NewsJuly 25, 2011

Sea Level Rise to Put the "Squeeze" on Coastal Georgia

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Sea Level Rise to Put the “Squeeze” on Coastal Georgia

By Bruce Dorminey

Sapelo Island Ferry. Credit: Bruce Dorminey.

Darien, Georgia – Some 60 miles south of Savannah, Dorset Hurley strides into chest-high cordgrass on the mainland side of the Sapelo Island ferry dock. Standing in elevated muck on a recent steamy summer’s afternoon, he gestures toward a tidal creek running along an isolated spit of road.

“At high tide here, we would be ankle deep in water,” says Hurley, an estuarine ecologist at Georgia’s Sapelo Island National Estuarine Research Reserve. He snaps off the end of a dead cordgrass blossom and looks east long enough to notice that the tide has turned. In a few hours time, this tidal bank will again be inundated, but never for more than an hour or so each day. Once the tide rolls in, striped mullet feed on decaying cordgrass on the marsh’s soggy bottom. 

Like the tides on which these estuaries depend, Georgia’s 100-mile coastline has waxed and waned for thousands of years, surviving by shifting miles in the process. And it's doing so once more; again driven by warming oceans and melting glaciers.

Mention “global warming” in this part of the world and most people's eyes will glaze over. Sea level rise, however, has tangibility that residents here experience with every high tide.

But that doesn’t mean they’ve thought about how global sea level rise will impact them personally. In a recession-weary economy heavily dependent on tourism, real estate and the fishing industry, sea level rise hasn’t exactly hit the top of the charts — yet.

The state’s susceptibility to sea level rise is a function of the lay of the land. Because Georgia lies on a low continental slope, ecologists say that salt marsh loss is expected to be quite significant, both in terms of the area lost, and in reduced habitat for economically important local fisheries, such as shrimp, oysters and blue crabs.

Since Georgia’s inception as a colony in 1732, this coast has depended on these marshes for a large part of its livelihood. A few miles south of here, the broad Altamaha River slips into tidal marshes near the old Hofwyl-Broadfield plantation. There, landowners once used slaves and the estuary’s natural ebb and flow to grow massive quantities of long-grained rice. Today, the plantation’s dike and levee system lies in ruins, a poignant reminder of days gone by and the power of the sea.

But at dusk, when shrimp boats line the Darien River, some 50 miles south of Savannah, it's clear that a large chunk of the coastal Georgia economy still depends on these marshes. And it’s these marshes, and in turn the fishermen and businesses that generate tourism dollars, that may suffer the most from the inexorable rise in sea levels as the oceans warm and polar ice sheets melt.

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Although several of Georgia’s barrier islands are highly developed, most remain undisturbed as wildlife refuges. This should only increase the ecological value of the state’s coastal areas. That’s at least until sea level rise arrives in full force. In addition to providing habitat for oysters, shrimp, crab and fish, Georgia's roughly 300,000 acres of marsh are quite important for trapping carbon, otherwise known as “carbon sequestration.” Clark Alexander, a coastal geologist at the Skidaway Institute of Oceanography, says the marshes’ surface sediments can contain as much as 10 to 15 percent natural organic carbon, which is a factor in lessening the area’s overall carbon footprint.

“White shrimp come in as larval shrimp and they find their optimal habitat in these tidal [marsh] creeks,” says Hurley, noting that the shrimp use these same salt marshes as nurseries. “As they get larger, their habitat expands until they are fully developed. Then in a year’s time, they use the currents to swim offshore.”

Shrimp boat fisherman then scoop them up from the sea floor a few miles off the Georgia coast. White shrimp, blue crabs, and intertidal Georgia oysters all thrive in the salt marsh. 

Still, the marsh can only grow so fast, and likely not quickly enough to keep up with the current rate of sea level rise. 

Alexander says the Georgia coast is already experiencing local sea level rise rates of 2.98 mm per year, as recorded at Ft. Pulaski, near Savannah. But he notes that the second half of this century will likely bring even more accelerated rates of sea level rise, adding that because of the inertia in the climate system, sea level rise is not going to cease in 2100.

Even if we stopped putting out excess carbon dioxide, the main greenhouse gas that is likely responsible for helping to warm the planet, today, says William Savidge, a chemical oceanographer at the Skidaway Institute, it would still be at least several hundred years before its atmospheric effects abated. Thus, coastal residents are left to prepare for the inevitable.

Can the coast migrate this time?

Prior to human habitation, Georgia’s coastal marshes moved with the coastline, but today, human roots in the area run deep. Efforts to protect local infrastructure are expected to interfere with the marsh’s ability to keep up with a migrating coastline.

