Thwaites Glacier loses ice even without ocean melting

Pine Island (PIG) and Thwaites (TG) glaciers currently dominate Antarctica's sea-level contribution. These glaciers began a synchronous retreat in the mid twentieth century when PIG ungrounded from a submarine ridge and Thwaites from its Western pinning point. The historical ice loss in this sector is ultimately caused by changes in ocean melting. However, it remains unclear the extent to which the ongoing ice loss is driven by anomalously warm present-day ocean conditions, potentially caused by anthropogenic climate change, or is an ongoing response to a natural climate anomaly in the 1940s, to which the ice sheet is still adjusting. Here, we probe these drivers of ice loss by completely removing the ocean forcing. We use three state-of-the-art ice sheet models to simulate the response of PIG and TG in an extreme hypothetical scenario of zero ocean melting, maintained over a policy-relevant timescale (150 years). We find that PIG thickens and re-advances to the prominent sea-bed ridge on which it was grounded prior to the 1940s. In contrast, Thwaites continues to lose ice (at a decreasing rate) over the next 150 years, despite the absence of sub-ice shelf melt. This tells us two things. Firstly, since all the forcing is removed in these experiments, the ice loss from Thwaites must have a substantial component that is an ongoing transient response to historical forcing. Secondly, the historical retreat of Thwaites has led to a present-day state that cannot re-advance under any ocean cooling measures. Thus, Thwaites is now in a dynamically different state to its 1940s configuration. While this does not imply commitment to irreversible future retreat, the historical mass loss from Thwaites is now built-in and cannot be reversed through reductions in ocean forcing alone. This further suggests that some future sea-level contribution is unavoidable on centennial timescales, even under strong policy interventions that result in ocean cooling.