Surface mass balance projections until 2100 for Folgefonna, a Norwegian ice cap

Abstract. Robust glacier projections are essential for mountain communities adapting to climate change, yet current projections are limited by climate data that either have coarse spatial resolution or span only a narrow range of future scenarios, thereby obscuring the true scale of predictive uncertainty. In this study, we quantify the cascading impact of uncertainty in climate projections on glacier surface mass balance (SMB) projections. We simulate the SMB of the Folgefonna glacier complex in western Norway for 1970–2100 using the energy-balance and snowpack model BESSI. To represent future climate forcing, we use the EURO-CORDEX ensemble, but show that the dataset is systematically too cold and too wet and exhibits unrealistic precipitation patterns for the western Norway region. We therefore develop a downscaling framework in which each EURO-CORDEX member is represented by analogs drawn from a high resolution convection-permitting model (NorCP). This enables SMB projections that account for a plausible spread in climate models and are based on high-resolution climate data with explicitly resolved physical processes. Using this method, we present the most detailed SMB projections of Folgefonna to date for three emission scenarios until 2100. Whether the glacier complex retains any accumulation zone by 2100 depends on the emission scenario. Ensemble medians indicate that Midtfonna loses its accumulation zone in all scenarios, Nordfonna does so in RCP4.5, and no accumulation zone remains on Folgefonna in RCP8.5. However, cumulative SMB (2026–2100) has an uncertainty of 65–75 m w.e. within each scenario due to climate model spread (25th to 75th quantiles). We furthermore find that the choice of global circulation model has a stronger influence on Folgefonna's SMB than the choice of regional climate model. These findings underscore the need for improving upon agreement between climate models. Detailed glacier mass change projections based on only a subset of the available and hence plausible climate projections underestimate uncertainty and should be considered with caution.