As sea level rises, Savidge says everyone will want to armor his or her piece of property. “As soon as that happens, the salt marsh no longer has an avenue of retreat,” he said, adding: 

“So, the real threat for the marsh is not sea level rise per se, but to where does it retreat?”

A 2007 report from the U.N. Intergovernmental Panel on Climate Change (IPCC) estimated that global average sea levels might increase by up to 0.6 meters by the end of the century, but that estimate did not take into account changes in polar ice sheet dynamics. Sea level rise may be even more pronounced in areas such as the Georgia coastline, where the curve of the land already amplifies high tides of 8.2 ft. On the Georgia coast, Alexander says a meter of sea level rise would mean that the actual shoreline would move about a half mile inland from its present location, on average.

The marsh and adjacent intracoastal waterway in coastal Georgia. Credit: Bruce Dorminey.

“We know that [rates of] sea level rise have increased by a foot over the last 100 years and it has been increasing in fits and starts since the last Ice Age,” said Chuck Hopkinson, a coastal systems ecologist at the University of Georgia in Athens. “Some areas of marsh are already showing signs of sea level rise-[related] degradation, with a lot of open water areas at the heads of some large tidal creeks.”

The formation of Georgia’s current 14 barrier islands dates back to some 12,000 years ago. But even within the last two million years, the location of the Georgia coast was as far west as Statesboro, more than 60 miles inland from today’s shoreline. The last time sea levels here were as high as today was roughly 125,000 years ago.

Chris Craft, a wetland scientist at Indiana University, who along with colleagues has predicted sea level rise on the Georgia coast of between 30 cm to 1 meter by 2100, is planning to re-run their simulations for coastal Georgia later this year.

“The [new] results should give us a better idea of how sea level rise-driven saltwater intrusion will alter wetland habitat and ecosystem services in the region,” said Craft.  “These data are critical for determining how coastal wetlands locally are responding to current rates of sea level rise and, hence, how they will respond to future rates.” 

Another recent study, based on data taken from microfossils in sediment cores from marshes on the North Carolina coast, demonstrates that “the rate of sea-level rise on the U.S. Atlantic coast [some 2 millimeters per year on average] is greater now that at any time in the past 2000 years.”

The research, which was published in the Proceedings of the National Academy of Sciences (PNAS), points to what the researchers termed “a consistent link between global mean surface temperature and sea level.” The scientists are already planning a follow-on study on the Georgia coast.

“Basically, we are doing the same study that we did in North Carolina, but this time in Georgia and other areas of the U.S. Atlantic coast,” said Ben Horton, one of the paper’s authors and an environmental scientist at the University of Pennsylvania. “Then we will be able to compare regions along the U.S. Atlantic coast to determine which are the most susceptible to sea level rise.”

Dorset Hurley, an estuarine ecologist at Georgia’s Sapelo Island National Estuarine Research Reserve, argues that, given the area’s dynamic environment of sand, silts and upland clays, it’s impossible to know exactly what such tidal basins will look like in a hundred years time. This poses challenges for coastal managers, who must prepare for a variety of scenarios.

“The question is how the area’s marsh chemistry and biology will respond to sea level rise,” said Savidge. “Several of us at Skidaway are trying to develop a study to examine the effects of simulated sea level rise on marsh ecosystems.”

“Marshes keep up with sea level rise in two ways,” said Hopkinson. “They trap sediments suspended in the water column and they accumulate organic matter in their soils from plants that don’t completely decompose.”

If plans for new dredging to deepen Savannah harbor come to fruition, Savidge says a lot more saltwater would move into freshwater habitat. That means that researchers like Savidge would potentially be able to study the equivalent of a hundred years of sea level rise over the course of just three to four years of dredging. Although it could harm the native ecosystems, that would give the scientists a big leg up in understanding the long term effects of sea level rise on marsh habitats.

“In the next 50 years, even if we do see a decline in marsh area, there’s going to be more marsh edge,” said Hopkinson, referring to the intersection between open water and the beginning of the marsh itself. “From a fisheries perspective, that edge is one of the most valuable habitats. That’s where organisms find refuge from predators. But at some point the loss of total marsh area relative to the increase in edge is going to cause a much more dramatic decrease in [marine life].”

With such prolonged marsh tidal flooding, Hurley says all these coastal Georgia ecosystems will be pushed westward. The marsh, he says, some 90 percent of which is covered by smooth cordgrass, will get squeezed by development and armoring, and in the process will likely become permanently covered by rising sea levels.

Although change is an inherent part of this coastal ecosystem, Hurley is clearly attached to the landscape as it is. No wonder — here at the end of a salt water river in the late afternoon’s lengthening shadows, the marsh’s windswept outline has a powerful allure. Lest it disappear before morning, Hurley pauses once more just to take it all in.