The United States has a debt, etched in stone, to pay back to the sea. Because of human-caused warming, global sea level has increased at 3 millimeters a year for the past two decades. Sucking in 90 percent of the excess heat trapped by greenhouse gases, the ocean is the world's thermometer, running hot. But as the surge sloshed into New York Harbor last week by Superstorm Sandy made clear, sea-level rise is also a deeply local phenomenon. There is no average ocean. There are only particular coastlines, shaped by geology, currents and gravity. The past matters. And in few places does it matter more than the United States.