U.S. coasts double-teamed by rising sea levels

(CBS/AP) Recent studies show the United States besieged by rising sea levels on both sides. Sea levels are rising much faster along the U.S. East Coast than they are around the globe, but the West Coast is not far behind - with water levels expected to rise by as much as five feet this century.

U.S. Geological Survey scientists call the 600-mile stretch along the Atlantic a "hot spot" for climbing sea levels caused by global warming. 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.

On the other side of the country, the news is similarly worrisome. A study by the National Research Council focused on the coasts of California, Oregon and Washington. The results suggest California could see ocean levels rise by half a foot by 2030. Long-term projections of water levels forecast that sea levels will be an average of 3 feet higher by 2100.

Globally, sea levels have risen about 8 inches over the last century, but the rate is increasing significantly, said Gary Griggs, one of the scientists assembled to produce the report. The U.S. Geological Survey reports global waters rising 2 inches since 1990.

But the American Atlantic coast is far exceeding the average. In Norfolk, Virginia, where officials are scrambling to fight more frequent flooding, sea level has jumped a total of 4.8 inches, the research showed. For Philadelphia, levels went up 3.7 inches, and in New York City, it was 2.8 inches.

The West Coast is seeing lower increases, but has the additional threat of periodic ocean-warming El Nino events.

"During those events, sea level is elevated as much as a foot above normal and then we've got typically larger waves coming in with the high tides," particularly in the Northwest, Griggs said

Storms during El Ninos in 2009 and 2010 ate away 40 feet from cliffs on the ocean side of San Francisco, leaving paved parking lots hanging over the void, said Ben Grant of the San Francisco Planning and Urban Research Association.

If a major earthquake occurs beneath the Pacific Ocean off Oregon and Washington, in what is known as the Cascadia subduction zone, that would cause the land to drop, allowing sea level to rise another 3 to 6 feet immediately, the National Research Council report said. Such a major temblor occurred 300 years ago, but becomes more likely as time passes.

In terms of a raw increase in sea levels, The USGS study suggests the Northeast would get hit harder than the West Coast 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, according to Asbury Sallenger Jr, an oceanographer with the USGS.

The two government studies may play an important role for state governments, because flood maps based on those predictions can result in restrictions on property development and affect flood insurance rates.