Book 2.indb - US Climate Change Science Program

Abrupt Climate Changetime. Degree-day models are advantageous forthe purposes of estimating worldwide glaciermelt, since the main inputs of temperature andprecipitation are readily available in griddedform from Atmosphere-Ocean General CirculationModels (AOGCMs).Techniques for measuring total mass balanceinclude:• the mass-budget approach, comparing gainsby surface and internal accumulation withlosses by ice discharge, sublimation, andmeltwater runoff;• repeated altimetry, or equivalently leveling orphotogrammetry, to measure height changes,from which mass changes are inferred;• satellite measurements of temporal changesin gravity, to infer mass changes directly.All three techniques can be applied to thelarge ice sheets; most studies of ice caps andglaciers are annual (or seasonal) mass-budgetmeasurements, with recent studies also usingmulti-annual laser and radar altimetry. Thethird technique is applied only to large, heavilyglaciated regions such as Alaska, Patagonia,Greenland, and Antarctica. Here, we summarizewhat is known about total mass balance,to assess the merits and limitations of differentapproaches to its measurement and to identifypossible improvements that could be made overthe next few years.3.1.1 Mass BalanceSnow accumulation is estimated from stakemeasurements, annual layering in ice cores,sometimes with interpolation using satellite microwavemeasurements (Arthern et al., 2006),or meteorological information (Giovinetto andZwally, 2000) or shallow radar sounding (TheISMASS Committee, 2004), or from regionalatmospheric climate modeling (e.g., van de Berget al., 2006; Bromwich et al., 2004). The stateof the art in estimating snow accumulation forperiods of up to a decade is rapidly becomingthe latter, with surface data being used mostlyfor validation, not to drive the models. This isnot surprising given the immensity of largeice sheets and the difficulty of obtaining appropriatespatial and temporal sampling of snowaccumulation at the large scale by field parties,especially in Antarctica.Ice discharge is the product of velocity andthickness, with velocities measured in situ orremotely, preferably near the grounding line,where velocity is almost depth independent.Thickness is measured by airborne radar,seismically, or from measured surface elevationsassuming hydrostatic equilibrium, forfloating ice near grounding lines. Velocities aremeasured by ground-based survey, photogrammetry,or with satellite sensors; the latter aremostly imaging radars operating interferometrically.Grounding lines are poorly known from insitu measurement or visible-band imagery butcan be mapped very accurately with satelliteinterferometric imaging radars.Meltwater runoff (large on glaciers and icecaps, and near the Greenland coast and partsof the Antarctic Peninsula, but small or zeroelsewhere) is traditionally inferred from stakemeasurements but more and more from regionalatmospheric climate models validated with surfaceobservations where available (e.g., Hannaet al., 2005; Box et al., 2006). The typicallysmall mass loss by melting beneath groundedice is also estimated from models.Mass-budget calculations involve the comparisonof two very large numbers, and small errorsin either can result in large errors in estimatedtotal mass balance. For example, total accumulationover Antarctica, excluding ice shelves,is about 1,850 Gt a –1 (Vaughan et al., 1999;Arthern et al., 2006; van de Berg et al., 2006),and 500 Gt a –1 over Greenland (Bales et al.,2001). Associated errors are difficult to assessbecause of high temporal and spatial variability,but they are probably about ± 5% (20–25 Gt a –1 )for Greenland. The errors for Antarctica(Rignot, 2006) range from 5% in dry interiorbasins to 20% in wet coastal basins. The totalaccumulation for Antarctica is approximately1,900 Gt a –1 (ranged from 1,811 to 2,076 Gt a –1between 1999–2006; van de Berg et al., 2006),with an overall uncertainty of 6% or 114 Gt a –1 ,derived from 93% dry interior region and 7 %wet coastal region, using uncertainties of 5%,and 20%, respectively.Broad interferometric SAR (InSAR) coverageand progressively improved estimates ofgrounding-line ice thickness have substantially41