"In the 21st century, as temperatures rise, what will sea level do? It doesn't take a rocket scientist to realize that sea level will rise, too."
The overall finding roughly matched previous predictions. But "it's nice to see this finally confirmed with real data," said Rob Thieler, a research geologist with the U.S. Geological Survey, which funded part of the work.
It also lends credence to the upper-range predictions of sea-level rise globally - about three feet by the end of the century - said Pennsylvania State University meteorology professor Michael Mann, who participated in the study.
With many coastal areas just slightly above sea level, more accurate predictions are crucial for flood planning.
Rutgers University sea-level expert Kenneth G. Miller said he recently told Gov. Christie to plan for a rise of about three feet - resulting in a loss of about 3 percent of New Jersey's land area - by 2100.
This would amount to about 170 square miles, most in ecologically critical marshlands, which would experience a 30 percent loss.
Flooding would be more common, said Miller, who was not involved with the new research. The current "100-year" storm causes a surge of about eight feet. Given the sea level rise he expects by 2100, such surges would recur annually, flooding access to bridges, tunnels and Newark International Airport, Miller said.
The new research was published Monday in the Proceedings of the National Academy of Sciences.
To come up with the 2,100-year timeline of sea-level change, the scientists analyzed core samples of sediment taken from salt marshes in North Carolina.
They were looking for fossils of microscopic creatures called foraminifera, planktonlike organisms that live in the oceans. Different species live at different depths, so by dating the sediment layer and identifying the species, they can tell how deep the ocean was at a particular time.
Core samples were central to the previous studies upon which this one was built.
Now, with the models more firmly established and calibrated, researchers are working to develop sea level records specific to New Jersey, Florida, and Connecticut.
Based on the core samples taken from marshes near the Outer Banks, the scientists found, the sea level was relatively unchanged from at least 100 B.C. until 950 A.D.
For the next 400 or so years, the sea rose about a quarter-inch per decade.
Then, sometime between 1270 and 1480, sea levels once again stabilized or even dropped slightly.
But between 1865 and 1892, in the years following the Industrial Revolution, the sea level began rising sharply to an average of nearly an inch per decade.
"This is a very important contribution, because it firmly establishes that the rise in sea level in the 20th century . . . is unprecedented for the recent geologic past," Miller said.
People who dispute a human role in global warming "always argue that climate change is part of a natural cycle," he said. The new report offers "a robust conclusion" to the contrary.
What was also significant, he and other scientists said, was the consistent link the researchers found between changes in sea level and changes in global temperature in the past - providing a kind of cross-checking and better calibration for models that could be used to predict the future.
Before this, scientists had a pretty good idea of above-the-surface temperature changes, based on tree rings, ice cores, and other data.
What they lacked was a clear grasp of sea-level change and how it varied over shorter spans of history - which is what they consider 2,000 years to be.
Current models predicting how sea level might change in an altered climate also have had difficulty accounting for the future impact of less-studied processes such as melting of ice sheets.
Because this new record of past sea level is based on observations, not predictions, it presumably includes all the relevant processes contributing to sea level rise - including the melting of the ice sheets atop Greenland and Antarctica.
Still, the study looked only at the historical record.
"If you take this relationship between temperature and sea level in the past, to extrapolate into the future, it may not be correct," Miller said. For instance, he said, ice might respond differently once the "easy" ice has melted.
Contact staff writer Sandy Bauers at 215-854-5147 or firstname.lastname@example.org. Visit her blog at http://go.philly.com/greenspace.