We're now a day out from Hurricane Irene’s projected landfall in the Outer Banks of North Carolina (or near-miss, and it doesn’t make a lot of difference which it is). A day later — sometime Sunday, that is — the storm’s assault on the New York metropolitan area will begin. And unless Irene’s track veers sharply and unexpectedly out to sea, the Northeast will suffer its worst hurricane in decades. Evacuations have begun from Cape Hatteras all the way up to New Jersey; in New York City, hospitals and nursing homes in low-lying areas in Brooklyn and Staten Island have begun emptying, and the city’s subway system could shut down entirely on Saturday, for safety reasons.
At the moment, the immediate question for anyone in the path of the storm is — or should be — “how can I keep myself and my loved ones safe?” But another question may be lingering in the background. It’s the same question that came up in April, when a series of killer tornadoes tore up the South in April, and in May, when floods ravaged the entire Mississippi River basin, and in July, when killer heat waves seared the Midwest and Northeast, and in August, when Texas officially completed its worst one-year drought on record — a drought that isn’t over by a long shot.
The question: is this weather disaster caused by climate change?
Here's the right question: is climate change making this storm worse than it would have been otherwise?
For one thing, sea-surface temperatures in the Atlantic Ocean are higher now than they used to be, thanks to global warming, and ocean heat is what gives hurricanes their power. All other things being equal, a warmer ocean means a more powerful storm. It’s hard to say that all other things are exactly equal here, but it’s certainly plausible that Irene would have been a little weaker if precisely the same storm had come through, say, 50 years ago.
What we know for sure, however is that thanks largely to climate change, sea level is about 13 inches higher in the New York area than it was a century ago. The greatest damage from hurricanes comes not from high winds and torrential rains — although those do cause a lot of damage. It’s from the storm surge, the tsunami-like wall of water a hurricane pushes ahead of it to crash onto the land. It was Hurricane Katrina’s storm surge, not the wind or rain, that destroyed New Orleans back in 2005.
With an extra foot of sea level to start with, in other words, Irene’s storm surge is going to have a head start. And climate change is a big part of the reason why.
The relationship between climate change and hurricanes is one that scientists are still trying to understand. As I mentioned above, warm ocean waters provide the energy that keeps a hurricane going. That's why the storms lose energy when they pass over land, and why they gain energy when they pass over warmer water (as Katrina did when it entered the Gulf of Mexico after crossing Florida).
Still the phrase “all other things being equal” is key. In a warming climate, all other things will not necessarily be equal. For one thing, wind patterns will probably change, and something called wind shear, which tends to snuff out hurricanes before they can fully form, may increase over the Atlantic as the climate changes. Moreover, some climate scientists argue that a key factor in hurricane formation is not simply the ocean temperature, but the differences in temperature from one ocean basin to another. One recent paper in Science concludes that the overall number of hurricanes in the Atlantic is likely to decrease over the coming century — but that the intensity of those that do happen is likely to increase.
But that says nothing — and nobody has a clue — about how many of those hurricanes will hit land, and if they do, whether it will be in densely populated areas or not (although more and more of the U.S. shoreline that lies in hurricane territory is filling up with people).
Nevertheless, one study has projected an overall 20 percent increase in hurricane-related damage based on population growth and sea-level rise alone, even if there were no change in hurricane frequency or strength.
Let's also not forget that while storm surges pose the biggest danger, Irene will almost certainly bring torrential rains to a part of the country that has already been drenched over the past couple of weeks. With saturated ground and a deluge that could add up to 10 or even 20 inches of rain in just a day or so, rivers and creeks will likely overflow their banks, causing widespread flooding. And then there's the wind, which will inevitably cut power to hundreds of thousands of people, at least (it can happen even when there isn't a hurricane).
To better understand how climate change-related sea level rise aggravates coastal flooding risks, see this graphic we produced based on 2010 Census and sea level rise data. For those who want to monitor Irene's progress, here's a map, a great liveblog, and the main tracking portal from the National Oceanic and Atmospheric Administration. You can also get updates by following us on Twitter at @ClimateCentral. Our managing editor Andrew Freedman, who also writes for the Washington Post's Capital Weather Gang blog, will be tweeting storm updates throughout the weekend via @afreedma.