On the edge (Image: Christophe Boisvieux/Hemis FR/Getty Images)

THE cracks are beginning to show. Greenland’s ice sheets slid into the sea 400,000 years ago, when Earth was only a little warmer than it is today. That could mean we are set for a repeat performance.

Greenland’s ice sheets slid into the sea 400,000 years ago, when Earth was only a little warmer than today

The finding, along with data from Antarctica, suggests both of Earth’s big ice sheets may have already passed a crucial tipping point, condemning them to collapse – either melting, or sliding into the ocean. That will mean sea levels rising by as much as 13 metres, leading to massive coastal flooding. So how fast will the ice collapse, and can we stop it?


Rising seas are an inevitable consequence of global warming as warmer water expands. But the big risk comes from the ice sheets of Greenland and Antarctica, which contain enough ice to force the oceans up by tens of metres if they melt completely. The Intergovernmental Panel on Climate Change has estimated that sea levels could rise by a metre this century, and that will just be the start.

The obvious danger is that low-lying coastal areas will be gradually swamped. That includes many major cities, such as New York, London and Rio de Janeiro. In the long run, we will have to choose between protecting them with elaborate sea defences or abandoning them to move inland. But long before that, rising seas will make coastal flooding worse. The deluge that swept through New York when superstorm Sandy struck in 2012 was worsened by sea level rise, as were the floods caused by typhoon Haiyan in the Philippines last year.

Now a team led by Alberto Reyes of the University of Wisconsin-Madison has evidence that the southern Greenland ice sheet melted during a relatively warm spell that interrupted the ice age. If they are right, the planet may already be warm enough to unleash a collapse.

Short of a time machine, we cannot see how big the Greenland ice sheet was in the past. But ice sheets grind away the rock they sit on, dumping sediment into the sea. So Reyes’s team studied a sediment core that had been drilled out of the Eirik Drift, a sandy hummock in the sea south of Greenland. The core contained sediments laid down over 440,000 years that chemical analysis showed came from Greenland. But hardly any sediment was deposited around 400,000 years ago, for a period of about 10,000 years, suggesting that at this time there was no ice sheet on Greenland to break up the rocks (Nature, DOI: 10.1038/nature13456).

“Our study provides the first evidence that the southern Greenland ice sheet has collapsed in the geologically recent past,” says team member Anders Carlson of Oregon State University in Corvallis.

Indirect evidence suggested the ice sheet shrank considerably back then, but there was always a question mark, says Richard Alley of Penn State University in University Park. “I would probably take the question mark off now.”

This is the latest in a flurry of work suggesting that Greenland’s ice is vulnerable. For instance, better mapping of the bedrock has revealed that the valleys in which the glaciers sit extend further inland, and deeper below sea level, than previously thought. That means warm sea water can creep many kilometres inland, melting the glaciers from beneath (Nature Geoscience, doi.org/s96).

All this means Greenland’s ice may be on course for full-scale melting. But it is difficult to pin down the temperature tipping point when a collapse becomes inevitable. “We might pass it before we know,” says Alley. We don’t know how much warmer Earth was 400,000 years ago. “It may have been as little as 1 °C warmer than present,” Carlson says.

News from the other end of the planet is no better. While most of Antarctica’s ice appears to be safe, the western fringes may have already passed a point of no return. Several west Antarctic glaciers have retreated by tens of kilometres (Geophysical Research Letters, doi.org/srf). If west Antarctica’s ice sheet does collapse, it will mean metres of sea level rise.

“We can only hope it takes a long time to happen,” says Tim Lenton of the University of Exeter, UK. The word “collapse” implies a sudden process, but in human terms ice sheets disappear slowly. In the case of Greenland, it will probably take centuries, says Alley. That’s because friction with the underlying rock slows ice flow into the sea, setting speed limits.

But Antarctica’s sheets may melt faster. Its valleys open out into much wider canyons than those in Greenland, so the ice feels less friction. “There remains a chance that west Antarctica doesn’t listen to the speed limits,” says Alley.

Can we stop the ice sheets collapsing? Some suggest cooling the planet by reflecting sunlight back into space, a controversial idea known as geoengineering. But it might not help the ice, says Lenton, as much of the melting is caused by warm sea water and that takes a long time to cool.

The best option is to stop emitting greenhouse gases, and then start removing them from the atmosphere, says Lenton. “We might be able to get back under the threshold in a couple of centuries,” he says. “Even if you can’t stop the meltdown you could slow it.” That might sound like admitting defeat, but every year we postpone the meltdown is another year in which to get ready for it, he says.

This article appeared in print under the headline “Ice sheets on course for collapse”