Assessing the attribution of alpine glacier mass loss to anthropogenic warming over the last millennium using ensemble paleoclimate reconstructions and GCM simulations

The central estimate of the Intergovernmental Panel on Climate Change is that the magnitude of anthropogenic warming since 1850 is equal to 100% of the observed warming. However, the IPCC is notably much more timid in attributing glacier mass loss to anthropogenic warming over the same period. Disagreements have arisen in previous research, primarily stemming from ambiguity in the dynamic disequilibrium of preindustrial glaciers and its lingering effects. Accounting for variability in glacier disequilibrium entering the industrial era, Roe et al., (2021) used simple glacier models and synthetic climate scenarios to estimate  a mass-loss attribution of ˜100% [90-130%, likely range] over the full industrial era. Our work further assesses this claim for a case study of glaciers in North America and the Alps using: i) realistic ice dynamics, ii) observed glacier geometries, iii) ensembles of last-millennium reconstructions (LMR) and GCM simulations, and iv) a comprehensive sensitivity and uncertainty analysis. In addition to CMIP6 past1000 simulations, we use recently developed LMR paleoclimate reconstructions, specifically adapted for melt-season temperatures. By using millennial-scale climate time series, we avoid the need for an accurate initial condition. We simulate glacier mass-balance and length fluctuations over the last millennium for a variety of potential climate histories to produce an uncertainty envelope for each glacier’s preindustrial state. For our case-study of glaciers, we find that all: i) exhibited slow growth over the last millennium, ii) have lost mass over the industrial era, and that iii) the magnitude of industrial-era mass loss for each glacier greatly exceeds natural variability over the last millennium. Given that 100% of industrial-era temperature change is attributable to anthropogenic activity, these results imply that mass loss for these glaciers can be confidently attributed to anthropogenic warming since the beginning of the industrial era (1850 vs. the IPCC’s 1990). Work is ongoing to expand the analysis scope to a larger network of well-observed glaciers, with potential for a global assessment in the future.