"When a storm, such as Hurricane Sandy, sets waters in New York Harbor rising, those sloshing seas are now 20 times more likely to overtop the Manhattan seawall than 170 years ago, a new study finds.
The increased risk comes from a combination of sea level rise — which has raised water levels near New York City by nearly 1.5 feet since the mid-1800s — and storm tide, or the amount that local seas rise during a storm. Storm tide is itself a combination of storm surge (the water that a hurricane pushes ahead of it) and the astronomical tide.
The rise in sea level and storm tide combined puts the odds of storm waters overtopping Manhattan’s defenses at one in every 4 to 5 years, compared to only once in every 100 to 400 years in the 19th century, the study found. (Put another way, the annual chance of a storm overtopping the seawall has gone from about 1 percent to 20-25 percent.)"