Are the big ice sheets in Antarctica stable in the face of the warming we've already committed to? That's a more serious question than it might sound. The continent is thought to hold enough ice to raise ocean levels by over 55 meters if it were to melt—enough to drown every single bit of coastal infrastructure we have and send people migrating far inland from the present-day shoreline.

But the melting of this ice is a complicated process, one that depends on things like the dynamics of glaciers as they push through coastal hills, the shape of the seafloor where the ice meets it, and the slope of the basins the ice sheets sit in. It's tough to reason out how much ice would be lost for a given bit of warming. As a result, we're left with historical comparisons—the last time it warmed by that amount, how much ice did we lose?

Lots of ice

This week, we got some new information on this topic courtesy of a detailed study of Antarctica's Wilkes Subglacial Basin. The work showed that it wasn't so much the amount of warming the ice experienced; it was how long it stayed warm.

The majority of the South Pole's ice is in the East Antarctic Ice Sheet, which has a structure that could leave it prone to instability. The ice sits on rock that's below sea level in large basins. The water is currently kept out by the large mass of ice that rises above sea level, pushing the rest down. If the ice thinned significantly and ocean water invaded the basin, the ice could float off the rock and break up, dramatically raising sea levels in a relatively short amount of time. The ice hasn't completely destabilized in millions of years, but there are indications that parts of the East Antarctic Ice Sheet have been lost in the past.

While the Earth has undergone a regular cycle of warmer and glacial periods for the past few million years, the details of the warm periods have varied due to differences in things like orbital configurations and greenhouse gas levels. Their length and maximum temperature differed, meaning we have multiple possible examples of what some degree of future warming might bring to ocean levels, even if the uncertainties are still significant. These mostly tell us that it may not need to get much warmer than the present to see over five meters of additional sea level rise.

The key question addressed in the current paper is "why?". Is that extra ocean coming from the destabilization of some of Antarctica?

To understand this, researchers relied on sediment cores taken from the ocean near the Wilkes Subglacial Basin, part of the East Antarctic Ice Sheet. (The cores come from a depth of nearly 3.5km.) These cores covered several recent glacial cycles, including two that are especially relevant, as they involved temperatures about 2°C above preindustrial conditions (or 1°C above present) and over six meters of additional sea level. These occurred approximately 125,000 and 425,000 years ago. The cores covered a number of additional cycles, allowing the two to be compared with different conditions.

The cores easily show the difference between glacial and interglacial conditions. To begin with, there are far more signs of life during the interglacials, indicating that the area was probably covered by sea ice during glacial periods; sediments were much siltier during these times. In addition, interglacial periods often contained "dropstones," which are rocks melted out of the underside of icebergs that drifted through the area.

Isotopes and erosion

To understand where the ice was during each period, the researchers turned to isotopes of the element neodymium, which vary in the different rock layers in Antarctica. They found that during these two critical interglacials, the ratio of different neodymium isotopes changed at the same time that the dropstones and other iceberg debris started appearing. Similar changes had been seen during warm periods in the Pliocene, when we know that the Antarctic lost significant ice.

The authors explain this difference by suggesting that the ice was eroding different rocks once these interglacial periods started; a similar conclusion is supported by strontium isotope data. Currently, the area at the edge of the glaciers, where most of the scoured material originates, is primarily granite. But we know that there's also large regions of basalt on the continent, and it's possible that some of these reside in the basin behind the current exit glaciers. That would suggest that the ice retreated significantly from the present coast during these warm periods.

Overall, they identify three different interglacials where this data suggests the ice sheets retreated, and at least one where they did not. The difference between them isn't the maximum temperature reached during the warm period; instead, in all cases where the glaciers retreated, the elevated temperatures lasted for at least 2,500 years. The present interglacial, which also has not featured a major ice sheet retreat, looks similar to the earlier one where the glaciers also remained stable.

Message for the present?

So what does that mean for the present? Potentially good things, assuming the world gets its act together and hits the target of limiting climate change to 2°C above preindustrial conditions. The results suggest that we could tolerate at least a thousand years of these sort of temperatures before the Wilkes Subglacial Basin saw significant ice retreat. Which means there is plenty of time to develop technologies that remove carbon dioxide from the air and lower the temperatures to prevent this event (assuming we still have coastal infrastructure we want to preserve).

The bad news is that we're not currently hitting our targets, raising the prospect that we'll overshoot the 2°C target, potentially by a wide margin. If that's the case, then it's possible we'll destabilize the East Antarctic Ice Sheet on a shorter time scale and have to scramble to develop negative emissions technologies to remove a lot of CO 2 .

But even here, there's a silver lining. The ocean levels during the interglacial periods are nothing like those that would be expected if the East Antarctic Ice Sheet had completely destabilized. So either it has at least semi-stable states in between present conditions and collapsed or the collapse is gradual enough and can be reversed under the right conditions. This means that while we might face sea level rise that's catastrophic for existing coastal regions, we won't be looking at a planet remade by 50 meters of additional ocean.

Nature, 2018. DOI: 10.1038/s41586-018-0501-8 (About DOIs).