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Antarctic ice dams at risk

Source: Fürst et al. (2016), The Safety band of Antarctic ice shelves. Nature Climate Change

The shrinking Antarctic

As our planet warms, the melting of glaciers and polar ice caps is causing sea level rise, threatening the future of coastal cities and low lying areas around the world. This melting includes Antarctica’s ice shelves, which add freshwater to the ocean, contributes to rising seas and changing the ocean currents. Measurements from satellites show that Antarctica’s ice is melting faster now than it was a decade ago, and as the Earth’s atmosphere continues to warm due to carbon dioxide emissions it will likely melt even faster. Scientists are working hard to understand what controls how fast Antarctica melts, so that they can provide accurate predictions of future sea level, allowing coastal communities and small island nations around the world to prepare and adapt to rising seas.

Figure 1 Image taken from the NASA MODIS satellite during the collapse of the Larsen B ice shelf in Antarctica in 2002

Holding back the flow

Ice shelves protruding out from the continental shelf over the ocean serve as natural dams that slow the flow of slowly moving ice sheets behind them. Before our planet started to warm, the amount of snow piling up on top of ice shelves balanced the amount of melting and icebergs which break off (in a process known as calving), so the ice shelves stayed the same size. Now that the planet is out of balance, the amount of snow piling on the ice shelves can’t keep up with the amount of melting and calving. This means that the ice shelves are shrinking, and with it the strength of the wall holding back the flow of the ice sheets. To figure out how much these ice walls will weaken as ice sheets melt, a group of scientists based in France set out to estimate how well the ice shelves hold back the flow.


Measuring ice strength

The researchers used a model of ice flow that includes measurements of ice from satellites to calculate how much force the ice shelves push back on the ice piling up behind them. They compared this to the amount of force the ocean would push back on ice if there were no ice shelf. If they are the same, then the ice shelf has no effect on ice flow. If the force from the ice shelf is much larger than if there were just ocean water, then it is a really effective wall and this effect is called ‘buttressing’. The amount of buttressing depends on both the size and shape of the ice shelf. Figure 2 shows the strength of buttressing in color on all the Antarctic ice shelves and shows that it tends to be stronger close to the continent and weaker near the edges of the shelf.

To determine exactly how much the level of buttressing impacts the speed of ice flow, the researchers ran a model experiment where they had icebergs break off the front of ice shelves and looked at how the speed of ice flow responded. From this they were able to separate passive shelf ice (PSI), which does not impact the speed of ice flow from the rest of the ice shelf (contoured in red on figure 2). In some regions, like the Amundsen and Bellinghausen Seas, there is almost no PSI, which means that the entire ice shelf is acting like a wall to hold back ice flow. This is also the region to Antarctica where the ice shelves are melting most rapidly and if they continue to melt rapidly this will cause the flow of ice from the continent to speed up. In other places, like the Larsen C ice shelf in the Weddell Sea (Figure 3), has a large portion of PSI at the front of the ice shelf, which if it breaks off (as is predicted to happen very soon) won’t affect the strength of the wall holding back the ice sheet.

Figure 2 Buttressing of ice shelves in color, with the red line separating the PSI from the rest of the ice shelf The pie charts show the fraction of each ice shelf that is passive in blue, and the amount that contributes to holding back ice flow in yellow.

Beyond the ice edge

The findings of Fürst et al. suggest that the regions where the ice shelves are most important for holding back the ice sheet flow are the same regions that are currently shrinking the fastest. However there are other ice shelves on the brink of collapse that are not buttressing the ice behind, so are unlikely to speed up the flow of ice off the continent. These insights are important for predicting how the rate of ice melting from Antarctic will change in the future and which ice shelf regions researchers should be focusing their efforts on understanding. This work illustrates the complexity of ice processes in Antarctica, and that if we are going to be able to accurately predict how sea level will change over the globe we need to continue to work to figure out what controls how quickly Antarctic ice melts, and how that could change in the future.

Figure 3 Zoomed in map of buttressing of the Larsen C ice shelf and ice velocity with the dashed line showing a crack that is spreading through the ice. Most of the ice that is breaking off is passive so it won’t impact the buttressing effect of the ice sheet.

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