Sea levels along the Atlantic coast of the US are rising faster than at any time in the past 4,000 years, an international team of scientists have found.

The team led by the University of Pennsylvania reports evidence showing that sea levels in that region rose 2 millimeters faster during the 20th century than the recent historic “background rate” of sea level rise.

Over the past several thousand years, sea levels have risen gradually because of land lost through coastal subsidence. The subsidence is a result of land masses rebounding after the melting of the massive ice sheets that existed during the last glacial period.

Using sediment cores from the US Atlantic coast, researchers discovered the mid-Atlantic coastlines of New Jersey, Delaware and Maryland are subsiding twice as much as areas to the north and south. Coastal subsidence enhances sea-level rise, which leads to shoreline erosion and loss of wetlands and threatens coastal populations.

The research team corrected relative sea-level data from tide gauges by taking into account the coastal-subsidence values. The result: a 20th-century rate of sea-level rise is 2 millimeters higher than the background rate of the past 4,000 years. Furthermore, the magnitude of the sea-level rise increases in a southerly direction from Maine to South Carolina.

This is the first demonstrated evidence of this phenomenon from observational data alone. The scientists believe it could be related to the melting of the Greenland Ice Sheet and ocean thermal expansion.

“There is universal agreement that sea level will rise as a result of global warming but by how much, when and where it will have the most effect is unclear,” said Benjamin P. Horton, assistant professor in the Department of Earth and Environmental Science at the University of Pennsylvania. “Such information is vital to governments, commerce and the general public. An essential prerequisite for accurate prediction is understanding how sea level has responded to past climate changes and how these were influenced by geological events such as land movements.”

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