New insights into the current dynamics of Thwaites and Pine Island Glaciers, West Antarctica.

We provide a summary of new insights into the contemporary dynamics of Thwaites and Pine Island Glacier, West Antarctica, obtained through series of recent modelling studies. By conducting ice-flow modelling studies with three independent models (ISSM, StreamIce, a), all initialized to current day conditions, we show that all models provide very similar future estimates of future mass loss for a given forcing scenario. Importantly, while description of basal processes does impact our results to some degree, we nevertheless find that estimates of mass loss over time scale of 50 to 100 years, are insensitive to the choice of basal sliding law. This can be understood to be related to the compensating impact of the initialisation process that in each case produces correct initial state for the ice sheet (i.e. surface velocities and current rates of mass loss) irrespectively of the sliding law used. Furthermore, we find that inversion produces (e.g. estimates of basal slipperiness and englacial ice rate factors) can be exchanged between models, showing that these products are to large degree model independent. A common feature found in all our ice-flow studies, and coupled ice+ocean studies, is the existence of multiple tipping points within the Thwaites and Pine Island system. In all our ice-flow simulations, the grounding line of Thwaites always becomes unstable once it has retreated by about 75km upstream from its current position. We also conclude that the grounding line of Pine Island Glacier has recently undergone a phase of irreversible retreat, and that the glacier has at least 3 further tipping points that can be crossed in the near future (centennial time scale). Additionally, the western ice shelf of Thwaites has been dramatically weakening over the past decade and we used our three independent models to assess the effect of a potential complete collapse of Thwaites floating extension. First, we find that this ice shelf provides limited buttressing, and disintegrations of the ice shelf, in its current configuration, will not significantly impact upstream grounded. Second, it has been suggested that Thwaites would be subject to the Marine Ice Cliff Instability (MICI) if its ice shelf collapses, as it would expose a tall ice cliff. Using recently proposed cliff-height dependent calving laws, we find no indication that such calving laws give rise to an irreversible or unstable calving-front retreat. Thus, those calving laws do not lead to a marine ice cliff instability despite prescribing calving rates as strongly increasing functions of cliff height.