Abstract
Abstract. Central Asian water resources largely depend on melt water generated in the Pamir and Tien Shan mountain ranges. To estimate future water availability in this region, it is necessary to use climate projections to estimate the future glacier extent and volume. In this study, we evaluate the impact of uncertainty in climate change projections on the future glacier extent in the Amu and Syr Darya river basins. To this end we use the latest climate change projections generated for the upcoming IPCC report (CMIP5) and, for comparison, projections used in the fourth IPCC assessment (CMIP3). With these projections we force a regionalized glacier mass balance model, and estimate changes in the basins' glacier extent as a function of the glacier size distribution in the basins and projected temperature and precipitation. This glacier mass balance model is specifically developed for implementation in large scale hydrological models, where the spatial resolution does not allow for simulating individual glaciers and data scarcity is an issue. Although the CMIP5 ensemble results in greater regional warming than the CMIP3 ensemble and the range in projections for temperature as well as precipitation is wider for the CMIP5 than for the CMIP3, the spread in projections of future glacier extent in Central Asia is similar for both ensembles. This is because differences in temperature rise are small during periods of maximum melt (July–September) while differences in precipitation change are small during the period of maximum accumulation (October–February). However, the model uncertainty due to parameter uncertainty is high, and has roughly the same importance as uncertainty in the climate projections. Uncertainty about the size of the decline in glacier extent remains large, making estimates of future Central Asian glacier evolution and downstream water availability uncertain.
Highlights
The fate of Asian glaciers under climate change has been the topic of a heated scientific debate (Cogley et al, 2010; Immerzeel et al, 2010; Kargel et al, 2011; Bolch et al, 2012; Sorg et al, 2012)
Though the climate projections of both ensembles mainly cluster around the same values, the new CMIP5 ensemble includes the possibility of more extreme climate change
Both CMIP3 and CMIP5 climate change projections point towards a decline of glacier extent in Central Asia
Summary
The fate of Asian glaciers under climate change has been the topic of a heated scientific debate (Cogley et al, 2010; Immerzeel et al, 2010; Kargel et al, 2011; Bolch et al, 2012; Sorg et al, 2012). Downstream water availability in several large Asian rivers is highly sensitive to changes in snow and glacier extent (Immerzeel and Bierkens, 2012), and large populations depend on the water generated upstream. This dependence is likely to increase as irrigated areas further expand under population growth (Wada et al, 2011). While mass balance modeling is rather straightforward to implement and approaches of different complexity can be used (from simple degree-day to energy-balance models for calculation of ablation), changes in glacier geometry due to ice flow are more
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