Runaway retreat concerning
Joy Gibson, of Oamaru, asked:
What is the impact of human activity on the Antarctic i.e. its glaciers, sea ice, etc)?
Christina Hulbe, a glaciologist at the University of Otago, responded:
The most important impact that people are having on Antarctic ice is global — humaninduced climate change due to burning fossil fuels.
Ice sheets, glaciers and ice shelves grow by snow accumulation on the surface. As new snow falls on top, the older snow is compacted. Eventually it becomes glacier ice and begins to flow towards the coast. It speeds up and stretches out as it goes, and if it reaches the coast, it can float, forming an ice shelf.
Ice can be lost either by melting or by iceberg‘‘calving’’ (breaking off). Because we have ice sheets and glaciers on land and floating ice shelves all around the Antarctic, changes in both the atmosphere and the ocean are important. Warmer air temperatures can cause more melting, and in some places, more melting can also cause a change in how the cracks that lead to icebergcalving form.
Two things are happening in the ocean. First, the ocean is warming. Second, the winds that drive ocean circulation around Antarctica can change over time. This second part is important because it means that the source of the water that contacts the ice shelves can change over time — possibly bringing warmerthanusual water to the ice — and this can yield more melting even without any warming of the water itself.
Once the ice sheet behind an ice shelf starts to change, it might be hard to stop. In most of West Antarctica, the land surface beneath the ice sheet gets deeper away from the coast. This is different from the situation for mountain glaciers here in New Zealand, where the edge of a shrinking glacier would retreat up to higher elevation. In West Antarctica, the edge of the retreating ice sheet stays below sea level and because the water depth grows deeper away from the coast, the tendency to float increases. Once the ice is floating, it can speed up and this can keep the retreat going.
The possibility for runaway retreat is one of the most concerning issues in Antarctic glaciology today. It may have already started on Thwaites Glacier but there are not enough measurements yet to be sure. Several years ago, a student (Scott Waibel, who lives in the United States) and I used computer models of Thwaites Glacier to investigate what happens when the retreat is started by different amounts of ocean warming. Scott found that retreats started with a larger ‘‘push’’ from the ocean went faster than retreats that started with a smaller push, even after the ocean warming was removed. This means that even if we can’t stop the runaway retreat entirely, the sooner we stop the warming, the more slowly it will happen.
Other ice sheet modellers have come to similar conclusions. The threshold for irreversible retreat seems to be between 1.5 and 2degC global mean warming, which means that the threshold is close but also that we might not be there yet. All the more reason to make the changes we can and to hold Government to account for more.
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