flooded Bronx ParkHurricane Irene flooded parts of the Bronx.Photo: Edwin MartinezClimate science suggests that global warming will make hurricanes like Irene more destructive in three ways (all things being equal):

  1. Sea level rise makes storm surges more destructive.
  2. “Owing to higher SSTs [sea surface temperatures] from human activities, the increased water vapor in the atmosphere leads to 5 to 10 percent more rainfall and increases the risk of flooding,” as NCAR senior scientist Kevin Trenberth put it in an email to me Saturday.
  3. “However, because water vapor and higher ocean temperatures help fuel the storm, it is likely to be more intense and bigger as well,” Trenberth writes.

On the third point, warming also extends the range of warm SSTs, which can help sustain the strength of a hurricane as it steers on a northerly track. As meteorologist and former hurricane hunter Jeff Masters has explained:

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… this year sea surface temperatures one to three degrees warmer than average extend along the East Coast from North Carolina to New York. Waters of at least 78 degrees F extend all the way to southern New Jersey, which will make it easier for Irene to maintain its strength much farther to the north than a hurricane usually can. During the month of July, ocean temperature off the mid-Atlantic coast (35°N – 40°N, 75°W – 70°W) averaged 2.6 degrees F above average, the second highest July ocean temperatures since record-keeping began over a century ago (the record was 3.8 degrees F above average, set in 2010.) These warm ocean temperatures will also make Irene a much wetter hurricane than is typical, since much more water vapor can evaporate into the air from record-warm ocean surfaces.

Also, hurricanes tend to be self-limiting, in that they churn up deeper (usually cooler) water that can stop them from gaining strength and also weaken them. So since global warming also warms the deeper ocean, it further helps hurricanes stay stronger longer.

One says “all things being equal”  because, among other things, it is possible that global warming will increase wind shear, which can disrupt hurricanes.

The media prefer to ask the wrong question — as Politico did Friday with its piece, “Was Hurricane Irene caused by global warming?” But they do have a good quote from perhaps the leading expert on the subject:

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“I think the evidence is fairly compelling that we’re seeing a climate change signal in the Atlantic,” said Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology. Citing other recent trends of extreme weather, including hailstorms and catastrophic tornadoes, “one begins to wonder, if you add all those up, maybe you are seeing a global warming effect.”

Still, he adds, “I would be reluctant myself to say anything about global warming and Irene” — but again, that, I think, is a function of asking the wrong question. That’s a point Climate Central makes in its post on this subject:

[On Friday], the immediate question for anyone in the path of the storm [was] — or should [have been] — “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, 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?

Wrong question.

Here’s the right question: Is climate change making this storm worse than it would have been otherwise?

Answer: Absolutely.

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.

Note that teasing out a relationship between global warming and hurricane damage is tricky, because “more than half the total hurricane damage in the U.S. (normalized for inflation and populations trends) was caused by just five events,” as Emanuel explained in an email to me a while back. Storms that are category four and five at landfall (or just before) are what destroy major cities like New Orleans and Galveston with devastating winds, rains, and storm surges. One extra category four or five hitting Miami, and you’ve obliterated the damage records.

Still, here’s a key finding of a 2009 study (subs. req’d):

In the period 1971-2005, since the beginning of a trend towards increased intense cyclone activity, losses excluding socioeconomic effects show an annual increase of 4 percent per annum. This increase must therefore be at least due to the impact of natural climate variability but, more likely than not, also due to anthropogenic forcings.

That isn’t definitive attribution — which the authors explicitly avoid — but it still is a statement of attribution.

We are facing 10 times as much warming this century as in the last 50 years, so the three factors described above are going to have a greater and great impact over time.

Here’s more from Climate Central’s post on Friday, which essentially predicted the floods now inundating Vermont and upstate New York:

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).

So this is another potential way that global warming can make hurricanes more destructive — by causing more deluges that can saturate the ground and worsen the flooding caused by a subsequent hurricane (whose deluge itself was likely worsened by global warming).

Finally, even without global warming, there are obviously good reasons for increased funding of early warning of hurricanes — which the Republican House has in fact reversed — and for improving the ability of coastal communities to deal with hurricanes. And, because of global warming, there are good reasons to plan for more extreme weather events, which, again, the GOP House bitterly opposes. As long as the Tea Party rules, coastal communities are unlikely to see any significant funding increases for monitoring, planning, resilience, or adaptation.

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