The Mont-Blanc massif, being iconic with its large glaciers and peaks of over 4,000 m, will experience a sharp increase in summer temperatures during the twenty-first century. By 2100, the impact of climate change on the cryosphere and hydrosphere in the Alps is expected to lead to a decrease in annual river discharge. In this work, we modelled the twenty-first century evolution of runoff in the Arve river, downstream of Mont-Blanc's French side. For the first time for this region, we have forced a hydrological model with output from an ice-dynamical glacier model and 16 downscaled climate projections, under RCP4.5 and RCP8.5 scenarios. By 2100, under RCP8.5 (high-emission scenario), the winter discharge of the Arve river remains low but is expected to increase by 80% when compared to the beginning of the century. By contrast, the summer season, currently the most important discharge period, will be marked by a runoff decrease of approximately 40%. These changes are almost similar according to a scenario with a lower warming (RCP4.5) and are mostly driven by glacier retreat. These shifts will have significant downstream impacts on water quantity and quality, affecting hydroelectric generation, agriculture, forestry, tourism and aquatic ecosystems. Global change and temperature increase are projected to lead to major environmental changes in mountainous regions 1 , including major changes in glacier extent 2,3 , permafrost 4 , ice and snow cover 5-7 , and vegetation 8,9. In the Alps, the cryosphere is crucial for water storage and for contributing to the total discharge of the main major European rivers 10. In this context, major changes are likely to occur in water discharge 11 in and near these vulnerable regions, mostly decreasing runoff in summer and modifying water resources 12. These changes could either be driven by (i) changes in precipitation 13,14 , especially in the proportion of liquid and solid water 15,16 , depending on the altitude of the 0 °C isotherm 17 ; (ii) a general warming trend, increasing snow and ice melt, hereby contributing to glacier mass loss 18 , but also increasing evapotranspiration from the surface; (iii) glacier retreat in response to such warming, ultimately leading to changes in water discharge as the available ice reserves gradually decrease 11. Of particular importance are the relative weight of each of these forcings over the coming years and decades, and their dependency on the greenhouse gas emission scenarios. These changes will have major effects on river discharges and water quality 19 , impacting hydropower generation, agriculture, forestry, tourism and aquatic ecosystems 8,20. Many studies have explored the impact of global warming and glacier retreat on glacier runoff, at global 21 and regional scales (e.g., High Mountain Asia 22,23 , the Andes 24,25 and the USA 26). In the European Alps, glacier runoff evolution under climate change has already been investigated in Switzerland 27,28 and Austria 29 for example. In the French Alps, assessments of future changes are sca...
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