Much of the population and economy of the country is concentrated on the East Coast, which the accumulating scientific evidence suggests will be a global hot spot for a rising sea level over the coming century.
Scientists say the East Coast will be hit harder for many reasons, but among the most important is that even as the seawater rises, the land in this part of the world is sinking. And that goes back to the last ice age, which peaked some 20,000 years ago.
As a massive ice sheet, more than a mile thick, grew over what are now Canada and the northern reaches of the United States, the weight of it depressed the crust of the earth. Areas away from the ice sheet bulged upward in response, as though somebody had stepped on one edge of a balloon, causing the other side to pop up. Now that the ice sheet has melted, the ground that was directly beneath it is rising, and the peripheral bulge is falling.
The evidence suggests that the sea-level rise has probably accelerated, to about a foot a century, and scientists think it will accelerate still more with the continued emission of large amounts of greenhouse gases into the air. The gases heat the planet and cause land ice to melt into the sea.
The official stance of the world’s climate scientists is that the global sea level could rise as much as three feet by the end of this century, if emissions continue at a rapid pace. But some scientific evidence supportseven higher numbers, five feet and beyond in the worst case.
Some degree of sinking is going on all the way from southern Maine to northern Florida, and it manifests itself as an apparent rising of the sea.
The sinking is fastest in the Chesapeake Bay region. Whole island communities that contained hundreds of residents in the 19th century have already disappeared. Holland Island, where the population peaked at nearly 400 people around 1910, had stores, a school, a baseball team and scores of homes. But as the water rose and the island eroded, the community had to be abandoned.
One of the most ambitious attempts to take account of all known factors came just a few weeks ago from Kenneth G. Miller and Robert E. Kopp of Rutgers University, and a handful of their colleagues. Their calculations, centered on New Jersey, suggest this is not just some problem of the distant future.
People considering whether to buy or rebuild at the storm-damaged Jersey Shore, for instance, could be looking at nearly a foot of sea-level rise by the time they would pay off a 30-year mortgage, according to the Rutgers projections. That would make coastal flooding and further property damage considerably more likely than in the past.