Maximum water levels in New York harbor during major storms have risen by nearly two-and-a-half feet since the mid-19th century. That means the chances of water spilling over the Manhattan seawall are at least 20 times greater than they were 170 years ago.

Manhattan Might Be Underwater Sooner Than We Expected

Stefan Talke—an assistant professor of civil and environmental engineering at Portland State University—came to that conclusion after studying hundreds of pages of handwritten tide data going back to 1844.

Since then, the global rise in sea levels has increased the height of water along New York Harbor by nearly one-and-a-half feet. On top of that, every few years, the city is hit by a storm large enough to temporarily raise water levels even further. The maximum height of this storm tide has increased by almost a foot since the 19th century.

Manhattan Might Be Underwater Sooner Than We Expected

By combining this data, Talke and his colleagues calculated that waters could be expected to spill over the lower Manhattan seawall once every four to five years. In the mid-1800s, when both sea levels and storm tides were lower, water was expected to overtop the seawall only once every 100 to 400 years.

The scientists speculate that climate change and increasing global temperatures are the primary culprits. But there could also be local factors—like the deepening of shipping channels around New York harbor—which could have affected storm tides in the area over the past 170 years.

Whatever the cause, New York City can expect more frequent and extensive flooding, similar to the aftermath of Hurricane Sandy last year.

Read the study in the journal, Geophysical Research Letters.