This paper has already caught on around the mainstream media – related articles can be found at CNN and an NPR blog – but I thought I’d add my two cents here. Some new research has just come out from UC-Irvine and NASA’s Jet Propulsion Laboratory showing that glaciers in West Antarctica will inevitably melt within the next century or so, based on forty years of evidence that their grounding line is rapidly retreating (published here in Geophysical Research Letters). So, what is a grounding line and what does this mean for sea level rise?First, let’s figure out where this is all happening – the South Pole, the continent of Antarctica. As you can see above, there are a host of glaciers – Pine Island, Thwaites, Pope, Smith, and Kohler – that reside along this western segment of Antarctica. Glaciers are enormous, dense blocks of ice that slowly move or change shape. They always form on land, but can move toward water over long (in the geological sense – meaning very long!) times. These glaciers gradually transition to ice shelves – the Eastern, Cross, Dotson, for example – again shown above, which are huge slabs of ice that completely rest on water. Glaciers transform to ice shelves as the ice moves along a coastline, moving from a substrate of land to water. The initial location where the glacier rests on land is known as the grounding line, shown below.
As this figure alludes to, once ice from a glacier has moved past the grounding line, now becoming an ice shelf, water can circulate beneath the ice. When this water is warm enough, ocean currents moving beneath the ice shelf can melt the shelf, turning the ice to water. There are massive amounts of water in these glaciers and ice shelves, so large-scale melting results in sizable, measurable, sea level rises. As the warm ocean water continues to move past the glacier, more and more ice melts, cutting away at the ice and moving the grounding line farther and farther back inland (to the left in the above figure) – this is known as grounding line retreat.
And this is exactly what the paper has found to be happening at a very quick rate over the past 40 years! The researchers used a very clever measurement tool (as scientists are known to develop) to measure the grounding line, which is often located under kilometers of ice and difficult to spot. Using satellite imaging, researchers can collect data of the horizontal and vertical movements of the glaciers. Of special importance here, ice shelves – and glaciers making this transition at the coastline – can be seen to bob vertically with the tides, whereas glaciers on land do not share this behavior. Therefore, the grounding line can be estimated as the location where this vertical oscillatory behavior ends.
Using this measurement and combing through the past forty years of data, the researchers have found that the glaciers shown above have had their grounding lines retreat on the orders of 10-40 kilometers. In addition, they have found no sign of hills or higher elevation upstream from the grounding lines, meaning that, once the glaciers begin to slide into the water, no geographical constraints can prevent them from falling all the way in. This, combined with previous measures of flow speeds, all combine to indicate that these glaciers are past the point of no return – eventually they will slide completely into the water and melt! The speed at which the grounding line is retreating is thought to be due to higher wind speeds at the ocean surface, leading to faster and warmer ocean currents, along with ozone depletion.
Using estimates based on the size of the glaciers under consideration, the researchers guess that sea level could rise about 3 feet by 2100 due to these glaciers alone. This would currently displace about 100 million people, not taking into account population growth over the next century. It’s time we start thinking about solutions!
Rignot, E., Mouginot, J., Morlighem, M., Seroussi, H., & Scheuchl, B. (2014). Widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith and Kohler glaciers, West Antarctica from 1992 to 2011. Geophysical Research Letters DOI: 10.1002/2014GL060140