People wade and paddle down a flooded street as Hurricane Sandy approaches on Oct. 29, 2012, in Lindenhurst, N.Y. (Jason DeCrow/AP)

In a new study, scientists say that the risk of major hurricane or storm-driven flooding in New York City is already considerably higher than it was 1,000 or even 100 years ago, thanks both to a considerable rise in sea level, but also, they say, to changes in the nature of storms.

“We see more intense storms with a greater ability to produce high storm surges at The Battery in NYC during the anthropogenic era than during the pre-anthropogenic era,” the researchers write.

The paper, just out in the Proceedings of the National Academy of Sciences, is by Andra Reed of Penn State University and a group of other researchers that includes Reed’s colleague Michael Mann, hurricane expert Kerry Emanuel of MIT, and Ning Lin of Princeton. Emanuel and Lin just published a separate paper this month suggesting that Tampa, Fla., also faces a major hurricane flooding risk.

The Atlantic ocean actually has considerably better hurricane records than many other parts of the world, but those still only go back to 1851. That’s still “too short” to really determine whether the storm risk to New York has changed, the researchers find. So they took a different approach — simulating very large numbers of “downscaled” Atlantic coast hurricanes for the period from 850 to 1800 in several large climate models, and then comparing the storm surges they produced with those produced by hurricanes for the period from 1970 to the present (which they dub the “anthropogenic era”).

But one key thing wasn’t static across this large time period — the level of the ocean off the New York coast. So the research also used geological records of more than 1,000 years of past sea levels from the coast of New Jersey as part of the analysis. Each synthetic storm’s surge was calculated based on the state of the sea level during the time when it occurred.

The result was that the storms and their consequences were pretty different across eras — most importantly, because sea levels have risen considerably since the year 850 A.D., with a particularly fast rate in the past 100 years or so. As a consequence, the study found that the average storm surge during the anthropogenic era was 1.24 meters higher than during the prior, and much longer, period. “These intuitive results stress the increasing risk that coastal regions of the United States face due to the combination of [sea level rise] and storm surges,” the authors wrote.

Referring to a flooding event on the scale of Superstorm Sandy, Mann says that “we estimate that that shouldn’t happen more than once every 3,000 years. With climate change, that becomes roughly a  one-in-a-century event. So it should have happened once in the last century, and it did.”

The bulk of the increasing risk is a simple result of rising seas. If they’re higher, then storm surges can attain greater absolute heights and travel farther inland. But there’s also a question of whether something about the characteristics of storms has also changed — bigger or more powerful storms can also fling greater storm surges.

To determine this, the researchers subtracted back out the role of changing sea levels and simply examined the surges generated by the storms. Here, there was not an average difference in storm surge heights across the two eras, but in statistics-speak, there was a “long tail” phenomenon, in which the very rare extreme storms appeared to be getting worse. “The storm surge heights in the tails of the anthropogenic distributions are significantly greater than the storm surge heights in the tails of the pre-anthropogenic storm surge distributions,” the paper concluded.

The study also found that very, very large storms that aren’t so intense in terms of sheer wind speed, but nevertheless drive very large surges, were also more likely in the current era. Storms like Sandy, that is.

“I think the punchline is, we made Sandy much more likely already,” says Mann. “We’re already dealing with greatly elevated risk. We’re not just talking about the future. Climate change is already costing us dearly, but it’ll be a whole lot worse if we do nothing about it.”