The stability regimes of Thwaites and Pine Island Glaciers, West Antarctica.

Thwaites and Pine Island glaciers, West Antarctica, are some of the most dynamical areas of the Antarctic Ice sheet and both currently are discharging more ice into the ocean than is replenished through surface snow accumulation. While the current imbalance does not represent a large contribution to global sea level rise, as compared to the Greenland Ice Sheet an Alpine Glaciers, both Thwaites and Pine Island glaciers have rightly received considerable attention by the glaciological community. This interest is related to the suggestion that these glaciers either have already, or are likely to become, dynamically unstable. Here we review the history of ice-dynamical studies of those systems and show how, in recent years, a more detailed picture of the contemporary dynamics of those systems has emerged.  Our new model-based consensus confirms some previously suggested dynamical behaviour, but also points towards a new understanding of the stability regime of those glaciers. The stability regime of marine-type ice sheet cannot be determined based on local geometry alone. In particular, the slope of the bed with respect to flow direction does not determine the stability of grounding lines. Neither Pine Island nor Thwaites glaciers are currently in a dynamically unstable state. However, Pine Island Glacier did go through a phase of unstable retreat during the 1970s. This unstable phase has now come to a halt. Thwaites glacier similarly appears currently to be responding to some past external forcing, which has yet to be fully identified. However, all studies published do indicate that Thwaites will enter a large-scale unstable and irreversible retreat once, or if, the grounding lines retreats about 75 km upstream of its current location. This recent progress can be described using the well-known IPCC confidence levels, as a shift in the confidence in the potential of near-future collapse of Thwaites from low agreement and low evidence to high agreement and medium evidence.  The biggest unknown is now no longer whether Thwaites glacier can in principle become unstable, but when and if this instability will arise.