Quantifying and attributing 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. Over one-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 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 the calibrate-emulate-sample (CES) uncertainty quantification framework. This procedure yields observationally constrained probability distributions of both model and climate forcing parameters. Our analysis suggest 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%. We also find significant retreat even with no anthropogenic trends in forcing, potentially highlighting the role of ice-sheet memory associated with long, slow retreat over the Holocene in controlling present retreat of the WAIS. We further find that significant anthropogenic signals in climate forcing only emerge in the middle of the 22nd century. An important caveat to this work is our choice of initial state, which is larger than expected in practice and may render the anthropogenic forcing contribution in our simulations to be smaller than it is in practice.