Why 0.5°C matter for the future evolution of Alpine glaciers

With the Paris Agreement, leaders of the world have recognized the urgency of limiting ongoing, anthropogenic climate change. In preparation of the upcoming 26^th UN Climate Change Conference of the Parties, discussions have been focusing on the difference of limiting the increase in global average temperatures below 1.0, 1.5, or 2.0°C compared to pre-industrial levels. Here, we assess the impacts that such different scenarios would have on both the future evolution of glaciers in the European Alps and the water resources they provide. We force the combined glacier mass balance and ice flow model GloGEMflow with climate projections from Coupled Model Intercomparison Project Phase 6 (CMIP6), and compute the area and volume evolution of all 3926 glaciers of the European Alps for the period 1990 to 2100. Our results show that the different temperature targets have important implications for the predicted changes: in a +1.0°C scenario, glaciers in the European Alpsare  projected to lose 44 ± 21 % of their 2020 ice volume; 68 ± 12 % in a +1.5 °C scenario; while 81 ± 8% in a +2.0°C scenario. The changes in glacier volume will strongly impact the water yield from presently-glacierized catchments, with 2080-2100 yearly average runoffs decreasing by 25 ± 6% (for a global warming of +1.0°C), 32 ± 8%, (+1.5°C) and 36 ± 10% (+2.0°C) when compared to 2000-2020 levels. Changes in peak runoff -- anticipated to occur 1 to 2 months earlier by the end of the century than it does today -- will be even more pronounced, with reductions of 23 ± 15 %, 29 ± 14 %, and 37 ± 15 % in the three warming scenarios, respectively.