North Miami shoreline from the sea
The sea level on a stretch of the US Atlantic coast that features the cities of New York, Norfolk and Boston is rising up to four times faster than the global average, a report said Sunday.
This increases the flood risk for one of the world's most densely-populated coastal areas and threatens wetland habitats, said a study reported in the journal Nature Climate Change.
Since about 1990, the sea level along the 1,000-kilometre (620-mile) "hotspot" zone has risen by two to 3.7 millimetres (0.08 to 0.15 inches) per year.
The global rise over the same period was between 0.6 and one millimetre per year, said the study by the US Geological Survey (USGS).
If global temperatures continue to rise, the sea level on this portion of the coast by 2100 could rise up to 30 centimetres over and above the one-metre global surge projected by scientists, it added.
The localised acceleration is thought to be caused by a disruption of Atlantic current circulation.
"As fresh water from the melting of the Greenland Ice Sheet enters the ocean, it disrupts this circulation, causing the currents to slow down," USGS research oceanographer and study co-author Kara Doran explained.
"When the Gulf Stream current weakens, sea levels rise along the coast and the greatest amount of rise happens north of where the Gulf Stream leaves the coast (near Cape Hatteras)."
The hotspot stretches from Cape Hatteras, Northern Carolina to north of Boston, Massachusetts and also includes other big cities like Philadelphia and Baltimore.
"Extreme water levels that happen during winter or tropical storms, perhaps once or twice a year, may happen more frequently as sea level rise is added to storm surge," Doran told AFP.
"Scientists predict that this will lead to increased beach erosion and more frequent coastal flooding."
Another study has shown a one-metre sea level rise to increase New York's severe flooding risk from one incident every century to one every three years.
The USGS report was based on actual tide level measurements, said Doran. Other studies have shown a similar hotspot using climate models.
In a 2007 assessment report, the United Nations' Intergovernmental Panel on Climate Change said global warming would cause the sea level to rise by up to 59 centimetres by century's end.
Even this relatively modest projection would render several island nations unlivable and wreak havoc in low-lying deltas home to hundreds of millions.
But reports since then have said that melting Arctic ice plays a greater role in sea level rise than previously suspected, and most climate change scientists now project the ocean will rise roughly a metre by century's end.
Climate warming causes sea levels to rise by melting land-ice and through the thermal expansion of water.
In a separate study in Nature Climate Change, European scientists said a 1.5-degree-Celsius rise in global temperatures would see sea levels peak at about 1.5 metres above 2000 levels.
But warming of two degrees would result in sea levels reaching 2.7 metres -- nearly double.
The UN is targeting a 2 C (3.6 degrees Fahrenheit) limit on warming from pre-industrial levels for manageable climate change.
"Due to the long time it takes for the world's ice and water masses to react to global warming, our emissions today determine sea levels for centuries to come," said lead author Michiel Schaeffer of Wageningen University in the Netherlands.