The sea level along much of the U.S. East Coast is rising three to four times faster than the global average, a rapid increase that government scientists expect to continue over the next several decades.
Since 1990, the world's oceans have risen 0.02 to 0.04 inch per year. But along 600 miles of coastline between Cape Hatteras, N.C., and Boston, the climb has occurred much faster -- 0.08 to 0.15 inch per year during the same period.
U.S. Geological Survey researchers have dubbed that stretch of the East Coast a sea-level rise "hot spot." They predict the area will experience 20 to 30 percent more sea-level rise, compared to the global average, by the end of the century, increasing the risks of flooding, erosion and storm surge.
Many scientists estimate that the world's oceans will rise an average of 1 meter, or roughly 3.2 feet, by 2100. If that is the case, the East Coast hot spot would experience an additional 7 to 12 inches of rise, according to the new findings, published yesterday in the journal Nature Climate Change.
"That just makes the storm surge higher, the reach of the waves that much higher," said lead author Asbury Sallenger Jr., a USGS oceanographer based in St. Petersburg, Fla. "It potentially makes a storm much more damaging."
The East Coast braces for a double hit
Sallenger and his co-authors analyzed data collected by tide gauges up and down the East Coast. Their results suggest the hot spot is caused by changes in ocean circulation caused by influxes of fresh water from the melting of the Greenland ice sheet, a scenario first suggested by modeling studies.
That fresh water leads to a slowdown in the Gulf Stream and its northern extension, the North Atlantic Drift.
Waves on the Gulf Stream. Photo courtesy of bunnygoth
Normally, those fast-flowing currents pull coastal water toward them as they flow, so that sea level slopes upward toward the currents as it moves away from the coast. But as the Gulf Stream and its north extension slow, that slope flattens, and sea level along the coast rises.
Jianjun Yin, a climate modeler at the University of Arizona, said the new research was the first to use observations to confirm the projections of the East Coast hot spot he and others have made.
"This is the first study based on tide-gauge data showing that the Northeast coast of the United States is very special in terms of sea-level rise," he said. "Regional sea-level rise there is much faster and larger than the global mean."
The research is especially important in light of previous studies that suggest climate change will increase the number of major hurricanes in the Atlantic Ocean off the East Coast, Yin said.
"Sea-level rise plus stronger storm surge will cause much more damage to this area," he said. "It's a real threat to this region."
Even N.C. must face reality, scientists say
Sallenger says his study is part of a growing body of evidence that calls for a new view of sea-level rise.
"We need to move away from talking about a bathtub filling with water and much more towards the idea of sea-level rise that varies regionally, which we will have to adjust to," he said.
That includes the state of North Carolina, where lawmakers have proposed legislation that would require state agencies to use linear, historical projections of sea-level rise -- in effect barring analyses that consider the possibility of accelerated sea-level rise as the Earth warms and its land ice melts.
The northern third of the state's coastline lies within the "hot spot" described in the new paper.
Proponents of the North Carolina legislation have based their arguments in part on a paper published last year in the Journal of Coastal Research that argues that global sea-level rise has slowed down since 1930.
"There are reports that say, 'Well, there is no acceleration within the United States,' but we show a case where sea level is accelerating within this hot spot," Sallenger said.
But the East Coast isn't the country's only sea-level rise hot spot.
West Coast worries include earthquake impacts
California's coastal waters will rise more than 3 feet during the next century, outpacing the global average, according to a report released Friday by the National Academy of Sciences.
The region south of Cape Mendocino, the northern boundary of the San Andreas Fault, is projected to experience the biggest rise -- anywhere from 16 inches to 4.5 feet by 2100.
The science academy said sea level north of Cape Mendocino -- including the Oregon and Washington coasts -- is likely to fall slightly through midcentury, because the land there is rising as the oceanic tectonic plate pushes under the coastal plate.
But that could change suddenly if there is a major earthquake in the region. A temblor of magnitude 8 or higher could cause a sudden, rapid rise in sea level of more than 3 feet.
"Historically, there have been major earthquakes there," said Robert Dalrymple, a professor of civil engineering at Johns Hopkins University who led the National Academy of Sciences committee that wrote the report. "The last one was 300 years ago, but these earthquakes appear at fairly regular frequency."
The state of California asked the science academy to conduct the study, which was also sponsored by the states of Washington and Oregon, the U.S. Army Corps of Engineers, the National Oceanic an Atmospheric Administration and the U.S. Geological Survey.
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