Up to 70 percent of Northeast U.S. coast likely to adapt to rising seas

“Geologists have always known that the coast has some potential for give and take,” said lead author Erika Lentz, a research geologist at the USGS Coastal and Marine Science Center in Woods Hole, Massachusetts. “But the standard bathtub models of sea level rise don’t reflect that. This approach couples what we do know about these systems with what we still need to learn — how different ecosystems may respond to different sea-level rise scenarios — to estimate the odds that an area will persist or change instead of simply drown.”

By casting results in terms of odds, the new model provides a more accurate picture of sea-level rise vulnerability for informing adaptation strategies and reducing hazards, the USGS researchers say. They make it clear, however, that just because an area is less likely to drown might not mean it is less vulnerable. “Our model results suggest that even natural changes may pose problems,” Lentz said. “For example, the likelihood that barrier islands will change could impact the infrastructure and economies of coastal communities, and the barrier islands or marshes may not protect coastal communities in the same way they do today.”

In fact, the outcome is uncertain for the Northeast’s low-lying developed coastlines, where seawalls, buildings and other immovable structures thwart some natural processes. The model found the region’s developed coastal lands lying 1 meter (about 3.5 feet) or less above sea level will likely face a tipping point by the 2030s, when humans’ decisions about whether and how to protect each area will determine if it survives or drowns.

A 2012 USGS study identified the densely populated region from Cape Hatteras to Boston as a hot spot where seas are rising faster than the global average, so land managers urgently need to understand how their coastal landscape may change, said John Haines, coordinator of the USGS Coastal and Marine Geology Program. 

“The model allows us to identify vulnerable areas, and that information can be very valuable to land managers as they consider whether to protect, relocate or let go of certain assets,” Haines said. “Even when the results are uncertain, it’s useful to know there’s a 50 percent chance that an important habitat or infrastructure project may be lost in a few decades.”

To come up with their model for the Northeastern United States, the researchers mapped all coastal land between ten meters (about 33 feet) above sea level and ten meters below it, from the Virginia-North Carolina line to the Maine-Canada border. They factored in a variety of forces that affect coastal change, from planetary phenomena like the movement of Earth’s tectonic plates to local ones like falling groundwater levels that cause land surfaces to sink. Looking at parcels of 30 meters by 30 meters — about the size of two NBA basketball courts side by side — they weighed the balance of forces on each parcel.

Using scenarios that assume humans will continue adding moderate to high levels of greenhouse gases to the atmosphere through the twenty-first century, the team projected global sea level rise for the 2020s through the 2080s, and applied that to the coast. The model then estimated the likelihood, from 0 to 100 percent, that each parcel will persist above sea level at the end of each decade.

Predictions for many parcels fell close to 50 percent in the first few decades, a tossup between drowning and surviving. The uncertainty was greatest when the researchers had to wrestle with more than one question that can’t yet be definitively answered. Among them are, how fast will seas rise, can coastal marshes make new soil quickly enough to stay above the waves, and what engineering strategies will people use to protect some shorelines?

“By building in our understanding of the sea level rise response of the coastal landscape, we’re providing a more realistic picture of coastal change in the Northeastern U.S. over the next several decades,” Lentz said.

USGS says that the researchers’ next step will be to group the basketball-court-sized parcels into larger areas, such as major coastal cities, national wildlife refuges, and national seashores, and assess the vulnerability of these areas to future change and drowning. This information may assist decisionmakers as they develop management priorities to address longer-term coastal challenges.

— Read more in Erika E. Lentz et al., “Evaluation of dynamic coastal response to sea-level rise modifies inundation likelihood,” Nature Climate Change (14 March 2016)