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The 2011 Hurricane Season is Over: Now for the Recap

Usually, those of us who live in the Northeast think of hurricanes as the payback Floridians have to endure to make up for their balmy winters. But this year, we were reminded that these monster storms can and do strike on our own home turf. On Aug 28, Tropical Storm Irene slammed into New Jersey — its second landfall, after hitting North Carolina the day before as a hurricane.

Flooding at the White River National Fish Hatchery in Bethel, Vt. after Hurricane Irene. Credit: flickr/USFWS Northeast Region's photostream

By the time it reached the Jersey Shore, Irene was "only" a tropical storm, with sustained winds of 65 mph, but it was still powerful enough to knock down trees and power lines. It also dropped enough rain to cause major flooding, especially in New Jersey and Vermont. All in all, Irene was blamed for 40 deaths, and caused upwards of $7 billion in damage.

But it could have been a lot worse. Wind and rain are bad enough, but the worst damage, and most deaths, from a hurricane often comes from the storm surge, the wall of water that a hurricane pushes in front of it. That's what destroyed the Mississippi coastline and helped flood New Orleans when Katrina hit in 2005. A few days before Irene made landfall, experts were worried about what was in store for New York City. Jeff Masters of Weather Underground wrote earlier today about a very plausible worst-case scenario, in which Irene would have hit New York head-on as a Category 2 hurricane:

Since Irene was an exceptionally large storm with winds that covered a huge stretch of ocean, the storm had a much larger storm surge than its peak winds would suggest, and could have easily brought a storm surge of 15 - 20 feet to New York City. The storm would arrive during the new moon, when tides were at their highest levels of the month, compounding the storm surge risk.

In the end, the surge was only about 4.3 feet at Manhattan's southern end — still formidable, wrote Masters:

enough to top the city's seawall and flood low-lying park lands and roads near the shore. Fortunately, the water was not high enough to flood New York City's subway system, which could have easily occurred had Irene's winds been just 5-10 mph stronger.

Then, just a week later, Tropical Storm Lee  came ashore in Louisiana to cut a drenching path from the Gulf Coast to Canada, adding ten or more inches of rain to the the already-soggy land and swollen rivers. 

Still, the U.S. got off relatively easy from a season that had a total of 19 named tropical storms, seven full-fledged hurricanes and three major hurricanes — the third-highest total since record-keeping began in 1851, says the National Oceanic and Atmospheric Administration (NOAA) in its end-of-season recap

NOAA animation of the 2011 hurricane season.

That may not last, however. Hurricanes seem to come in natural boom-and-bust cycles. Lately, according to NOAA, we've been in something of a boom — a "trend of active hurricane seasons that began in 1995," says the agency's website. 

But like other natural climate cycles (the El Nino-Southern Oscillation, or ENSO, is another), the hurricane trends are likely to be affected over the coming century by anthropogenic — that is, human-caused — climate change. The greenhouse gases we keep pumping into the atmosphere are warming the planet, and that in turn is likely to change weather patterns. A new report from the Intergovernmental Panel on Climate Change (IPCC), for example, has taken a look at all the available evidence and concluded that extreme weather — especially heavy rainfall, heat waves and droughts — is likely to come more often and more severely in many parts of the world over as the century unfolds.

What about hurricanes? The picture there is less clear: as best they can determine, hurricanes may actually become fewer in number in the North Atlantic over the century, but more of the ones that do happen are likely to be Category 4 and 5 storms — the most powerful and dangerous kind.

Nobody knows how many of these will make landfall in the U.S., or anywhere else, for that matter: the tracks of these hypothetical future storms are impossible for science to predict at this point. If they do strike, odds are they'll do more damage than a similar storm might today, simply because the population is growing, putting more and more people and property in harm's way. 

At least the end of this hurricane season (they run from June 1 through Nov. 30 each year) lets us breathe a sigh of relief for the moment.

Or...maybe not. Mother Nature can't read the calendar, after all, and every so often, a tropical cyclone shows up after the party was supposed to over. In 2005, for example, the cyclone named Zeta achieved tropical storm status on December 30. It was only the second named storm in recorded history to start in one calendar year and end in another.