Sea levels are rising much faster along the U.S. east coast than they are around the globe, putting some of the world's most prized coastal properties in danger of flooding, government researchers report.
U.S. Geological Survey (USGS) scientists call the 965-kilometre swath extending from Cape Hatteras, N.C., to Boston a "hot spot" for climbing sea levels caused by a combination of global warming and changes in the structure and speed of ocean currents in the region.
Along the region, the Atlantic Ocean is rising at an annual rate three to four times faster than the global average since 1990, according to the study published Sunday in the journal Nature Climate Change.
It's not just a faster rate, but at a faster pace, like a car on a highway "jamming on the accelerator," said the study's lead author, Asbury Sallenger Jr., an oceanographer with USGS. He looked at sea levels starting in 1950 and noticed a change beginning in 1990.
Since then, sea levels have gone up globally about five centimetres. But in Norfolk, Va., where officials are scrambling to fight more frequent flooding, the sea level has jumped a total of 12.19 centimetres, the research showed. For Philadelphia, levels went up 9.4 centimetres, and in New York City, it was 7.11 centimetres.
Canada will see similar acceleration
Other places in the U.S. and around the globe have witnessed increases in sea level that are much larger or happened much faster than those along the Atlantic coast, but what's unique about the hot zone is the rate of acceleration at which the rise is occurring, study co-author Peter A. Howd told CBCNews.ca.
"There are regions in the United States where the rates of relative sea level rise — how fast water is coming up relative to the land — are much faster than those that we're seeing now within this hot spot," Howd said.
"It's just that those rates are increasing the fastest within this hot spot. Essentially, they're playing catch-up to the worst places around the globe."
Howd is a research oceanographer who works for Cherokee Nation Businesses but was contracted to work on the study by the USGS.
He told CBCNews.ca that the researchers also examined data from four tide gauges in Canada: in St. John's, Charlottetown, Halifax and Port-aux-Basques, N.L.
They found that the Atlantic coast of Canada is likely to see similar accelerations in sea level rise as the hot zone in about 10 to 20 years.
"They will be just as large — they're just delayed a little bit," Howd said.
Similar changes in the rate that sea level is rising are also probably occurring south of the hot zone, Howd said.
"The same changes are likely to be occurring south of Cape Hatterras; we just can't measure them yet, because the signals are small relative to the other processes that are incorporated in those tide gauge records," he said.
Higher sea levels cause more damage during storms
Climate change impacts the sea level in several ways: by accelerating the melting of glaciers and ice caps, by reducing the ice mass of the Greenland and west Antarctica ice sheets and by increasing ocean temperatures and causing water to expand through the process of thermal expansion.
Computer climate models have long projected higher levels along parts of the U.S. east coast because of changes in ocean currents from global warming, but this is the first study to show that's already happened.
By 2100, scientists and computer models estimate that sea levels globally could rise as much as 1.01 metres. The accelerated rate along the east coast could add about 20 to 28 centimetres more, Sallenger said.
"Where that kind of thing becomes important is during a storm," Sallenger said.
That's when it can damage buildings and erode coastlines.
On the west coast, a U.S. National Research Council report released Friday projects an average rise in sea level in California of nearly 1-metre by the year 2100 and 0.61 metres in Oregon and Washington. The land mass north of the San Andreas Fault is expected to rise, offsetting the rising sea level in those two states.
The USGS study suggests the northeast of the U.S. would get hit harder because of ocean currents. When the Gulf Stream and its northern extension slow down, the slope of the seas changes to balance against the slowing current. That slope then pushes up sea levels in the northeast. It is like a see-saw effect, Sallenger theorizes.
Scientists believe that with global warming, the Gulf Stream and other ocean currents are slowing and will slow further, Sallenger said.
Real estate at risk
Jeff Williams, a retired USGS expert who wasn't part of the study, and Stefan Rahmstorf, a professor of ocean physics at the Potsdam Institute in Germany, said the study does a good job of making the case for sea level rise acceleration.
'Somewhere between Maryland and Massachusetts, you've got some bodaciously expensive property at risk.'— Margaret Davidson, National Oceanic and Atmospheric Administration
Margaret Davidson, director of the Coastal Services Center for the National Oceanic and Atmospheric Administration in Charleston, S.C., said the implications of the new research are "huge when you think about it. Somewhere between Maryland and Massachusetts, you've got some bodaciously expensive property at risk."
Sea level projections matter in coastal states, because flood maps based on those predictions can result in restrictions on property development and affect flood insurance rates.
Those estimates became an issue in North Carolina recently when the state legislature proposed using historic figures to calculate future sea levels, rejecting higher rates from a state panel of experts. The USGS study suggests an even higher level than the panel's estimate for 2100.
The North Carolina proposal used data from University of Florida professor Robert Dean, who had found no regional differences in sea level rise. Dean said he can't argue with the results from Sallenger's study showing accelerating sea level rise in the region, but he said it's more likely to be from natural cycles. Sallenger said there is no evidence to support that claim.