21^st century global glacier evolution under CMIP6 scenarios and the role of glacier-specific observations

Abstract

Abstract. Projecting the global evolution of glaciers is crucial to quantify future sea-level rise and changes in glacier-fed rivers. Recent intercomparison efforts have shown that a large part of the uncertainties in the projected glacier evolution is driven by the glacier model itself and by the data used for initial conditions and calibration. Here, we quantify the effect that mass balance observations, one of the most crucial data sources used in glacier modelling, have on glacier projections. For this, we model the 21st century global glacier evolution under CMIP6 climate scenarios with the Global Glacier Evolution Model (GloGEM) calibrated to match glacier-specific mass balance observations, as opposed to relying on regional mass balance observations. We find that the differences in modelled 21st century glacier changes can be large at the scale of individual glaciers (up to several tens of percent), but tend to average out at regional to global scales (a few percent at most). Our study thus indicates that the added value of relying on glacier-specific observations is at the subregional and local scale, which will increasingly allow projecting the glacier-specific evolution and local impacts for every individual glacier on Earth. To increase the ensemble of models that project global glacier evolution under CMIP6 scenarios, simulations are also performed with the Open Global Glacier Model (OGGM). We project the 2015–2100 global glacier loss to vary between 25±15 % (GloGEM) and 29±14 % (OGGM) under SSP1-2.6 to 46±26 % and 54±29 % under SSP5-8.5 (ensemble median, with 95 % confidence interval). Despite some differences at the regional scale and a slightly more pronounced sensitivity to changing climatic conditions, our results agree well with the recent projections by Rounce et al. (2023), thereby projecting, for any emission scenario, a higher 21st century mass loss than the current community estimate from the second phase of the Glacier Model Intercomparison Project (GlacierMIP2).

Inter-model differences in 21st Century Glacier Runoff for the World’s Major River Basins

“…Zekollari et al, 2019). Below we describe the key features of each glacier model; for further details, we refer readers to the associated description papers cited above and to Zekollari et al (2024). Average runoff values are derived from the median of each glacier model's multi-GCM median under the SSP2-4.5 scenario.…”

“…We analyse the standard OGGM projection option of OGGMv1.6.1 (Maussion et al, 2023), which performs a bias correction between the reanalysis data and each GCM/scenario based on 2000-2019. OGGMv1.6.1 uses a dynamic spinup and calibration routine to initialise glacier states in the year 2000 and ensure that glacier mass balance during the 2000-2019 historical simulation still matches observations taking elevation feedback into account Aguayo et al (2023); Zekollari et al (2024). The spin-up ensures that glacier areas at the inventory date are matched within 1%.…”