Detection and attribution of the role of anthropogenic climate change in industrial-era retreat of Pine Island Glacier

Abstract. The West Antarctic Ice Sheet (WAIS) has undergone rapid change over the satellite era, characterized by significant thinning, grounding-line retreat, and mass loss. More than a third of the ice loss from this region is from Pine Island Glacier (PIG). However, robust causal links between anthropogenic climate change and PIG ice loss have yet to be established. Here we attempt to quantify the role of anthropogenic climate change in observed retreat of PIG over the 20th century and how this may evolve up to 2200. To do so, we use an ensemble Kalman inversion data assimilation method embedded into an uncertainty quantification framework, called calibrate-emulate-sample (CES). This procedure, which assimilates observations of grounding-line retreat and ice volume, yields observationally constrained probability distributions of both model and climate forcing parameters. Our analysis suggests that it is unlikely that the extent of 20th century PIG retreat would have taken place without anthropogenically driven trends in ice-sheet forcing and that anthropogenic forcing exacerbated the extent of PIG retreat over the 20th century, by approximately 18 %. These results are, importantly, conditional on our choice of initial state. For our chosen initial state, we find that the parameter combinations compatible with these observational constraints require PIG to lose mass (but not experience grounding-line retreat) over the entire simulated period since 1750, not just after the 1940s when grounding-line retreat was initiated. This preconditioned ice mass loss introduces significant uncertainty into our quantification of 20th century forcing contributions. In simulations with no anthropogenic trend in forcing, we still observe significant retreat; this may result either from a larger-than-actual initial state, or may suggest that the earlier ice state preconditioned the industrial era retreat, possibly implicating longer term changes to WAIS in the present retreat.