With a rising tide lapping at their edges, tidal marshes on many parts of the Eastern Shore are being swallowed by the Bay at a rate faster than they can be replaced.

"A lot of people think the marshes will move inland, but that doesn't happen," said Court Stevenson, a scientist at the University of Maryland Center of Environmental Science Horn Point Laboratory who has seen rising waters engulf hundreds of acres of the nearby Blackwater National Wildlife Refuge over the years. "The water levels have been rising too fast."

This is nothing new. Globally, sea level rise has been observed for about 18,000 years, with the rate accelerating since about 1600. The Chesapeake, in fact, is actually a section of the Susquehanna River that was drowned by rising water after the last ice age.

But today, water levels in the Bay are increasing by about 3 millimeters a year - twice the global average. Scientists believe that is likely because the sea is not only rising, but the coastal areas in the Mid-Atlantic are slowly sinking at the same time.

To better understand what is going on, the U.S. Geological Survey has started a long-term analysis of sea level change in the Bay over the past 6,000-8,000 years, based largely on a detailed study of sediments.

The idea, said Curtis Larsen, who is heading the USGS project, is to determine how the rate of sea level rise has varied over time. If levels begin rising at a faster pace than has been observed in the past, it could signal that effects of climate change are being felt in the Bay.

While few things are certain about global climate change, there is close to universal agreement on this: If the Earth warms, sea levels will go up. The reasons are clear. First, warmer water expands. Second, glaciers on land will continue to melt away as they have for the past century, putting more water into the oceans.

In the next century, sea levels could increase from 18 inches to 3 feet, according to estimates by a panel of scientists assembled by the United Nations. That would be in addition to the effects of land subsidence already taking place in the Bay, meaning that the level of the Chesapeake would be rising even faster than it is today.

Even at the present rate, the Bay is swallowing up islands. Some - such as Sharps Island, which once measured 890 acres - are gone. Others are going fast: 91 percent of Poplar island is gone, 64 percent of Barren Island, 21 percent of Hoopers Island, and so on.

"That's what's happening to the Bay and that's what's happening to low-lying areas," Larsen said. "Sea level rise in the Chesapeake is a natural, ongoing process that we can do little about." That's bad news for many coastal marshes. While new marshes can be formed over time, existing ones are being lost faster than they can be replaced, Stevenson said.

Some, like those at the Blackwater refuge and a few other "hot spots" around the Bay, are "sediment starved" - that is, too few sediments are entering the marsh to allow new, higher wetlands to be built up to replace the drowned marshes.

In other areas, the reaction of homeowners may also doom marshes. Often, they protect their land by building concrete barriers - something that prevents marshlands from migrating inland, as they would naturally.

The result is that tidal marshes - historically thought to be the wetlands most protected by regulation - are starting to lose ground, literally, around the Bay.

"The normal human action is to armor the shoreline," said Victor Kennedy, a professor at Horn Point. "People set up armor or dikes or levees to keep the sea at bay, and you lose your marshes because they have nowhere to go inland."

When those barriers end up at the water's edge, they create more problems because they reflect waves back into shallow areas where they erode grass beds and cloud the water with sediment churned up from the bottom. "It degrades the shallow water habitat in the vicinity," Kennedy said. "It is not the kind of habitat that is conducive for nursery areas for the young of many species of aquatic animals."

The situation for islands and marshes has been getting worse in recent years as the rate of erosion has increased, Stevenson said. In part, that is because there has been a period of unusually high tides, influenced in part by astronomical events that run in cycles.

But Stevenson said he believes the increased rate of erosion is more than can be explained by those factors alone, and that it is a symptom of a warming climate. "It's happening now," he said. "It's not 'what if.' We're seeing it now, here, in a big way."

Not only that, he said, the effects are slowly reaching inland. Tides are reaching farther up drainage ditches, slowly killing loblolly forests at the edge of marshes, and reaching into the fields beyond. Stevenson said some people are starting to sell out and - literally - head for higher ground. "When I started research at Blackwater, people didn't want to sell because they had pretty nice, fertile, flat land," he said. "They were looking at the refuge as the enemy that kept wanting to buy their land. But for the last 10 to 15 years, everybody's been trying to get out of it because they realize that they can't keep farming it."