Rethinking Antarctic Sea Level Projections (RASP)

One major consequence of global warming is the rising of sea levels that threaten coastal communities, ecosystems and industries worldwide. Since the beginning of the industrial revolution, global sea-levels have risen by about 20cm, largely through four components: the expansion of warming ocean waters, the melting of glaciers and the thinning of the Antarctic and Greenland ice sheets. In the recent 2021 report from the International Panel on Climate Change summarising the physical understanding of the Earth System, it is emphasised that future ice loss of the Antarctic Ice Sheet is the most uncertain of the four components above. Computer simulations suggest that the Antarctic Ice Sheet could slightly lower sea level or, more likely, raise sea level by more than 50 cm by 2100. Antarctica is particularly relevant for the UK, since northern hemisphere sea level responds over proportionately to ice loss in Antarctica, due to a reduction of the local gravitational pull. Furthermore, a research briefing for UK parliament (POSTnote 555) discusses that 50 cm of local sea level rise would make about 200 km of UK coastal flood defences vulnerable to failure. Further commissioned research suggests that by 2080, higher end sea-level estimates cause yearly damages in the UK that are £540m higher than those expected for the lower end estimates. Clearly, there is an urgent need to reduce uncertainty in sea-level projections for UK coastal planning. In the project "Rethinking Antarctic Sea-level Projections" (RASP), we propose a new approach to better understand and constrain the uncertainty for the Antarctic component.

One reason why the future evolution of the Antarctic Ice Sheet is so uncertain is a gap in the scientific understanding, and thus representation in computer models, of how the surrounding Southern Ocean melts the Antarctic Ice Sheet in the future. Warmer ocean waters are found offshore of the Antarctic continent in the deeper, open ocean. In some regions, they already access the continental shelf and reach the ice, causing high melting. This is for example the case in the Amundsen Sea, where the bulk of Antarctica's ice loss is observed. Other regions, such as the Weddell Sea, are currently protected by colder waters. If, and how, those warm water masses access the continental shelf is dependent on a complex interaction of regional climate drivers such as winds, precipitation and air temperatures. But how important these different climate drivers will be for ice loss in the different Antarctic regions in the future, is unclear. We here propose to answer this question using a numerical model that represents the relevant Southern Ocean processes. Importantly, it will include the continental shelf and the ice-shelf cavities where the ice is in contact with the ocean in Antarctica. This gives us the unique opportunity to make a suite of targeted numerical experiments to identify the most relevant of those regional climate drivers, and how they interact.

Using this novel understanding, we can then bridge the gap between the far-field, open ocean and the vicinity of the ice sheet. We will use a numerical ice flow model to make Antarctic future projections. This model represents how the changes in the atmosphere and ocean-driven melting affect the ice flow in Antarctica, and thereby lead to sea-level rise. We will run a range of experiments testing for uncertainties in future changes in the atmosphere and ocean, as well as uncertainties in the model physics. In particular, we can map out how much each climate driver contributes to the uncertainty in Antarctic sea-level projections.

Thereby, we can fill the knowledge gap in the physical links between the Southern Ocean and the Antarctic Ice Sheet, and how much each link will contribute to sea-level rise over the coming decades to centuries.