| 英文摘要: | Over the past 40 years Pine Island Glacier in West Antarctica has thinned at an accelerating rate1, 2, 3, so that at present it is the largest single contributor to sea-level rise in Antarctica4. In recent years, the grounding line, which separates the grounded ice sheet from the floating ice shelf, has retreated by tens of kilometres5. At present, the grounding line is crossing a retrograde bedrock slope that lies well below sea level, raising the possibility that the glacier is susceptible to the marine ice-sheet instability mechanism6, 7, 8. Here, using three state-of-the-art ice-flow models9, 10, 11, we show that Pine Island Glacier’s grounding line is probably engaged in an unstable 40 km retreat. The associated mass loss increases substantially over the course of our simulations from the average value of 20 Gt yr−1 observed for the 1992–2011 period4, up to and above 100 Gt yr−1, equivalent to 3.5–10 mm eustatic sea-level rise over the following 20 years. Mass loss remains elevated from then on, ranging from 60 to 120 Gt yr−1. At present Pine Island Glacier (PIG) is responsible for 20% of the total ice discharge from the West Antarctic Ice Sheet2, 3 (WAIS). The accelerated thinning observed since the 1980s has essentially been attributed to enhanced sub-ice-shelf melting12 induced by the recent alteration of Circumpolar Deep Water circulation13. This has reduced the buttressing exerted by the ice shelf, leading to the acceleration of the ice stream and the ongoing retreat of the grounding line along the glacier’s trunk observed since 19925. Today the grounding line lies over bedrock that has a steep retrograde slope14 (Fig. 1c) raising the possibility that PIG may already be engaged in an irrevocable retreat. Assuming that ice flow is dominated by basal sliding and lateral variation can be ignored, grounding lines located on retrograde slopes are always unstable6, 7, but in realistic, three-dimensional geometries lateral drag and buttressing in the ice shelf can act to prevent unstable retreat11. Assessing the stability of PIG therefore requires numerical models that accurately represent these additional forces. Models designed to study the evolution of PIG have been reported, though limited to flowline geometries15 or extreme forcings8. Overall, the short-term behaviour of PIG is not well understood and projections vary wildly, ranging from modest retreat to almost full collapse of the main trunk within a century8, 15.
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