As Hurricane Sandy lashed the East coast last October, the ocean invaded New Jersey. It revealed a snapshot of a possible future with a warmer climate, where the ice on Greenland and Antarctica melt and sea levels rise globally.

That future has already begun.

Global average sea level is rising at about 2.39 millimeters per year; at the local level, the ups and downs that build that average have far greater impacts.

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New Jersey is one of those places. Today the ocean is lapping at the Jersey shore at triple the rate seen in the past 2,000 years, suggests a study published in the Journal of Quaternary Science on May 28.

The last time the seas were rising this fast off New Jersey, it was between 10,000 and 6,000 years ago, at the beginning of the Holocene epoch. Glaciers were melting. The ice sheet that had covered large parts of North America was receding, and it caused the land mass to subside. Mammoths, mastodons and saber-toothed cats were dying out. Sea levels rose the world over. Off New Jersey, the rate was 4 millimeters a year.

This tale is told in the fossilized sediments of coastal wetlands in New Jersey. Benjamin Horton, a researcher at the University of Pennsylvania and Rutgers University, and his colleagues drilled out sediment cores from the wetlands. They examined the older layers, which they dated through radiocarbon measurements.

Fossils of protozoans called foraminifera revealed how high the ocean once was. These critters are sensitive to particular salinities, so a change in the sea level would mean a species better suited to the changing salinity would move in. Horton and his colleagues collected a series of these index points that showed sea levels at various points in history.

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They found that the oceans calmed down between 6,000 and 2,000 years ago to 2 millimeters a year. The oceans were still rising, but much of the relative sea level rise was because the land mass of New Jersey was subsiding.

From 2,000 years ago until the year 1900, sea levels rose by 1.3 millimeters a year, primarily due to land subsidence.

Today, the sea level rise at Atlantic City is about 4 millimeters, and much of it is due to human activity, said Horton.

The sea is rising at 2 millimeters a year as ice sheets are melting due to climate change. Another 1 millimeter is likely due to the rapid withdrawal of groundwater. The remaining 1 millimeter is due to land subsidence caused by natural geological processes.

The EPA finds that if the ocean off New Jersey rises by an additional 2 millimeters a year, it will threaten the wetlands themselves. And as Hurricane Sandy demonstrated, it will test the resiliency of our coastal cities.

IMAGE: Workers on lunch break watch the removal of the Star Jet roller coaster on May 14, 2013 in Seaside Heights, New Jersey.

Mark Wilson/Getty Images