More floods in the future?

On a moonlit night almost 20 years ago, a geologist sneaked onto private land deep in the Calverton woods, canoed to the middle of a large pond and began lowering a long rope he had carefully knotted at three-foot intervals. He never got to the second knot.

When Steve Englebright’s rope touched the bottom of Swan Pond after just four feet, instead of the 50 feet he was expecting, his secret midnight trip with two other environmental activists to study the pond suddenly became a mystery as well as a mission.

The mission was accomplished soon afterward when Suffolk County bought the property to block a planned golf course expansion. And the mystery is being unraveled two decades later as Englebright, now a state assemblyman, and other naturalists discover just how profoundly Long Island is being reshaped by a relentless but often overlooked force: the rising sea.

Sea level around the world has risen about 350 feet since the climax of the last Ice Age 22,000 years ago, as gradually increasing global temperatures have melted polar ice and swelled the oceans. Few places on Earth have been changed as dramatically by the rising tide as Long Island.

Twenty thousand years ago, Long Island wasn’t an island at all, but an elevated area of a broad coastal plain that extended as far south as Atlantic City. Since then, the rising sea has moved the southern shoreline 80 miles north, and flooded the valley we know as Long Island Sound.

If sea level continues to rise more than an inch per decade, and especially if that rate doubles or triples because of pollution-induced global warming, as many experts predict, Long Island will become a very different place over the next century.

The sea may split the narrow South Fork into at least three sections, and shoreline residents from Montauk to Long Beach will face a grim choice of either abandoning their property or building barriers to hold back the rising sea, as in Holland.

“In the long term, you can envision dikes around places like Oakdale and Bay Shore. All that would be missing are the tulips,’’ said Henry Bokuniewicz, a professor of oceanography at the State University at Stony Brook and an expert on sea-level changes.

Windmills may be there, too, he said, because the shrinking coastline is only the most obvious of the many ways in which rising sea level is transforming Long Island. As in Holland, the rising ocean waters also are pushing up the underground water table, flooding formerly dry areas. That’s why the Dutch use windmills: to pump out water-saturated land.

Some scientists think the water table may be rising even faster than sea level. Here’s why: Long Island’s freshwater aquifers are like a bubble floating on a curved bed of salt water. When sea level rises, the salt water compresses the bubble from below and from all sides, forcing the fresh water upward even faster than the rising sea.

Of course, none of these changes is happening overnight, and there are other complicating factors. Heavy rains raise the water table, and heavy pumping by water wells lowers it. The land isn’t stable, either. Various parts of Long Island are still slowly rising or falling, in a residual effect of the last glacier. That ice sheet was so heavy that it temporarily distorted the Earth’s crust, which is still gradually returning to its former shape 1000 generations after the glacier receded north.

Nor has the 20,000-year trend of warmer weather and higher seas been a steady rise. Almost all of the increase came during the first 14,000 years. Since then, the up-and-down oscillations have been small, although the experts do agree that the current trend is upward — abetted, most likely, by ever-increasing emissions of carbon dioxide and other so-called “greenhouse gases.’’ These gases, byproducts of burning coal and oil, trap heat in the atmosphere like the window panes of a greenhouse.

It’s possible, experts say, that the warming trend will end soon, and sea level will begin falling again. Rhodes Fairbridge, a retired professor of geology at Columbia University, has become famous among his peers for suggesting that temperature and sea level rise and fall in complex but predictable cycles. The cycles, he said, may follow shifts in the orbits of the nearby planets, since planetary alignment affects sunspot activity and the amount of solar radiation reaching Earth, two factors known to influence climate. Markings on cliffs near his Amagansett home, Fairbridge said, show that sea level was actually about 10 feet higher 5,000 years ago than it is today, and has gone through several ups and downs since then, including a decline during the so-called “Little Ice Age’’ of 300 to 400 years ago and a subsequent rebound that is still continuing.

Until the next shift, the rising tide will continue to reshape the Island. The ocean will drown existing beaches and create new ones farther inland, while dry areas will become flooded marshes.

That’s exactly what has happened at Swan Pond, the place Englebright discovered was only four feet deep when he made his secret expedition there 20 years ago.

Scientists have known for a long time that many of Long Island’s lakes and ponds, including Lake Ronkonkoma and Lake Success, were so-called “kettlehole lakes’’ formed by large chunks of ice left behind by the last glacier. When those chunks melted, they left deep depressions that groundwater filled.

But Swan Pond and many nearby ponds in Calverton, Englebright discovered, were shaped like frying pans, not kettles.

Several years later, he began to figure out why. Digging 24 feet down into a Shelter Island swamp, Englebright uncovered dry sandy soil at the bottom that looked exactly like the sand now found in the pine barrens of Brookhaven and Southampton. Over hundreds or even thousands of years, he decided, the gradually rising water table must have transformed a dry area into a bog.

Englebright believes something similar happened at Swan Pond. Like the kettleholes, the pond was a product of the last Ice Age, but was formed by a thin layer of melting permafrost, not a big chunk of glacial ice. The shallow depression was exposed for thousands of years until the rising water table finally claimed it.

If the tide keeps rising, he said, many more formerly dry areas will eventually flood, including the backyards of suburban homes in communities farther inland, such as Middle Island and Yaphank. “Those neighborhoods are the next Calverton ponds,’’ Englebright said, “whether they like it or not.’’