We’re on thin ice when it comes to sea level rise Colin Monteath/Hedgehog House/Minden Pictures

A massive rise in sea level is coming, and it will trigger climate chaos around the world. That was the message from a controversial recent paper by climate scientist James Hansen. It was slated by many for assuming – rather than showing – that sea level could rise between 1 and 5 metres by 2100.

But now, just a week after being formally published, it is being backed up by another study. “He was speculating on massive fresh water discharge to the ocean that I don’t think anybody thought was possible before,” says Rob DeConto of the University of Massachusetts Amherst. “Now we’re publishing a paper that says these rates of fresh water input are possible.”

DeConto’s findings suggest that even if countries meet the pledges made as part of the UN climate agreements in Paris last year, global sea level could still rise 1 metre by 2100. If emissions keep climbing it could go up more than 2 metres. North America would be especially hard hit, because gravitational effects mean that ice loss from Antarctica will lead to bigger local increases for the US East Coast.


“Today we’re measuring global sea level rise in millimetres per year,” DeConto says. “We’re talking about the potential for centimetres per year just from [ice loss in] Antarctica.”

Exposed ice

So far, almost all estimates of global sea level rise by 2100 have assumed that Antarctica will gain rather than lose ice, thanks to increased snowfall compensating for any melting at the margins. However, the Intergovernmental Panel on Climate Change warned in its latest report that this could be an underestimate, because the computer models used may not be able to predict rapid changes in Antarctica’s ice.

Now DeConto has included factors in his model that other studies have lacked. First, floating ice shelves around Antarctica will soon be exposed to above-zero summer air temperatures, speeding their melt, he says. Second, once the shelves are gone, the huge ice cliffs that remain will begin to collapse.

Working with David Pollard of Pennsylvania State University, DeConto calibrated this model using data on past sea level rises during warm periods 120,000 and 3 million years ago. Their model is the first to match what is thought to have happened during these periods.

They applied their model to several greenhouse gas emission scenarios and found that if emissions rapidly grow in future years, Antarctica alone could contribute well over 1 metre to global sea level by 2100. When this finding is combined with IPCC estimates, it suggests that we could be looking at a rise of over 2 metres by the end of the century, and 20 metres by 3500.

If countries meet the commitments they made as part of the Paris climate agreement, and continue to cut emissions after the agreement ends in 2030, then the world should be on track for a less severe rise in sea level. But DeConto’s model suggests that even this might not be enough to prevent rises that would devastate Miami and other low-lying cities within the lifetime of young people today.

Rapidly reducing global emissions could, in principle, avoid most Antarctic ice loss, says DeConto. In practice, we wouldn’t be able to replace fossil fuel infrastructure fast enough, even if the political will was there. “That’s not going to happen,” he says.

Collapsing cliffs

DeConto’s model doesn’t include some mechanisms that Hansen argues would make ice melt even worse, but it also omits some processes that could reduce ice loss. As Richard Hindmarsh of the British Antarctic Survey in Cambridge notes, the falling ice debris might build up in front of the collapsing cliffs, and buttress them against further collapse.

“I don’t believe sea level will rise more than a metre by 2100,” says Hindmarsh, whose recent research has suggested that ice loss from Antarctica this century will be minimal. How ice breaks is much less well understood than how it melts, and Hindmarsh thinks DeConto’s model overestimates the impact of ice cliffs collapsing.

But DeConto says that Hindmarsh’s work predicts future ice loss based on what’s happened so far, and doesn’t take into account processes like ice surface melt that haven’t kicked in yet.

In fact, glaciologist Richard Alley of Pennsylvania State University says DeConto’s model may even be an underestimate. “The new work should not be considered to be a ‘worst case’ scenario,” he says.

Journal reference: Nature, DOI: 10.1038/nature17145