Peak water from glaciers: advances and challenges in a global perspective

Mountain glaciers show a high sensitivity to changes in climate forcing. In a global perspective, their anticipated retreat will pose far-reaching challenges to the management of fresh water resources and will raise sea levels significantly within only a few decades. Different model frameworks have been applied to simulate melt water contributions of glaciers outside the two ice sheets for the recent IPCC report. However, these models depend on strongly simplified, and often empirical descriptions of the driving processes hampering the reliability of the results. Thus, a transition from the physically-based mass balance-ice flow models developed for single glaciers to the application at the global scale is urgently needed. The challenges are manifold but can be tackled with the new data sets, methods and process-understanding that have emerged during the last years.

Here, we present a novel glacier model for calculating the response of surface mass balance and 3D glacier geometry for each individual glacier around the globe. Our approach accounts for feedbacks due to glacier retreat and includes models for mass loss due to frontal ablation and refreezing of water in the snow/firn. This allows the calculation of the components of proglacial runoff for each individual glacier in a process-based way. The current surface geometry and thickness distribution for each of the world’s roughly 200’000 glaciers is extracted from the Randolph Glacier Inventory v3.3 and terrain models. Our simulations are driven with 14 Global Circulation Models from the CMIP5 project using the RCP4.5, RCP8.5 and RCP2.6 scenarios.

We focus on the timing of peak water from glacierized catchments in all climatic regions of the earth and the corresponding importance of these regime changes on hydrological stress. Peak water represents a crucial tipping point for sustained water supply even for regions with only a minor glacier coverage, and is relevant to the dynamics of sea level rise. The maximum rate of water release from glacial storage is subject to a high spatio-temporal variability depending on the glacier-specific geometry and its transient response to climatic change.