Modelling the present-day imbalance of the Antarctic Ice Sheet

Recent human-driven climate change has very likely caused more frequent heatwaves, extreme weather events, and rising global sea levels. When it comes to rising sea levels, two primary drivers are at play: thermal expansion and the loss of freshwater from the cryosphere due to melting glaciers and ice sheets. Rising seas pose a serious threat to coastal communities worldwide. The three main sources of sea level rise from the cryosphere are land glaciers, the Greenland Ice Sheet, and the Antarctic Ice Sheet. Currently, the Greenland Ice Sheet is the largest contributor to global sea level rise. However, the Antarctic Ice Sheet, poses an even greater long-term threat. Although it contributes less to sea level rise at present, it holds around eight times (~ 58 m) more potential for sea level increase than Greenland (~ 7 m). This potential collapse can be triggered by a number of feedback mechanisms. One potential feedback mechanism is known as the Marine Ice Sheet Instability (MISI), which describes the unstable nature of marine-terminating glaciers that rest on bedrock below sea level and slope downward inland. This configuration makes them highly vulnerable to retreat and collapse. Two of Antarctica’s largest outlet glaciers, Thwaites Glacier and Pine Island Glacier, are situated in such precarious positions. These glaciers currently drain a significant portion of the West Antarctic Ice Sheet, and their retreat could destabilize the entire region. Satellite observations have shown that the highest thinning rates across the entire Antarctic Ice Sheet are occurring at Thwaites and Pine Island Glaciers. In this thesis, we present a method for incorporating the observed imbalance of the Antarctic Ice Sheet into ice sheet models. This approach allows future projections to begin from a state that closely matches present-day mass change rates. The method is relatively simple and adaptable to various ice sheet models employing a spin-up strategy. We first tested and validated this method using two different ice sheet models. These models were then used to run future projections under a scenario without any additional climate change (i.e., no further warming). The aim was to investigate whether the current imbalance alone could trigger an accelerated, MISI-driven mass loss, ultimately leading to the deglaciation of major parts of the West Antarctic Ice Sheet. Our results show that the present-day imbalance is sufficient to cause the complete collapse of both Thwaites Glacier and Pine Island Glacier, even in the absence of further climate forcing. We used the updated models to investigate how including the present-day imbalance affects projections that incorporate oceanic and atmospheric climate forcing. We found that accounting for the observed imbalance significantly accelerates the collapse of the West Antarctic Ice Sheet and results in roughly double the sea level contribution of the Antarctic Ice Sheet by 2100, compared to projections that do not include the imbalance. However, over the longer term, as climate forcing intensifies, the influence of the present-day imbalance diminishes.