Global-scale 21st-century glacier mass change projections from six published global glacier models are systematically compared as part of the Glacier Model Intercomparison Project. In total 214 projections of annual glacier mass and area forced by 25 General Circulation Models (GCMs) and four Representative Concentration Pathways (RCP) emission scenarios and aggregated into 19 glacier regions are considered. Global mass loss of all glaciers (outside the Antarctic and Greenland ice sheets) by 2100 relative to 2015 averaged over all model runs varies from 18 ± 7% (RCP2.6) to 36 ± 11% (RCP8.5) corresponding to 94 ± 25 and 200 ± 44 mm sea-level equivalent (SLE), respectively. Regional relative mass changes by 2100 correlate linearly with relative area changes. For RCP8.5 three models project global rates of mass loss (multi-GCM means) of >3 mm SLE per year towards the end of the century. Projections vary considerably between regions, and also among the glacier models. Global glacier mass changes per degree global air temperature rise tend to increase with more pronounced warming indicating that mass-balance sensitivities to temperature change are not constant. Differences in glacier mass projections among the models are attributed to differences in model physics, calibration and downscaling procedures, initial ice volumes and varying ensembles of forcing GCMs.
The automatic weather station (AWS) on the snout of the Vadret da Morteratsch, Switzerland, has delivered a unique 12 year meteorological dataset from the ablation zone of a temperate glacier. This dataset can be used to study multi-annual trends in the character of the surface energy budget. Since 2003 there has been a substantial darkening of the glacier tongue due to the accumulation of mineral and biogenic dust. The typical surface albedo in summer has dropped from 0.32 to 0.15. We have analysed the implications of the lowered albedo for the energy balance and the annual ablation. For the 4 year period 2003–06, the decreased albedo caused an additional removal of about 3.5 m of ice. Calculations with an energy-balance model show that the same increase in ablation is obtained by keeping the ice albedo fixed to 0.32 and increasing the air temperature by 1.7 K. Our analysis confirms that for retreating glaciers the deposition of dust from exposed side moraines on the glacier surface constitutes an important feedback mechanism. The mineral dust stimulates the growth of algae, lowers the surface albedo, enhances the melt rates, and thereby facilitates the further retreat of the glacier snout.
We present a record of almost six years of data (2000–2006) from an automatic weather station (AWS) in the ablation zone of Midtdalsbreen, a glacier in southern Norway. Measured incoming longwave radiation is used to estimate cloudiness, revealing that high cloud fractions occur almost 50% of the time in all seasons. Measured wind speeds and humidity are higher for cloudy conditions, especially in winter. Net solar radiation dominates the surface energy balance in summer, contributing on average 75% of the melt energy. The turbulent fluxes supply 35% of the melt energy while net longwave radiation and the subsurface heat flux are energy sinks of 8% and 2%, respectively. Although the melt rate is generally larger under clear skies, almost 60% of the melt occurs under cloudy skies, a consequence of the prevailing cloudy conditions. Interannual variability in the total melt is found to be equally determined by variations in the date of ice reappearance and differences in the meteorological conditions during melt. Comparing the results for Midtdalsbreen with measurements from an AWS on Morteratschgletscher, Switzerland reveals that the larger ice ablation on Morteratschgletscher primarily results from an earlier start of the melt season and larger net solar radiation. The energy balance model used in this study is found to be more sensitive to changes in the stability correction than to an order‐of‐magnitude change in the roughness length for momentum.
Abstract. Glaciers respond to mass balance changes by adjusting their surface elevation and area. These properties in their turn affect the local and area-averaged mass balance. To incorporate this interdependence in the response of glaciers to climate change, models should include an interactive scheme coupling mass balance and ice dynamics. In this study, a spatially distributed mass balance model, comprising surface energy balance calculations, was coupled to a vertically integrated ice-flow model based on the shallow ice approximation. The coupled model was applied to the ice cap Hardangerjøkulen in southern Norway. The available glacio-meteorological records, mass balance and glacier length change measurements were utilized for model calibration and validation. Forced with meteorological data from nearby synoptic weather stations, the coupled model realistically simulated the observed mass balance and glacier length changes during the 20th century. The mean climate for the period 1961-1990, computed from local meteorological data, was used as a basis to prescribe climate projections for the 21st century at Hardangerjøkulen. For a linear temperature increase of 3 • C from 1961-1990 to 2071-2100, the modelled net mass balance soon becomes negative at all altitudes and Hardangerjøkulen disappears around the year 2100. The projected changes in the other meteorological variables could at most partly compensate for the effect of the projected warming.
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