New Antarctic spin-up method results in committed Thwaites glacier collapse

Projections of sea level rise are subject to large uncertainties in the contribution of the Antarctic Ice Sheet (AIS), as it is unclear how AIS dynamics will evolve over time. Ice sheet models use spin-up techniques to initialize the ice sheet to the present-day state. Previous attempts using the Community Ice Sheet Model (CISM) assumed that the ice sheet is in equilibrium at the end of the spin-up. This assumption limits the contribution of model drift, but does not match observations and might bias future projections.

 

For this reason, we have incorporated present-day thickness change rates from Smith et al. (2020) in our Antarctic spin-ups. As in previous spin-ups, we tune basal friction coefficients beneath grounded ice, and ocean temperatures beneath floating ice, to match observed present-day thickness. In the new procedure, CISM is also forced to match thickening and thinning rates, with the surface mass balance (SMB) adjusted to allow the AIS to maintain the observed thickness and grounding-line locations. This technique improves the modelled velocities in regions with substantial thinning. When the SMB adjustments are removed, the modelled ice sheet exhibits the observed thickness change rates. We use this initialised state to project AIS evolution without additional forcing (‘committed climate change’). For a range of parameter settings, this causes Thwaites glacier to collapse irreversibly, without further ocean warming. The time of initiation of collapse is sensitive to model parameters, but once initiated the collapse is largely complete within two to three centuries. The sensitivity tests are carried out for a range of parameters related to basal sliding and various ocean warming scenarios.