The Flood Next Time

by Juston Gillis,

The little white shack at the water’s edge in Lower Manhattan is unobtrusive — so much so that the tourists strolling the promenade at Battery Park the other day did not give it a second glance.

The equipment inside is linked to probes in the water that keep track of the ebb and flow of the tides in New York Harbor.

While the gear today is of the latest type, some kind of tide gauge has been operating at the Battery since the 1850s, by a government office originally founded by Thomas Jefferson. That long data record has become invaluable to scientists grappling with this question: How much has the ocean already risen, and how much more will it go up?

The work starts with the tides. Because of their importance to navigation, they have been measured for the better part of two centuries. While the record is not perfect, scientists say it leaves no doubt that the world’s oceans are rising. The best calculation suggests that from 1880 to 2009, the global average sea level rose a little over eight inches.

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.

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.

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.

Eventually just a single, sturdy Victorian house, built in 1888, stood on a remaining spit of land, seeming at high tide to rise from the waters of the bay itself. A few years ago, a Washington Post reporter, David A. Fahrenthold, chronicled its collapse.

… some places sit on soft sediments that tend to compress over time, so the localized land subsidence can be even worse than the regional trend. Much of the New Jersey coast is like that. The sea-level record from the Battery has been particularly valuable in sorting out this factor, because the tide gauge there is attached to bedrock and the record is thus immune to sediment compression.

These numbers are profoundly threatening, and among the American public, the impulse toward denial is still strong. But in towns like Norfolk — where neighborhoods are already flooding repeatedly even in the absence of storms, and where some homes have become unsaleable — people are starting to pay attention.

“In the last couple or three years, there’s really been a change,” said William A. Stiles Jr., head of Wetlands Watch, a Norfolk environmental group. “What you get now is people saying, ‘I’m tired of driving through salt water on my way to work, and I need some solutions.’ ”

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