[NERC-NSFGEO] BISTO: Better Ice Sheet forecasts via Transient assimilation and refined Ocean forcing

The rate of loss of ice from the Antarctic Ice Sheet has tripled in the last three decades, and is expected to continue increasing – potentially leading to several tens of centimetres of sea level rise by 2100 – with important implications for planning and adaptation in coastal communities, particularly in the Northern Hemisphere. These losses are largely attributed to increased ocean-driven melting of floating ice shelves, which causes fast-flowing outlet glaciers to speed up. Warming oceans could further increase this melt under climate change, so it is critically important that models deliver credible assessments of the ice-sheet response to different greenhouse-gas emissions scenarios. A major limitation arises from the wide disparity in responses of current ice-sheet models to future climate warming. This disparity is due in large part to uncertainty in ice-sheet models’ representation of the current ice state, owing to ice sheets’ long memory of past changes. It also arises from uncertainty in how the future melting of ice shelves responds to ocean warming, and poor knowledge of key physical processes which ice-sheet models do not fully resolve. The key advance of this project will be to reduce uncertainty in 21st century ice loss from Antarctica through (i) next-generation assimilation of satellite observations into ice sheet models and (ii) advanced climate downscaling through high-resolution ocean simulation. This will be achieved through the following objectives: Improve Ice-sheet Data Assimilation to capture the current dynamic state of Antarctic glaciers and reduce model uncertainty. Improve model representation of poorly constrained ice processes by leveraging the results of assimilation. Develop an effective modelling treatment for ocean-driven melt through high-resolution simulations. Improve projections of 21st century sea-level contributions through coupled modelling of dynamic Antarctic glaciers under greenhouse-gas emission scenarios. We will, for the first time, produce 21st century projections of ocean-driven ice loss from Antarctica that are consistent with the full range of satellite observations of ice-sheet thinning, ice-stream acceleration, and ice-shelf melt. By focusing on regions in West and East Antarctica with the strongest thinning rates and oceanic forcing, we will greatly reduce uncertainties in century-scale ice loss and provide step-change improvements in ice-sheet data assimilation and ice-ocean modelling to the Antarctic modelling community. The proposed contributions are both novel and timely. The modern satellite record offers an ever-growing wealth of information on ice-sheet evolution, yet most ice-sheet models remain incapable of integrating it. In the Ice Sheet Model Intercomparison ISMIP6 (part of the Coupled Model Intercomparison CMIP6), most models did not reproduce the observed ice loss, and ice-loss forecasts had a incredibly wide range. Our preparations for ISMIP7 will commence shortly. BISTO will provide methodologies of how to formally constrain ice sheet models to match the observed ice loss, and demonstrate the impact on ice-sheet forecasts and their uncertainty. Many leading climate modelling centres are developing interactive ice sheets and ice shelves in their modelling frameworks. However, they face issues of resolution (due to the small-scale ocean processes under ice shelves) and initialization (due to the long time scales inherent in both ocean and ice sheets and the tight coupling between them). The objectives of BISTO will provide roadmaps for these large-scale ice-ocean coupling efforts, greatly improving societal capacity to project future sea level rise.