High elevation or high latitude hydropower production (HP) strongly relies on water resources that are influenced by glacier melt and are thus highly sensitive to climate warming. Despite of the wide-spread glacier retreat since the development of HP infrastructure in the 20th century, little quantitative information is available about the role of glacier mass loss for HP. In this paper, we provide the first regional quantification for the share of Alpine hydropower production that directly relies on the waters released by glacier mass loss, i.e. on the depletion of long-term ice storage that cannot be replenished by precipitation in the coming decades. Based on the case of Switzerland (which produces over 50% of its electricity from hydropower), we show that since 1980, 3.0% to 4.0% (1.0 to 1.4 TWh yr-1 30 ) of the country-scale hydropower production was directly provided by the net glacier mass loss and that this share is likely to reduce substantially by 2040-2060. For the period 2070-2090, a production reduction of about 1.0 TWh yr-1 is anticipated. The highlighted strong regional 34 differences, both in terms of HP share from glacier mass loss and in terms of timing of production decline, emphasize the need for similar analyses in other Alpine or high latitude regions.

Introduction

Hydropower provides around 16% of the world’s total electricity [1]. In the European Union, hydropower represented 11% of the gross electricity consumption of the 28 member states in 2016 [2] and high shares of hydropower production (HP) can in particular be found in high latitude and high elevation regions [3], where part of HP relies on water resources that are temporarily stored in the form of snow and ice, and are thus particularly vulnerable to climate warming [4]. Despite the well-known inherent variability of water resources availability, fundamental energy market models [5] but also large-scale hydropower assessments usually only account for selected baseline water years [6]. The notable exception are recent continental to global 50 scale studies of HP potential [1] or of climate change impact on HP [7, 8]. While giving the broader picture at continental scales, these studies cannot yet adequately resolve the natural variability of water resources at the HP catchment scale or in mountainous regions in general [9]. Accordingly, the impact of climate warming on HP in snow- and glacier influenced 54 regions still essentially relies on individual case studies [namely from the Alps and US, see, 55 9], with some regional analyses of the effect of climate warming on snow- and glacier 56 influenced HP available for the US [10, 11]. Glacier retreat has in this context long been recognized as potential threat to HP around the world [12-14]. Studies quantifying the actual impact of glacier mass loss on HP are, however, extremely rare. Existing impact studies in the Alpine region focus on the quantification of water resources regime changes [15, 16] rather 60 than on quantifying the impact of actual glacier mass loss on hydropower production, as for example in the work of Vergara et al. [17] for the tropical Andes. They showed that glacier retreat might reduce HP for the Cañon del Pato HP plant on the Rio Santa by 570 GWh yr-1 if 63 the glacier contribution disappears.