Losing all the ice shelves of Antarctica would be like losing each flying buttress that supported a gothic building. Collapse is the inevitable result. The question is how fast is the collapse in the case of an ice sheet that would, as Richard Alley told Congress in February 2007, slowly spread outwards and flatten like pancake batter that was just plopped on a griddle.
Nearly a decade later, the latest science indicates a critical threshold may have already been crossed.
Glaciologist Eric Rignot described this threshold—the retreat of ice in this part of Antartica and its draining into the Amundsen Sea could be “unstoppable.” Many scientists think this is a key region that can lead to the disintegration of the vast stores of marine ice in the West Antarctic Ice Sheet (Figure 1). The latest study by Khazendar, Rignot, and others adds to the mounting evidence that the threshold for an irreversible disintegration has begun.
Figure 1. One panel depicts glaciers of the Amundsen Sea sector of the West Antarctic Ice Sheet. The floating ice shelves and adjacent tributary glaciers of the West Antarctic Ice Sheet have flow lines indicating areas of faster flow toward the sea. The second panel depicts the same region with the bathymetry revealed by airborne radar surveys. Brown indicates bathymetry below current sea level and green indicates topography above current sea level. Deep brown regions are areas that would likely not be able to stop the flow of the ice as it becomes unhinged from the bottom bathymetry, allowing seawater to flow beneath the ice. Labels added to the juxtaposition of the two original figures from NASA http://bit.ly/2f1jjFR
New evidence published on October 25, 2016 by Khazendar and colleagues in Nature Communications suggests that the buttress effect of the ice shelves of the Amundsen Sea Embayment of the West Antarctic Ice Sheet may be in jeopardy.
The most likely culprit is warm ocean water melting the underside of the ice shelves that are floating over the shallow sea and attached to the ice sheet on the landward side. This can become a “runaway” situation as each ice shelf thins and becomes separated from the bottom bathymetry that previously helped keep the warm seawater away. More of the seawater can then flow further underneath the ice shelf which in turn leads to rapid shrinking of ice shelf volume.
How do we know this?
There are two telltale signs that this is occurring:
- There has been a shift in the position of the ice shelf and bottom bathymetry contact point, known as the grounding line.
- The ice shelves have thinned.
Evidence mounts regarding the first sign. NASA’s airborne Operation IceBridge used radar to penetrate through the ice sheet to reveal the grounding zones of the Crosson and Dotson ice shelves. These buttress the Smith, Pope, and Kohler glaciers of the West Antarctic Ice Sheet. The grounding lines retreated 40 km ( ~25 miles) since 1996 at Smith Glacier and 11 km (~7 miles) since 2011 at Pope glacier. The same study recorded a 2 km (~1 mile) re-advance since 2011 of the Kohler glacier grounding line after a prior study logged dramatic retreat between 1992 and 2011. Over that same time period, two major glaciers nearby also retreated significantly. The center of the Pine Island Glacier retreated 31 km (~19 miles) and the core of the Thwaites Glacier retreated 14 km (>8 miles).
As for the second telltale sign, a few definitions are in order. Ice freeboard is the elevation of the ice above the local sea level and ice draft thickness is the length of the ice below local sea level. The latest study calculates that between 2002 and 2009, 300 to 490 meters of draft thickness of ice was removed from beneath the Smith Glacier grounding zone. Concurrent laser altimetry measurements of the floating freeboard surface showed a lowering of 30 to 60 meters over the same time period.
This rapid ice shelf thinning was surprising over such a short period. Recently, the National Science Foundation (NSF) and the British National Environment Research Council released an urgent and massive call for proposals to study the shelf and glacier region of the West Antarctic Ice Sheet undergoing the most rapid change. I call this the “No surprises” investment in societally relevant research with near-term and long-term implications for coastlines around the world. The good news is that NASA scientists have already started the eighth Antarctic Ice Change Airborne Survey.
What does this mean for sea level rise?
The most sophisticated ice sheet models to date suggest that once the West Antarctic Ice Shelf destabilization begins, the initial contributions to global sea levels are at rates we can likely adapt to, followed by a jump to major rates of sea level rise. The area of rapid change, the Amundsen Sea sector, gets the most attention—it has the potential to release ice volume equivalent to around 1.2 meters (~4 feet) of sea level rise.
The Amundsen Sea region is the key to unleashing the deep innards of the West Antarctic Ice Sheet. The Thwaites Glacier in this region could contribute less than 0.25 millimeters per year (mm/yr) over this century according to one ice model. Given that current sea level rise rates are around 3 mm/yr, that would represent around 8 percent of the current sea level rise from just this one region of Antarctica. The estimate from that ice model study is just shy of the 10% contribution to global sea level rise that NSF just announced is already observed coming from the region. This ice model of the Thwaites Glacier region projects that the rate during this century would likely jump to 1 mm/yr sea level rise rate starting anywhere between 200 to 900 years. The earlier onset is based on results for the ice model using the highest melt rate assumption which matched the rate of observed losses between 1996 and 2013 in the region. A different ice model investigated the region of the entire West Antarctic Ice Sheet.
This research suggests that local destabilization in the Amundsen sector can ultimately lead to complete disintegration of the marine ice of West Antarctica contributing around 3 meters (nearly 10 feet) to global sea level rise over centuries to millennia (Figure 2).
The main reason this can occur is that much of the West Antarctic Ice Sheet has bathymetry below sea level (Figure 1), and once the grounding lines shift past bathymetric “sticking points” the disintegration accelerates as the ice sheet flattens and spreads like pancake batter. This is where the analogy breaks down, since batter cooks and hardens into a solid pancake, whereas ice melts and flows into the sea. For coastal communities, it is imperative that future research sheds light on what factors could potentially slow down or speed up the pace of sea level rise contributions from the West Antarctic Ice Sheet this century.
West Antarctic Ice Sheet contributions to sea level rise as calculated by Feldmann and Levermann (2015) using an ice model under different durations of perturbation applying observed melt rates in the Amundsen Sea sector of the West Antarctic Ice Sheet. Figure source: http://www.pnas.org/content/112/46/14191.abstract