A Bayesian Framework for Anthropogenic Attribution of Sea Level Rise Contributions from the West Antarctic Ice Sheet

Satellite observations show rapid retreat of many outlet glaciers in West Antarctica, corresponding to a significant proportion of the contributions to global sea level rise in recent years. These changes have not been formally attributed to anthropogenic climate change, primarily because of the potential for positive feedbacks on ice sheet mass loss, which may have been triggered even within the limits of natural variability. This naturally leads to the attribution question: “how much more (or less) likely have anthropogenic changes made a specified contribution to sea level rise?” In this talk, I shall describe a Bayesian framework to address this question, which uses ensembles of many simulations with independent realizations of ice-sheet forcing with, and without, anthropogenic changes. Enhanced melting of ice shelves is thought to be the key forcing contribution responsible for recent retreat of the West Antarctic Ice Sheet; we include a consideration of the accuracy of melt rates in this framework by updating our prediction of sea level rise according to the agreement between the parametrized melt rate in the simulations and the output from a numerical ocean circulation model, at various time points. Experiments in an idealized setup point elucidate the dependence on the forcing timescale in the changes in likelihood of various contributions and demonstrate the feasibility of attribution studies for the Antarctic ice sheet.