The Antarctic icecap shrank when it was not much hotter than it is now

Are the large ice sheets of Antarctica stable in the face of the warming to which we are already committed? This is a more serious issue than it might seem. It is thought that the continent contains enough ice to raise the level of the oceans by more than 55 meters if it melted – enough to drown out every element of coastal infrastructure we have and send people migrating away from the sea. current coast to the interior of the land.

But the melting of this ice is a complicated process, which depends on factors such as the dynamics of the glaciers as they cross the coastal hills, the shape of the seabed where the ice meets it, and the slope of the basins in which the ice falls. ice caps. It is difficult to determine how much ice would be lost for a little warming. As a result, we still have historical comparisons – the last time it went up, how much ice did we lose?

This week, we received new information on this topic from a detailed study of the Wilkes Antarctic subglacial basin. Work has shown that the warming of the ice is less important. it was how long it was hot.

A lot of ice cream

The majority of ice at the South Pole lies in the East Antarctic Ice Sheet, whose structure could make it vulnerable to instability. The ice rests on a rock below sea level in large basins. The water is currently maintained by the large mass of ice rising above sea level, repelling the rest. If the ice clears up considerably and the ocean water invades the basin, the ice could float out of the rock and break, dramatically increasing the sea level in a short period of time. relatively short. The ice has not completely destabilized for millions of years, but all indications are that parts of the East Antarctic ice sheet have been lost in the past.

While the Earth has been experiencing a steady cycle of warmer, more glacial periods over the last few million years, the details of warm periods have varied due to differences in orbital configurations and greenhouse gas levels. Their length and maximum temperature differed, which means that we have several possible examples of what a certain degree of future warming could bring to the ocean level, even though uncertainties are still important. These tell us especially that it is perhaps not necessary to heat up much more than the present to see more than five meters of sea level rise.

The key issue addressed in this paper is the following. Does this extra ocean come from the destabilization of a part of Antarctica?

To understand this, the researchers used sediment cores taken from the ocean near the Wilkes subglacial basin, which is part of the East Antarctic ice sheet. . (The cores originate from a depth of nearly 3.5 km.) These covered several recent glacial cycles, two of which were particularly relevant, as they involved temperatures above 2 ° C at pre-industrial conditions (or 1 ° C at above the present) and additional sea level meters. These occurred approximately 125,000 and 425,000 years ago. The nuclei covered a number of additional cycles, allowing comparison with different conditions.

Carrots easily show the difference between glacial and interglacial conditions. For starters, there are many more signs of life during the interglacials, indicating that the area was probably covered with sea ice during the ice ages; sediments were much more limpid during these periods. In addition, the interglacial periods often contained "tombstones", which are melted rocks under the icebergs that drifted in the region.

Isotopes and erosion

To understand where the ice was in each period, the researchers turned to the isotopes of the neodymium element, which vary in the different rocky layers of the Antarctic. They found that during these two critical interglacials, the ratio of different isotopes of neodymium changed at the same time that droplets and other iceberg debris began to appear. Similar changes have been observed during warm periods in the Pliocene, when Antarctica is known to have lost much ice.

The authors explain this difference by suggesting that ice eroded different rocks once these interglacial periods began; a similar conclusion is supported by isotope data for strontium. Currently, the area on the edge of the glaciers, from which most of the washed material comes, is mainly granite. But we know that there are also large areas of basalt on the mainland, and it is possible that some of them reside in the basin behind the current outlet glaciers. This would suggest that the ice is significantly removed from the present coast during these warm periods.

Overall, they identify three different interglacials where these data suggest that the ice caps have receded and at least one where they have not been. The difference between them is not the maximum temperature reached during the hot period; on the contrary, in all cases where the glaciers withdrew, the high temperatures lasted at least 2,500 years. The current interglacial, which has not experienced significant retreat of the ice cap, is similar to the previous one, where glaciers have also remained stable.

Message for the present?

So what does this mean for the present? Potentially good, assuming the world pulls together and aims to limit climate change to 2 ° C above pre-industrial conditions. The results suggest that we could tolerate at least a thousand years of such temperatures before the Wilkes subglacial basin undergoes significant ice removal. Which means that there is enough time to develop technologies that remove carbon dioxide from the air and lower temperatures to avoid this event (assuming we still have the coastal infrastructure we want preserve).

The bad news is that we do not currently achieve our goals, which makes us fear that we are going beyond the 2 ° C goal, potentially significantly. If so, then we may destabilize the East Antarctic Ice Sheet over a shorter period of time and try to develop negative emissions technologies to eliminate a lot of CO ._{2 persons}.

But even here, there is a silver lining. Ocean levels during the interglacial periods are nothing like those that might be expected if the East Antarctic icecap were completely destabilized. So either it has at least semi-stable states between the present conditions and has collapsed, or the collapse is quite gradual and can be reversed under the right conditions. This means that even if we could cope with a catastrophic sea level rise for existing coastal regions, we will not see a planet reconstituted by 50 meters of additional ocean.

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