Looking deep into the geologic past, researchers at the University of Pennsylvania have learned that along the Atlantic Coast, including New Jersey, sea level rose three times faster during the 20th century than it did during the previous 4,000 years.

At one location in North Carolina, they fixed the date of the rapid acceleration to between 1879 and 1915, after the Industrial Revolution had taken deep hold, lending credence to the connection between the rising temperatures that occurred then and rising sea levels.

"If that happened in the past, it gives you strong confidence that the predictions of increased sea-level rise in the 21st century are true," said lead researcher Benjamin P. Horton, a professor in Penn's sea-level research laboratory.

Scientists predict sea levels will rise as a result of global warming, "but by how much, when, and where it will have the most effect is unclear," Horton said.

"Lots of people are looking at the future," he said. "The problem is, they've been looking at the future without having the information to understand the past."

Horton also can't predict the rise in particular Jersey Shore towns. Still, he and others said the studies suggested a strong acceleration in overall sea-level rise along the U.S. Atlantic Coast, including in New Jersey and Delaware.

Levels there are rising not only because of higher water - due to melting polar ice and expansion of a warmer ocean - but because the land is sinking.

In two papers published recently in the journal Geology, the researchers and collaborators teased out details of the complex relationship between the land and sea on the East Coast, mapping an epic geologic history that could help scientists predict what might happen under various climate-change scenarios.

They examined a century's worth of tide-gauge data and scrutinized marsh muck thousands of years old.

Thomas M. Cronin, a geologist with the U.S. Geological Survey in Reston, Va., called their work "critical for establishing baseline sea-level information."

Their research also suggests that the Greenland ice sheet may have begun melting before scientists think it did, a possibility that Rutgers University's Kenneth G. Miller termed "new and pretty darn interesting."

What makes studying sea level so difficult is the fact that sea level isn't really level at all, like some big bathtub.

It varies from place to place due to a variety of factors, including the "gravity" of Greenland and polar ice masses, which attract water much like planets in space influence one another.

Hard-boiled egg

So while a research paper published in the Proceedings of the National Academy of Sciences this month estimated that sea level could rise as much as six feet above 1990 levels by 2100, it would be much more than that in some places, much less in others.

And solid land isn't really the immobile hunk most people imagine. Along the coast of New Jersey, it's sinking.

Here's why: Horton likens the earth to a hard-boiled egg. The firm exterior where we live is like the shell. The interior, like the egg, is malleable.

About 20,000 years ago, much of North America was covered by ice, some of it 2.5 miles thick. Its massive weight caused the earth underneath to compress.

South of the ice - in this region, roughly at the top of what is now New Jersey - the earth squished up in a formation called the "forebulge."

After the ice melted, places such as Newfoundland began rising again. New Jersey and parts south began sinking.

The rate in New Jersey is about one inch every 10 years, faster than anywhere along the coast from Maine to South Carolina and double the rate in Boston or Charleston, S.C., Horton says.

"The important thing to realize," Horton said, "is that sea level is going up due to two factors." Not just because the ocean is rising, but also because the land is subsiding.

In the first of the two studies, Horton, postdoctoral scientist Andrew C. Kemp, and others looked at sea levels in North Carolina dating back to 1500.

They took core samples from two marshes - one at Cedar Island near Beaufort, the other near the lost colony of Roanoke, where North America was first colonized.

Just half a century ago, it would have been impossible to accurately date the layers of sediment. But that changed with the discovery of carbon dating in the 1950s, Kemp said.

The researchers also used lead concentrations from auto emissions, cesium spikes from nuclear detonations, and pollen changes from when early Europeans replaced the natural vegetation with crops - all detectable in the sediment.

Horton and Kemp's work used a relatively new method of mapping sea levels within core samples by looking at the proliferation of two microscopic organisms that live on the marsh - diatoms, which are algae, and foraminifera, single-celled organisms that, under magnification, resemble snail shells.

Both of them respond to even minor salinity changes or to being submerged under water for too long, so they are natural recorders of past sea levels.

Using those creatures, the researchers found that sea level rose at a consistent rate of half an inch per 10 years - the "background" rate due to the land sinking.

But then, between 1879 and 1915, it began an abrupt increase that eventually reached 11/2 inches per 10 years, triple the former rate. This they attributed to the water rising from melting glaciers and ice caps, plus the fact that the ocean expands when it gets warmer.

When that first study came out, climate-change skeptics responded within a matter of hours, Horton said.

A former television meteorologist posted on his Web site graphs of sea-level rise mapped from University of Colorado data, showing a slowing of the rise. He said it disproved Horton.

Other skeptics commented. Believers jabbed back. Insults were lobbed.

Horton was aghast at the response. He said - and the University of Colorado confirmed - that to understand what's happening requires looking at long stretches of geologic time, as he did, not the comparative blip of a decade, as the skeptic did.

'Two take-homes'

A month later, in the second paper, Horton and Penn postdoc Simon E. Engelhart, along with others, looked further back in time, to 4,000 years ago. They also expanded their coastal view from Maine to South Carolina.

They were able to substantiate the results of the first study on this bigger canvas.

And Engelhart, who crunched the numbers, "was able to produce the first accurate estimates" of rates of sinking along the coast, Horton said.

Scientists were aware of the basics, "but nobody ever pulled the numbers," said West Chester University geomorphologist Daria Nikitina, who was not involved with their studies but is looking at sea-level rise along the Jersey shore of Delaware Bay. "They compiled all the data, everything that was ever collected, reported, and published for the whole Atlantic Coast, then very, very carefully looked at every single data point."

"There are two take-homes here," added Kenneth Miller, a Rutgers University researcher who reviewed the studies. "There's been an acceleration of sea-level rise from pre-Industrial Revolution rates.. . . And we're throwing on one to two millimeters a year in New Jersey due to sinking."

The Penn group's conclusions were all the stronger because they were based on observational data, not models, Miller said.

By factoring in the data from tide gauges and the land subsidence, they also were able to show that the portion of sea-level rise from the oceans also was different along different stretches of the coastline.

The rise was lowest in Maine, highest in South Carolina.

Looking at where the water could have come from, Horton said the data suggest - "and I strongly use the word suggest" - that Greenland may have been a major source of sea-level rise in the 20th century.

Horton and his students also have studied sea-level rise in relationship to cataclysmic events, including the 2004 tsunami in the Indian Ocean and Hurricane Katrina along the Gulf Coast.

Next, with a federal grant, he plans to find out in more detail what has been happening in New Jersey, including what additional effect major storms that hit the coast in the geologic past may have had.

To him, it's a way to marry his captivation with history to what's been going on with the earth.

"When you stand on the surface," Horton said, "you don't know what's under your feet. At all."

But when you examine layers of sediment that go back centuries, you notice "how rapidly things change."

Contact staff writer Sandy Bauers at 215-854-5147 or sbauers@phillynews.com.