Sea-level rise accelerates faster on US east coast

Sea level rise off the US east coast is accelerating up to four times faster than the global average. The finding confirms model predictions that the east coast will bear the brunt of the unevenly rising waters. It also suggests that a major Atlantic current may be slowing down, which could see temperatures drop in Europe.

The "hotspot" of rising sea levels extends over 1000 kilometres from Cape Hatteras in North Carolina to Boston, Massachusetts, and possibly further north into Canada. It was identified by Asbury Sallenger and colleagues at the United States Geological Survey in St Petersburg, Florida, using tide gauge data from 1950 to 2009.

News of the hotspot may come as a surprise in North Carolina, where the state Senate recently passed a bill forbidding the use of models that include accelerating sea level rise. The bill has since been rejected by the state's House of Representatives.

Up, up and up

Sea levels are rising around the world, driven by climate change. Much of the rise comes from water expanding as it heats up, but melting ice caps are also contributing. The rise is accelerating: during the 20th century levels rose at an average of 1.7 millimetres per year, but since 1993 the rate has reached 3 mm/y.

But sea levels do not rise evenly around the world. In 2009 Jianjun Yin, now at the University of Arizona in Tucson, used a climate model to predict that the US east coast would see more than its fair share of rising sea levels (Nature Geoscience, DOI: 10.1038/ngeo462).

Sallenger's data confirms that this is already happening. "This is consistent with our climate model projections," Yin says.

Conveyor stalling?

The sea level rise hotspot could be the result of a major current, the Atlantic Meridional Overturning Circulation (AMOC), slowing down. The AMOC carries warm tropical waters into the North Atlantic, keeping western Europe warmer than other northerly regions.

Back in 2005, oceanographers led by Harry Bryden of the University of Southampton, UK, claimed that it was slowing down, potentially cooling western Europe. But further studies showed that the current is highly variable, so the results were dismissed.

A slowing AMOC would cause sea levels to shoot up on the east coast of North America, so Sallenger's hotspot suggests that the current really is losing power ? especially as other factors, such as land subsidence in North America, cannot account for the accelerated sea level rise.

Bryden says that a slowdown in the AMOC should have affected sea levels as far south as Florida. "I'm not sure they've explained why it happens north of Cape Hatteras but not south," he says.

"It could be AMOC, but it may not be," says Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany. Natural variability in currents and winds could also be responsible.

Sea level 2300

In a second paper, Rahmstorf and colleagues have modelled sea levels up until 2300 under various scenarios in which greenhouse gas emissions are cut to limit the global temperature rise. Limiting the temperature rise to 1.5 ?C ensures that sea level rise will peak around 1.5 metres. By contrast, 2 ?C leads to a rise of 2.7 metres by 2300, and no sign of the rise stopping.

Rahmstorf also tried a wildly optimistic scenario in which all greenhouse gas emissions stop in 2016. Even then, levels rose more than 1 metre. "There is nothing we can do about that, except actively remove CO₂," he says.

"This highlights the very long-term impacts of global warming," Rahmstorf adds. "Everyone is fixated on 2100, but the timescales of sea level rise are much longer."

Journal: Nature Climate Change, Sallenger: DOI: 10.1038/nclimate1597; Rahmstorf: DOI: 10.1038/nclimate1584

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