New computer models have shown that Antarctica’s ice sheets, which contain 70% of the worlds fresh water, are on the verge of a breakdown that could push seas to heights not experienced since prehistoric times.
From the abstract published in Nature on March 30.
Polar temperatures over the last several million years have, at times, been slightly warmer than today, yet global mean sea level has been 6–9 metres higher as recently as the Last Interglacial (130,000 to 115,000 years ago) and possibly higher during the Pliocene epoch (about three million years ago). In both cases the Antarctic ice sheet has been implicated as the primary contributor, hinting at its future vulnerability. Here we use a model coupling ice sheet and climate dynamics—including previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine-terminating ice cliffs—that is calibrated against Pliocene and Last Interglacial sea-level estimates and applied to future greenhouse gas emission scenarios. Antarctica has the potential to contribute more than a metre of sea-level rise by 2100 and more than 15 metres by 2500, if emissions continue unabated. In this case atmospheric warming will soon become the dominant driver of ice loss, but prolonged ocean warming will delay its recovery for thousands of years.
Penn State News reports on the grim findings.
Ocean warming has previously been identified as the main cause of ice retreat occurring today. Warmer water quickly erodes the underside of floating ice sheet portions. Floating ice shelves act as buttresses for the grounded ice inland, whose base is below sea level. Once the shelves are gone, the grounded ice can move faster. However, in previous models, this process did not simulate enough melting to explain the past sea levels, with only West Antarctica collapsing even though similar areas in East Antarctica with huge amounts of ice could collapse in the same manner.
Pollard, working with Robert M. DeConto, professor of geosciences, University of Massachusetts, Amherst, looked at two further mechanisms that could account for greater melting. The first mechanism is fracturing and deepening of crevasses on the low-lying floating ice shelves by pooling of surface meltwater and rainfall caused by warming air temperatures. If emissions continue unabated, this process will begin to dominate ocean warming within 100 years. It already caused the disintegration of the Larsen B Ice Shelf in 2002.
The second mechanism comes into play once floating ice sheets disintegrate back to the grounding zone, leaving extremely high walls of ice. These walls are so high that simple physics says they cannot structurally support their weight, and then collapse into the sea, eroding the cliff further and further inland as long as the bedrock stays deep enough below sea level. Similar cliffs, with about 328 feet of ice above sea level and 2625 feet below, exist today at a few of the largest outlet glaciers in Greenland and the Antarctic Peninsula, where huge calving events occur regularly.
Both of these mechanisms are known, but neither has been applied to this type of ice-sheet model before. The researchers incorporated the physics and tested the model, driven by high-resolution climate models and past climate data. The updated model reproduced ice-sheet retreat consistent with geologic sea-level data for the warm Pliocene and also for the last interglacial period around 125,000 years ago. Then they applied the model to the future, forcing it with various greenhouse-gas emission scenarios.
These same models show that Antarctica’s ice sheet would remain largely intact if the most ambitious goals of last year’s Paris agreement on climate change are achieved. We are making some progress, but is it enough?
The AGU reports on the surprisingly stable 5,000 square mile Stange ice shelf located at the base of the Antarctic Peninsula on its west side.
So just perhaps we may just be able to slow some of the worst impacts of global warming if we get our act together and fast.
They examined the ice shelf for the four key precursor symptoms of an ice sheet collapse. 1) Significant thinning due to surface or basal melt, which can structurally weaken the ice sheet. 2) Structural weakening along suture zones. 3) Sustained retreat and development of a concave front that has less connection to pinning points. 4) Increase in velocity. Examples where weakness is evident are en Verdi Ice Shelf, Wordie Ice Shelf and Jones Ice Shelf to the north. Here we use Landsat imagery from 1989, 2003 and 2016 to examine the south and central ice front, which illustrates what Holt et al (2014) concluded that the ice shelf is currently stable.
Larger versions of these images can be seen at the AGU link above.
Stange Ice Shelf, Antarctica in 2016 Landsat image. Five rift zones are mentioned two by the southern ice front R3 and R4. Two by the central ice front R1 and R2. Purple dots mark ice front and yellow and red arrow the 1989 frontal positions on the north and south side of Case Island
Stange Ice Shelf, Antarctica in 1989 Landsat image. Five rift zones are mentioned two by the southern ice front R3 and R4. Two by the central ice front R1 and R2. Purple dots mark ice front and yellow and red arrow the 1989 frontal positions on the north and south side of Case Island. IB= Ice berg that calves in 2001
Stange Ice Shelf, Antarctica in 2003 landsat image. Five rift zones are mentioned two by the southern ice front R3 and R4. Two by the central ice front R1 and R2. Purple dots mark ice front and yellow and red arrow the 1989 frontal positions on the north and south side of Case Island. IB= Ice berg that calves in 2001
Dr. David Pollard discusses the new models linked in the Penn State story.
