STORYLINES IN LAST INTERGLACIAL ANTARCTIC ICE-SHEET MASS LOSS INFORM FUTURE EXTREME SEA LEVELS

Global and local sea levels are projected to rise considerably this century, amplifying the frequency of extreme sea levels that can result in deadly and costly coastal flooding. But projections of future sea level rise are highly uncertain, due primarily to uncertainties in human decisions (through emissions pathways) and uncertainties in the mechanisms that could drive Antarctic ice-sheet (AIS) mass loss. In some coastal regions, the differences between best and worst case scenario projections can be a meter of sea level rise or more depending on the ice-sheet physics considered. AIS projections strongly depend on model calibration that are often based on varying interpretations or distinct storylines in paleo sea levels. Understanding and differentiating the impact of AIS paleo-storylines remains a research priority, especially to inform coastal adaption and decision-making. In this study we use supervised machine learning to emulate an Antarctic ice-sheet model. The model is forced with a high and low emissions pathways and model run ensembles are constructed assuming two different interpretations of last interglacial (LIG; ~128 kya) AIS mass loss. In particular, relatively high (>6 m) and relatively low (<3.5 m) LIG AIS mass loss storylines inform whether or not marine ice-cliff instability is considered a primary contributor to sea-level rise. Projections following these AIS paleo-storylines are integrated with a holistic sea level projection framework to produce localized sea level projections around the globe. We then calculate an amplification factor---an increase in expected frequency of extreme sea level events due to local sea level rise---associated with each LIG AIS storyline through 2100. While not a direct measure of coastal flooding (because it does not account for local factors such as terrain or infrastructure), the amplification factor is a useful quantification of how different storylines in the Antarctic ice-sheet influence local extreme sea levels. We find that LIG estimates and modern day observations of Antarctic mass loss can inform the physical mechanisms driving Antarctic contributions to sea level rise. Extreme sea level projections are sensitive to both the emissions pathway and Antarctic ice-sheet physics. From these projections it is also difficult to distinguish whether the planet is following a relatively high or relative low future AIS mass loss storyline until at least 2060.