Future ice-sheet surface mass balance and melting in the Amundsen region, West Antarctica

Abstract. We present projections of West-Antarctic surface mass balance (SMB) and surface melting to 2080–2100, under the RCP8.5 scenario and based on a regional model at 10 km resolution. Our projections are built by adding a CMIP5 (5th Coupled Model Intercomparison Project) multi-model-mean seasonal climate-change anomaly to the present-day model boundary conditions. Using an anomaly has the advantage to reduce CMIP5 model biases, and a perfect-model test reveals that our approach captures most characteristics of future changes, despite a 16–17 % underestimation of projected SMB and melt rates. SMB over the grounded ice sheet in the sector between Getz and Abbot increases from 336 Gt yr−1 in 1989–2009 to 455 Gt yr−1 in 2080–2100, which would reduce the global sea level changing rate by 0.33 mm yr−1. Snowfall indeed increases by 7.4 to 8.9 % per °C of near-surface warming, due to increasing saturation water vapour pressure in warmer conditions, reduced sea-ice concentrations, and more marine air intrusion. Ice-shelf surface melt rates increase by an order of magnitude along the 21st century, mostly due to higher downward radiation from increased humidity, and to reduced albedo in the presence of melting. Eastern ice shelves (Abbot, Cosgrove and Pine Island) experience significant runoff in the future, while western ice shelves (Thwaites, Crosson, Dotson and Getz) remain without runoff. This is explained by the evolution of the melt-to-snowfall ratio: below a threshold of 0.60 to 0.85, firn air is not entirely depleted by melt water, while entire depletion and runoff occur for higher ratios. This suggests that western ice shelves might remain unaffected by hydrofracturing for more than a century under RCP8.5, while eastern ice shelves have a high potential for hydrofracturing before the end of this century.