Glacier mass balance

Glacier mass balance, b, is a measure of the net accumulation minus ablation of snow and ice, b = a - m. The most common convention is to consider net annual mass balance, where a represents the total annual snow/ice accumulation and m is the total annual snow/ice loss through ablation. Sources of accumulation are primarily meteoric, although 'internal accumulation' occurs through the refreezing of surface meltwater that percolates into the snow or firn. Internal accumulation represents an important mass balance term above the glacier or ice-sheet equilibrium line in subpolar and polar regions, where up to 100% of summer meltwater can percolate and refreeze. Snow and ice ablation occur through iceberg calving, surface melting, sublimation and basal melting. Note that these are all represented as annual rates. Each variable is typically measured in myr-1 ice- or water-equivalent, although this can also be expressed as kgm-2yr-1.

In practice, the glacier mass balance year runs from ca. September (start of the snow accumulation season) to the following August (end of the melt season) in the Northern Hemisphere, but this varies regionally and is an idealization, because summer snowfall and non-summer snow melt occur in most glacial settings. In glacier-climate modelling studies, it is more common to consider calendar years, with net mass balance estimated from monthly or annual climate fields. In all cases, net annual mass balance is a direct measure of thinning or thickening of an ice mass. Measurements are local—at a particular location on an ice mass—and must be spatially integrated to provide an estimate of the large-scale mass balance of a glacier, icefield or ice sheet.

The following subsections discuss the different components of mass balance and the methods being used to estimate mass balance for glacier-climate modelling. I restrict the discussion to contemporary ice masses, for both present-day reconstructions and century-scale climate change scenarios. I focus on the modelling of spatially integrated mass balance, at scales ranging from individual glaciers to continental ice sheets. Mass balance models for local- or regional-scale glacier and icefield complexes com monly make use of observational climatology (networks of local monitoring stations). For global-scale icefield analyses or climate change scenarios, mass balance fields are typically estimated from modelled or reanalysed climatology (e.g. Kalnay et al., 1996). Extrapolation of climate data from monitoring stations and interpolation of climate fields from models pose similar challenges for glacier-climate modelling, discussed further in section 32.5.

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