Summary

Glacier mass balance is monitored at numerous sites worldwide, and remote sensing methods for monitoring snow accumulation and snow/ice surface melting are rapidly improving. The latter offers the potential to monitor glacier, icefield and ice-sheet changes with unprecedented spatial coverage, although field-based calibration and validation studies are still needed at present. Mass balance processes are relatively well-understood and are amenable to modelling, with the notable exception of marine ablation through iceberg calving and basal melting beneath ice shelves. At sites where extensive meteorological and snow survey data are available, successful mass balance models have been developed. That is to say, with knowledge of snow distribution and spatial-temporal energy balance components, surface melt rates and net annual mass balance can be well quantified through theoretical energy balance modelling.

Unfortunately, detailed snowpack and energy balance data are not available for most ice masses, nor are they well-predicted by global- or regional climate models at the scale of relevance for glaciers and icefields. This has led to the development of reduced models for snow and ice melt, parameterizing melt rates as a function of cumulative positive degree-days and/or incoming solar radiation. Temperature is generally considered to be more easily downscaled or interpolated over the landscape than other terms in the local energy balance (e.g. winds, relative humidity), and (potential direct) solar radiation can be theoretically predicted at a site. Hence, these simplified parameterizations of snow and ice melt permit distributed modelling in a region where ground observations are scarce. Despite their simplicity, these models have been extremely successful at melt modelling. Degree-day based mass-balance models are presently used by most glaciolog-ical models, at scales that range from individual glaciers to continental-scale ice sheets.

Mass balance sensitivity to climate change has been explicitly simulated for the Greenland and Antarctic Ice Sheets, based on either simple sensitivity tests (e.g. a 1°C warming) or climate change scenarios generated by climate models (e.g. 2 x CO2 experiments). Similar mass-balance sensitivity tests have been carried out for a number of individual glaciers, with explicit representation of the glacier geometry. This is not possible on a global basis, but regional-scale numerical experiments have investigated the general mass balance sensitivity of most of the world's glacierized regions, based on regional climatic and topographic environments. Collectively, these numerical experiments give a broad idea of regional mass-balance regimes, glacierized regions that are expected to be most sensitive to climate change, and the global-scale mass balance impact of a given climate change scenario.

Long-term simulations of glacier and icefield response to climate change require consideration of ice dynamics. Changing glacier geometries, through both glacier dynamics and cumulative mass-balance response, introduce feed-backs that will alter future mass balance as well as local and regional climate. Numerous studies have included glacier dynamics and simple elevation-based mass balance feed-backs in simulations of glacier response to climate change, but climatic feed-backs that arise due to changing albedo and topographic forcing conditions are seldom included. There is a need for improved two-way coupling in glacier-climate modelling, where changing surface boundary conditions are fed back into the climate model. It is generally believed that most climatic feed-backs will be positive (e.g. melting increases surface albedo which begets greater melting; melting lowers the surface which increases temperatures which increase melting; decreased surface slopes will lower orographic forcing, diminishing precipitation, and so on). Hence, two-way coupling may be important in some regions, and in long-term forecasts it is expected to improve quantitative estimates of glacier response to climate change.

Coupled modelling of glacier-climate evolution is rapidly developing, driven in part by intense efforts to improve the representation of the hydrological cycle in weather forecast and climate models. Advances are being made across the board, including model physics, technical capabilities, limits of resolution and the sophistication of climate system coupling (e.g. atmosphere-ocean-land-surface schemes). With the current emphasis on improving regional-scale precipitation and climate change forecasts, via both subgrid physical parameterizations and climate downscaling strategies, glacier-climate models can be expected to evolve and improve for the foreseeable future.

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