NEW YORK -- Manhattan is 20 times more likely to see its busy, low-lying downtown flooded than it was in the mid-19th century, a new analysis has found.

In a paper set for publication in the American Geophysical Union's research journal, Geophysical Research Letters, scientists at Portland State University claim storm surges and tidal waters are now likely to top the Big Apple's downtown seawall -- at just under 6 feet -- every four or five years.

In 1844, residents could expect the same seawall to be crested once every 100 to 400 years, they said.

"What we are finding is that the 10-year storm tide of your great-, great-grandparents is not the same as the 10-year storm tide of today," said report lead author Stefan Talke, an engineering professor at Portland State in Oregon, in a release from the organization.

Talke's team say they went further back in time than any other study to look at tidal reports throughout the late 19th century and the 20th century to compile their data. The research indicated sea-level rise has increased waters in New York harbor by about a foot since 1844, while a "once-every-decade" storm tide has risen by an additional foot.


The study comes as New York continues to struggle with the aftermath of Superstorm Sandy, which produced the largest storm surge the city had seen since at least 1821. Mayor Bill de Blasio (D) has been in the news the last few days for discussing how he intends to handle destroyed homes still not fixed well enough to weather the long-term climate threat (ClimateWire, April 23).

Talke says there's now a 10 percent chance that a storm tide will top the downtown seawall in any given year. That kind of storm tide was once thought of as a "10-year storm" but with sea levels rising the new normal has shifted, the report said.

As part of the study, researchers took photographs of hundreds of pages of handwritten hourly and daily tide gauge notes dating back to 1844 stored in the U.S. National Archives in College Park, Md. The researchers then used newspaper accounts to plug in the holes and compared the data to tidal and storm information from the modern era.

Proposed solutions go back to 1901

A storm tide is the amount of water that rises during a weather event due to both the storm surge and normal tides. During the worst of Sandy, the storm surge rose above 9 feet but the effect was exacerbated by rising tides and wind as well.

Whether to extend the downtown seawall to other parts of Manhattan or other boroughs is a question that will have to be answered in time. According to The New York Times archive, policymakers discussed extending the seawall to the entire Manhattan island as far back as 1901.

Other, more aggressive options -- like building a sea barrier right in the mouth of the harbor -- would likely costs in the tens of billions (ClimateWire, Nov. 15, 2012).

Climate change is not the only factor that may be influencing the tidal shift. Talke said the changes could be the result of several overlapping factors, among them "decades-long variations in the North Atlantic Oscillation [and] an irregular fluctuation of atmospheric pressure over the North Atlantic Ocean that has a strong effect on winter weather in Europe, Greenland, northeastern North America, North Africa and northern Asia," the report said.

The authors added that climate change and increased temperatures are likely causes, as well. There could also be local, man-made elements, like the deepening of shipping channels over the last 170 years, which could influence the direction of tidal waters.

"If it turns out to be a local reason ... there could be local solutions," Talke said in the release. "In some cases, we may be able to turn back the clock on that a bit."