Sunday, September 6, 2015

What is happening to Antarctic ice shelves and why should we care?

Front of Ross Ice Shelf
Front of Ross Ice Shelf from research icebreaker Palmer.
Photo © Bruce Luyendyk

Antarctic ice shelves are thinning. What are ice shelves and are they important in any way? These massive floating sheets of ice border over a third of the Antarctic coast. 

Ice shelves are not frozen ocean. That is sea ice. In the simplest notion they are floating glaciers and part of the cryosphere cycle in Antarctica and Greenland. The largest Antarctic ice shelf is the Ross Ice Shelf, about the size of Texas (or France) and formed by the merging of glaciers flowing off the Antarctic continent. This shelf for example, is hundreds of meters thick (up to two thousand feet or more) and floats over sea floor hundreds of meters deeper.

This spring Science magazine published a research study on the state of the Antarctic ice shelves1.
Using satellite data scientists looked at the thickness of ice shelves around Antarctica and how that has changed over the past eighteen years. They found the shelves remained unchanged over the period 1994-2003 but thinned (shrunk) rapidly during 2003 to 2012 losing over three hundred cubic kilometers of volume. Their interpretation is that the rate of mass loss (thinning) is accelerating. The cause believed responsible for the thinning of the floating ice shelves is warming from beneath by an ocean heated by global warming.

Loss of the ice shelves themselves will not affect global sea levels because they float in the Southern Ocean like immense ice cubes in a pot of water. When floating cubes melt the water level in the pot remains unchanged – try it. But the shelves hold back the ice on the continent that rests above sea level. The term for this is buttress; the shelves hold back the continental ice – sort of. They control the rate the ice flows off the continent. Ice on the continent will raise sea level if more of it makes its way into the ocean – like adding an ice cube to a glass of water. Try that!

At the front of ice shelves calving takes place – icebergs break off. All of this is part of the ice cycle on the continent (see figure); snow falls and compacts to ice; ice flows downhill into glaciers and glacier merge into ice shelves; ice shelves advance at a few meters per day (five to ten feet); shelves calve at their front creating icebergs that float away and melt in the sea. When all is in balance the size of the continental ice sheet stays the same more or less and sea level doesn’t change. But when ice shelves loose mass then imbalance occurs. If ice flows faster off the continent - more cubes are added to the glass and sea level rises.

Processes around an Antarctic ice shelf. © Hannes Grobe. Alfred Wegener Institute, Germany.

Ice shelves hold back the continental ice and slow it down as it runs off the continent. This happens where the ice shelf touches bottom either at the grounding line – where it starts to float next to the continent – or grounds on sea floor hills and mountains (see figure). Any process that causes an ice shelf to thin causes it to loose contact with the bottom, or the grounding line to retreat inland. This results in a decrease in friction that can cause the flow of ice off the continent to speed up. Ice is lost faster than it can be created. Under some circumstances this can become a runaway situation. This appears to be happening as is revealed in this study and earlier ones2.

The major finding of the research is that these shelves are shrinking - that is bad news for the future of the Antarctic ice sheet and global sea level. My previous posts (May 20, 2014) (June 9, 2014) have explained that the mass (ice) balance of Antarctica could not be taken into account by the Intergovernmental Panel on Climate Change when they predicted the rise of sea level this century. That was due to a lack of information and understanding. But now we know more. We know the West Antarctic Ice Sheet has become unstable by the process of ice shelves loosing mass. And now we know just how fast the ice shelves are shrinking. The meaning? New predictions can be made on the rise of sea level and the rate. These predictions I expect, will be larger than before.

What do we do with this news? We have three choices; ignore it, mitigate the causing phenomena, or adapt to it. Ignoring is not smart unless you are a climate change denier - then no information matters. Mitigation by decreasing carbon dioxide emissions would help to decrease global temperatures but not immediately affect global ice sheets. There is too much momentum in the system. Melting and ice mass loss will continue for hundreds of years. Adaptation is the only practical action – building sea walls, dikes, and migration of populations inland to higher elevations. It won’t be easy but it can be done.

 1. Fernando S. Paolo, Helen A. Fricker, and Laurie Padman. (2015), Volume loss from Antarctic ice shelves is accelerating. Science 17 April 2015: 348 (6232), 327-331.Published online 26 March 2015 [DOI:10.1126/science.aaa0940]

2. Rignot, E., J. Mouginot, M. Morlighem, H. Seroussi, and B. Scheuchl (2014), Widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith, and Kohler glaciers, West Antarctica, from 1992 to 2011, Geophys. Res. Lett., 41, 3502–3509, doi:10.1002/2014GL060140.

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