100 years ago the sea level was a full 9 inches lower and a lot of NYC’s waterfront had just been reclaimed from the water to begin with, because most of it is unnatural. huge portions of the modern end of Manhattan and adjacent territory used to be water and were made into port infrastructure that isn’t exactly ideal for keeping out water (and that has subsequently been redeveloped into other stuff). if we didn’t artificially impose land there, a lot of NYC’s coast wouldn’t exist at all–and since we have and sea level rise is accelerating, those areas are extremely and increasingly prone to flooding.
it’s pretty likely most of NYC will need to actively be placed behind levees or other structures mid-century because just the “expected” sea level rise by 2050 is another 8 inches–not accounting for sinking–and anywhere from 15 to 75 inches (2 entire meters) by the end of the century. (and the most doomer estimates place potential rise at up to 5 meters!)