The Chesapeake has been losing ground to the sea for 10,000 years, but the pace of sea level rise during the past century—and the projected rise in the coming 100 years— threatens to wash more than just dirt into the Bay.

Water levels have risen about a foot in the past 100 years, wiping some islands off the map, eroding shorelines and washing away ecologically valuable marshes. Sediment from that erosion clouds the water, contributing to the loss of underwater grasses.

But water levels in the coming century are expected to rise two to three times as fast, threatening not only coastal ecology, but also human development, which is moving to the waterfront.

“We’re on a collision course in many ways,” said Stephen Leatherman, director of the Laboratory for Coastal Research at Florida International University. “We’ve got a huge rush of population toward the coast.”

Leatherman, one of the nation’s leading experts on the impacts of sea level rise, spoke at a November conference that brought together Bay region researchers and state officials to improve their understanding of storm effects and rising water levels and to increase coordination among agencies and scientific institutions.

One impact of sea level rise was experienced during Hurricane Isabel on Sept. 19, 2003. In many areas, the storm surge that accompanied Isabel rivaled or exceeded that of an unnamed hurricane of similar strength that followed a similar path up the Bay in 1933.

Because Bay water levels were about a foot higher when Isabel hit, it flooded areas left untouched by the 1933 hurricane.

But, Leatherman said, “Isabel was just a taste of it.” People pressing to live near the Bay can expect more in the future. And as sea levels rise, the next hurricane that comes up the Chesapeake will push water even further inland.

While catastrophic events like Isabel grab headlines, the accelerated rate of sea level rise is a larger threat to coastal ecosystems, including the Bay, according to Leatherman.

Yet coastal residents and government planners rarely take rising water levels into consideration when making decisions, he said, because it seems so slow, and insidious. “These things creep up on people,” Leatherman said. “We are a crisis driven society. It’s easy to forget about sea levels.”

Over time, the impacts can be dramatic.

Sea levels have been on the rise since the end of the last ice age, 10,000 years ago. But the pace has been accelerating. The past century has seen the total loss of some islands in the Bay, such as Sharps Island near the mouth of the Choptank River. Huge amounts of other islands have washed away. Blackwater National Wildlife Refuge on Maryland’s Eastern Shore has lost about a third of its total marsh area between 1938 and 1988.

As shorelines erode, they unleash tons of sediment that smother nearshore underwater grass beds, one of the Bay’s most important habitats. When grasses disappear, they no longer help to buffer the shore against wave action, creating the potential for even more erosion.

The increasing rate of sea level rise is primarily the result of warming temperatures, which cause water to expand and increase the melting of glaciers and polar ice sheets.

Ocean levels are expected to rise another two feet by the end of this century—and an increase of three feet is considered possible. The increase in the Chesapeake could be even greater because much of the land around the Bay is also subsiding.

Leatherman said that on average, a foot of sea level rise means water erodes inland by about 150 feet, but the exact distance varies by geology. In highly erodible areas like the Eastern Shore, which has fine clay soils, one foot of rise could mean water moving hundreds of feet inland, he said.

“If you are living on an island or low-lying land, you ought to think of putting your house up a little higher than you would have before,” Leatherman said. He said dikes protecting some cities along the Bay will likely need to be built higher. In the countryside, farmers near the water’s edge are already losing access to parts of their fields because their tractors can no longer drive over the soggy ground. “Salt marsh grasses are going to march over their fields,” he said.

While it’s impossible to stem erosion in an area as big as the Bay, he said efforts were warranted to buffer shorelines with underwater grass beds in highly erodible areas.

As with Isabel, the impact can by magnified by hurricanes. Right now, the frequency of hurricanes appears to be on the increase, which may be part of a natural cycle which recurs every few decades, said Hans Paerl, a professor of marine and environmental Sciences at the University of North Carolina-Chapel Hill, a speaker at the conference.

The number of hurricanes was also high in the 1930s and early 1960s. And in 1893, four hurricanes hit near Beaufort, NC, where his lab is located, Paerl said. “We may not be living in such an unusual period when we look at long-term cycles.”

What has increased is the number hurricanes which become expensive natural disasters. But that increase is caused by the huge influx of people into hurricane-prone coastal areas in recent decades. “These are natural events,” Paerl said. “They become disasters when people are involved.”