Glacier projections sensitivity to temperature-index model and climate downscaling parameter calibration choices

A recent large-scale glacier model intercomparison revealed a strong influence of model design choice on glacier projections. Here we examine the influence of various temperature-index mass-balance models and calibration options. With the Open Global Glacier Model (OGGM) framework, we compare the performance and projections of model options such as the use of surface-type dependent degree-day factors as well as varying temporal climate resolution (daily, monthly) and downscaling strategies (temperature lapse rates, temperature and precipitation correction). We focus on 88 glaciers with long term observations of mass-balance profiles and seasonal mass-balance, allowing us to assess the added value of using multiple mass-balance statistics in the calibration process. We find that using interannual mass-balance variability to calibrate otherwise fixed parameters generally leads to an improved representation of the mass-balance gradient, which in turn is a crucial explanatory variable for future glacier evolution. Therefore, we also find a strong influence of the calibrated temperature lapse rates on future glacier volume. Using different degree-day factors for snow, firn, and ice leads to nonlinear sensitivities, where future glacier loss depends on how the accumulation area changes compared to the calibration period. Our study illustrates the strong impact of temperature-index model choice on projected glacier volume and runoff. But it cannot clearly demonstrate the added value of additional model complexity due to the lack of independent observations.