Inter-model differences in 21st century glacier runoff for the world's major river basins

Projected glacier change has important downstream consequences, including sea level rise, changing freshwater supply, and loss of important cultural sites. While the glacier contribution to global sea level rise and associated uncertainties have been quantified in model intercomparison studies, comparatively less focus has been directed towards the interannual changes in runoff caused by glacier recession. The observed effect of glacier runoff on basin-level water availability makes simulated future runoff a particularly consequential target for analysis. In this study, we compare century-scale runoff simulated by three global glacier evolution models. Aggregating annual glacier runoff contributions to 75 globally distributed major river basins, we find that the three models agree closely in some basins but differ dramatically (up to a factor of 3.8) in others. However, when we analyze century-scale runoff changes relative to a glacier model's historical runoff baseline, annual runoff projections are much more consistent across glacier models. Glacier models project broadly consistent relative changes in seasonal runoff supply, with some differences across climatic regions. Estimates of the year of peak water are more consistent across glacier models (when driven by a climate model ensemble) than across individual climate forcings within a single glacier model. We identify the glacier models' different approaches to modifying precipitation forcing as the dominant source of inter-model differences in projected runoff. Our findings highlight the comparative roles of glacier evolution model, global climate model forcing, and emissions scenario as important sources of uncertainty across different metrics of projected glacier runoff. For example, inter-glacier-model uncertainty in absolute annual runoff is large, but the year of projected peak water has much greater inter-climate-model uncertainty. We recommend that users pay particular attention to how a selected glacier model parameterizes and calibrates the glacier climatic mass balance in glaciohydrological modeling efforts.