew strength from two different sources. Second, the storm swept through the northeast during a Full Moon, so the tides were near their peak level, and the majority of the storm surge arrived right at high tide, driving the total storm tide up to 14 feet (4.2 m). Third, there was a 'blocking high' parked over the Maritimes — a region of strong high pressure that is prone to diverting other weather systems around it — that forced Sandy to make the hard-left turn it pulled on Monday. These factors are completely natural though, and likely have nothing do to with anything except timing.

However, there were a few things that may be attributed to climate change that could have contributed to the effects of the storm.

From records and observations, the Gulf Stream — the current of warm water running south-to-north along the east coast of North America, which provides Atlantic tropical cyclones with much of their energy as they move up the coast — was warmer this year than it was at the same time last year. Also, from the Climate Science blog of Roger Pielke Sr, this map of sea surface temperature changes from 1982 to 2011, and this one from 2003 to 2011, show that sea surface temperature have increased along the coast of the U.S. Northeast, by about half a degree C on average over the longer trend, and by a degree or more on the shorter trend. These higher sea surface temperatures would have provided more energy to Sandy, allowing the storm to maintain its strength further north than normal and possibly contributed to its overall size and extent.

Also, based on data from the National Oceanic and Atmospheric Administration (NOAA), the sea level around New York City has risen by roughly 30 cm in the past 100 years (and continues to rise by about 3 mm/year). Sea level rise is linked to climate change, as ocean temperatures rise (causing the water to expand) and more polar ice melts (adding more water to the oceans). This rise in sea level would have contributed to the storm surge from Sandy.

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The politicians take a slightly different stance on the issue, of course.

According to a CTV News article from yesterday, New York State governor Andrew Cuomo said "Anyone who says that there's not a dramatic change in weather patterns I think is denying reality. I told the president the other day: 'We have a 100-year flood every two years now."'

According to that same article, New York City Mayor Bloomberg took a slightly more cautious stance: "What is clear is that the storms that we've experienced in the last year or so, around this country and around the world, are much more severe than before. Whether that's global warming or what, I don't know. But we'll have to address those issues."

Public perception is very important to politicians, so it's understandable that they'd take this stance, regardless of whether or not science backs them up. Bias always has a chance of creeping in (because, hey, we're all human), but scientists try to rely on the data, regardless of whether or not it supports their opinion. Putting the puzzle together to form the 'big picture' is much harder when the puzzle pieces fit together in 4 dimensions (rather than just 2), so it's going to take more time for science to answer these kinds of questions.

So, did climate change cause Hurricane Sandy? No. Did it contribute? Probably. By how much? According to Texas A&M University climate professor Gerald North, maybe 10 per cent, possibly up to 20 per cent. Is Hurricane Sandy what hurricanes will look like from now on? There's speculation that the answer to that is "yes", but there's no way to be sure right now. As the science advances, we'll be able to answer that with more confidence ahead of time, but unfortunately, as of now, only time will truly tell.