Effect of climate policies on the long-term equilibration of glaciers

Glaciers adapt to changing climatic conditions by losing or gaining mass, translating into a geometry change. Due to ice-dynamical processes within glaciers that gravitationally transport mass from high to low elevation, the adaptation of the glacier geometry to changing climatic conditions is not immediate but requires timescales ranging from decades up to millennia. As a consequence, today glaciers are in imbalance with current climatic conditions and will respond on timescales that extend beyond the 21st century time horizon that is typically considered in todays glacier evolution studies. As part of the Glacier Model Intercomparison Project Phase 3 (GlacierMIP3), a global glacier modelling community effort, we quantify how glaciers will stabilize under a wide range of climatic conditions. Using 8 large-scale glacier models, we estimate the committed loss of all glaciers on Earth outside the ice sheets under current climate conditions (corresponding to +1.2C above pre-industrial levels) and their long-term stabilization under various policy-relevant global warming scenarios, such as +1.5C and +2C (Paris agreement), and the projected warming following current policies (+2.7C). The forcing is derived from historical and future simulations (3 SSP emission scenarios) from 5 GCMs, from which climatic conditions for given time periods are continuously repeated over millennial time scales, leading to an eventual glacier stabilization. We find that the committed glacier loss is substantial, with about one third of global glacier volume to be lost under current climatic conditions. The final (steady state) global glacier volume strongly depends on future temperatures, with an increase in the order of 2-3% of global glacier loss per 0.1C warming. We also evaluate regional differences and quantify the time scales involved in glacier stabilization. Here we find that the topographical features such as the elevation range and the surface slope of glaciers play an important role.