If water rises too quickly, it may reach a threshhold beyond which the marsh quickly deteriorates.
Along the edge of a marsh on the Eastern Shore of Maryland, the grasses give way to mud, and then more mud, and then muddy water. Suzanne Baird is the refuge manager for Blackwater National Wildlife Refuge, and Matt Whitbeck is a wildlife biologist here. They offer a word of caution: "It's gonna be muddy, you're gonna sink up to your butt if you walk out there."
Blackwater National Wildlife Refuge has forests, waterways, and currently about 8,000 acres of marsh. Once upon a time, Whitbeck says, it had 13,000.
"In 1933 when this refuge was established, this was a broad, vast, almost prairie-like expanse of sedges and grasses that are just critical to wetland-dependent species -- and now all you see is a vast expanse of shallow, open-water habitats," he says.
The reason? Sea level is rising. And the geology of the region is such that the ground is sinking. Conservative measurements put the water-level rise so far at one foot every hundred years. Striking aerial photos show this lake springing into existence from the 1930s to the 1950s to 2005.
"It is amazing. There are a lot of places you can go in this refuge, out in that open water you're running the boat across the shallow water, and the boat is bumping against stumps," he says.
We're talking tree stumps, as in parts of that lake used to be forest.
"It's happened so fast those stumps haven't decayed. It's phenomenal the rate at which these habitats are changing," he says.
Humans made this significantly worse when they introduced the nutria, an invasive giant swamp rat from South America used for its fur. It ate the very root system out from under the marshes, making them even more susceptible. The nutria has just about been exterminated. But for the creatures that belong here, all the muddy water that remains is a problem.
Whitbeck points to a line of about 5,000 birds a few hundred feet away.
"You have the snowgeese in the background -- that's most of the chatter -- and then you have a few tundra swans here talking amongst themselves," he says.
Two-hundred-fifty bird species -- ducks, swans, bald eagles -- call this place home, or at least stop by to visit while migrating.
"Hundreds of thousands of birds that come through here at various times," he says.
With less space to feed, there are fewer birds than before. Back in the '30s, there were more black ducks here than anywhere south of Labrador, Canada. There's nothing like those numbers around now. Many of the Tundra swans now overwinter in North Carolina and Virginia. And even fish don't do well in the murky, muddy water that's overtaking the marshes -- there's little in the way of underwater plants to shelter them.
"This is the future for the Eastern Shore," says Stephen Gill, a chief scientist with NOAA's National Ocean Service, where they've been measuring sea-level rise for a hundred years.
"A large part of that Eastern Shore of Maryland on the Chesapeake Bay is very low-lying marshland," Gill says. "It's acres and acres and hundreds of thousands of acres of marsh all very low-lying that are at risk."
In principle, marshes can move to higher ground as water rises. But if the water rises too quickly, like it has at Blackwater, the marshes can't keep up. They no longer hold the soil down, and the low-lying shore just unravels like a sweater.
Plus, Gill says, in a lot of areas on the Eastern Shore, there's just not that much higher ground to migrate to. He can't predict how much of the shore will disappear, because different scientists predict different trends in future sea-level rise. Some say 7 inches in a hundred years, others say 5 feet. But, he says, "there's no trends going down."
Back at Blackwater, refuge manager Baird is not giving up.
"We know we can build marsh," she says.
Man-made restoration, though not always economical, is possible.
"Part of this along the fringe of this forest here is actually built," Baird says.
Biologists here are trying everything -- hay bales to hold in dredge material, replanting aquatic grasses, they even burn the marsh every year to promote root growth.
The idea is not necessarily to rebuild those lost 5,000 acres of wetland, but to at least give what's left time to move -- if it has a place to move to, says Whitbeck.
"These are changes that a lot of coastal habitats are gonna face eventually. So it really gives Blackwater an opportunity to be a role model on how to adapt to these changes," he says.
This, after all, is the future.