Once-Rare Flooding Could Hit NYC Every 5 Years with Climate Change, Study Warns

New York City is at increasing risk from storm surge flooding as sea level rises. Hurricane Sandy provided a preview of what the city could soon see more often.

New York City was worried about sea level rise when it issued new recommendations this year that future buildings and other structures that are expected to last through the end of the century be raised 3 feet above current requirements. Credit: FEMA National Flood Hazard Layer

Climate change is dramatically increasing the risk of severe flooding from hurricanes in New York City, to the extent that what was a once-in-500-years flood when the city was founded could be expected every five years within a couple of decades.

Throughout the century, of course, the risk of flooding increases as sea levels are expected to continue to rise.

These are the findings of a study published today that modeled how climate change may affect flooding from tropical cyclones in the city. The increased risk, the authors found, was largely due to sea level rise. While storms are expected to grow stronger as the planet warms, models project that they'll turn farther out to sea, with fewer making direct hits on New York.

However, when sea level rise is added into the picture, "it becomes clear that flood heights will become much worse in the future," said Andra J. Garner, a postdoctoral researcher at Rutgers University and the lead author of the study.

The paper, published in the Proceedings of the National Academy of Sciences, combines the high-emissions scenario from the Intergovernmental Panel on Climate Change with newer research that assumes more dramatic melting of Antarctic ice sheets to come up with a worst-case scenario for sea level rise. The projection shows waters surrounding New York rising anywhere from about 3 to 8 feet by 2100.

To put that in perspective, New York City's subway system starts to flood at about 10.5 feet above the average low water mark, as the city saw during Hurricane Sandy five years ago, and Kennedy Airport is only about 14 feet above sea level.

"If we want to plan for future risk, we don't want to ignore potential worst case scenarios," Garner said.

In May, the city published guidelines for builders and engineers recommending that they add 16 inches to whatever current code requires for elevating structures that are expected to last until 2040, and 3 feet to anything expected to be around through 2100.

That falls in the lower half of the range projected by the new study. By the end of the century, it says, the flooding from a once-in-500-years storm could be anywhere from about 2 feet to 5.6 feet higher than today.

Garner said that while the models consistently showed storms tracking farther out to sea, it's possible that changing ocean currents could cause the storms to stay closer to shore. If that were to happen, flooding could be even worse.

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