If we carry on as we are, the land on which nearly 30 million Americans now live will end up below the sea’s high-tide line. Even with drastic action to slash carbon emissions – more drastic than some think possible – 10 million Americans’ homes will be submerged (see map, below).

That’s the conclusion of the latest study to look at how much sea level rise we are committing ourselves to over the coming centuries. “It’s hard to imagine how south Florida and New Orleans can survive in the long run,” says team member Benjamin Strauss of Princeton University.

The US is set to lose a state-sized chunk of land, he says. “That should concern any American of any political stripe.”


Business-as-usual scenarios are likely to raise the sea level by around a metre by 2100, but the waters will keep rising for centuries or even millennia. According to a 2013 study by Anders Levermann of the Potsdam Institute in Germany every 1 °C of warming will lead to a rise in sea level of roughly 2.3 metres over the next 2000 years.

Now Strauss, Levermann and their colleagues have updated this study and worked out what it means for the US. Half their scenarios assume that the West Antarctic ice sheet collapses, as several recent studies suggest is now inevitable.

If that happens, we are already committed to nearly 5 metres of sea level rise, and this figure will rise even higher if we don’t curb our emissions – very much in line with what an analysis by New Scientist concluded earlier this year.

The best case, according to Strauss and Levermann, is that sea level could be limited to an increase of around 2 metres. This depends on a number of optimistic assumptions. The first is that the rapid melting of parts of the West Antarctic ice sheet already under way does not continue. This could happen if changes in ocean circulation stop warm currents reaching the base of the ice, Strauss says.

But without “aggressive action” to curb emissions, they say, not even changes in ocean circulation will stop the ice sheet’s collapse. So to limit sea-level rise to 2 metres, there must be rapid emissions cuts in line with what’s called the RCP2.6 scenario that would keep global warming below 2 °C.

Cuts on that scale look increasingly unrealistic. According to Kevin Anderson of the University of Manchester, UK, reducing emissions in line with RCP2.6 now requires either immediate action to curb the lifestyles of the rich, jet-setting elite producing most emissions – which would be politically unacceptable – or sucking carbon dioxide out of the atmosphere – which would be exorbitantly costly on the scale required.

Underlying this study is an assumption of the Earth’s climate sensitivity. This is a measure of how much warming the planet will experience with a doubling of CO 2 in the atmosphere. This study uses a figure of 3 °C. However, some studies suggest that over timescales of several centuries or more, sensitivity could be as high as 4.5 or 6 °C.

It is possible that sensitivity could be higher because of effects such as outgassing methane – a potent greenhouse gas – from melting permafrost, Levermann says. “But the uncertainties are large,” he says.

Rob DeConto of the University of Massachusetts-Amherst says his computer models of Antarctica back the idea that slashing emissions in line with RCP2.6 might prevent the loss of the West Antarctica ice sheet. “It hangs in there,” he says.

But for high emission scenarios – the path the world is on – his team’s work suggests there will be much higher sea level rises than Strauss and Levermann expect.

Journal reference: PNAS, DOI: 10.1073/pnas.1511186112

(Image: Robyn Beck/AFP/Getty Images)