Sea levels on the mid-Atlantic coast are rising faster than the global average, and global warming is to blame, according to a U.S. Environmental Protection Agency report released last week.
Tide-gauge records show sea levels rose by 2.4 to 4.4 millimeters per year over the 20th century, well above the 1.7 mm per year observed worldwide.
That translates to waters about a foot higher than they were at the turn of the last century, an increase that the EPA report pins on increased melting of glaciers and ice caps and rising ocean temperatures, since water expands as it warms.
It's a trend that's expected to continue over the next century, increasing the densely populated mid-Atlantic states' vulnerability to severe storms, storm surges and coastal erosion.
"We chose to study this region because it's experiencing the loss of wetlands, increasing population, and subsidence," or sinking, of some coastal land, said Steve Gill, a senior scientist with the National Oceanic and Atmospheric Administration who helped write the report.
It's a potent cocktail of factors that could swamp estuaries and coastal wetlands, disrupting wildlife habitat too quickly for species to adapt, or erode sand dunes that protect coastal communities. Higher sea levels also increase flooding risk in coastal areas by providing "an elevated base for storm surges," the report says, and gradually encroach on coastal roads -- like Interstate 95, which passes through low-lying Baltimore and Wilmington, Del.
If sea level rises quickly enough, traditional coastal engineering -- like dikes and sea walls -- may not be economically or environmentally feasible, and abandoning land to the sea may become the best option, the EPA report warns.
But the risk hasn't slowed coastal development, the agency concludes.
A first attempt to give local agencies guidance
The report doesn't attempt to predict how high sea levels will rise in the future, but sets out three likely scenarios based on the Intergovernmental Panel on Climate Change's 2007 report and scientific studies released since then.
The projections range from a low of 12 inches to about 40 inches by the end of the century. The more conservative numbers come from the IPCC report, which pegged the likely sea level rise from warming at between 7 and 23 inches by 2100. But the analysis also cautions that an additional rise could come from rapid and unpredictable melting of polar ice.
The higher number is based on new evidence that the behavior of ice sheets at both poles is changing in ways scientists didn't anticipate -- and that reality isn't well reflected in current climate models used to predict sea-level rise (ClimateWire, Dec. 17, 2008).
The new report is a first attempt to provide state and local governments with the information they will need to protect their communities, Gill said.
"We know that when it comes to decision-making on land use, it's really the county and state planners who make those decisions," he said. "And they don't really have the tools necessary to meaningfully account for sea-level rise."
That's true in Delaware, said Dave Carter, environmental program manager for that state's coastal program.
"There's really good stuff in this report," he said. "But we still have to build on even more detailed data to actually do adaptation at the local level." That means information about the risks faced by individual towns or even single parcels of land, he added.
As seas rise, so do populations along the shore
Despite the lack of information, some communities are already taking action, says the report, which recommends retrofitting buildings located close to shorelines and reconsidering how flood insurance rates are calculated, among other measures.
One state that's moving aggressively is Maryland, the EPA analysis says. The state's coastal program has developed an initial strategy for dealing with sea-level rise -- including erosion and flooding -- and improving its geographic data and climate models.
That's a natural for a state where sea level rise is among the top threats, said Matthias Ruth, director of the University of Maryland's Center for Integrative Environmental Research.
"If I had to rank all the different impacts climate change could have on the state, sea level rise would be among the top," he said.
That's because much of the state's infrastructure and population is concentrated along its coast. Two-thirds of Marylanders live in coastal counties or the low-lying city of Baltimore, a proportion that is growing. In one particularly vulnerable area -- Maryland's eastern shore, on the Chesapeake Bay -- population density increased 30 percent between 1985 and 2002, according to a report Ruth produced last year for the Maryland Commission on Climate Change.
Despite the uncertainties about how much or how quickly the seas will rise, the task for coastal managers is in some sense simple, said Carter.
"We just have to learn how to do better what we've been doing for decades," he said. "The real issue we have now is that we have an order of magnitude of people living in harm's way."