Book 2.indb - US Climate Change Science Program

Abrupt Climate Changetion, and accuracy is affected by atmosphericconditions and particularly by laser-pointingerrors. The strongest limitation by far is thatexisting laser data are sparse compared toSRALT data.Airborne laser surveys over Greenland in1993–94 and 1998–89 yield elevation estimatesaccurate to ~10 cm along survey tracks (Krabillet al., 2002), but with large gaps between flightlines and an incomplete coverage of the glaciers.ICESat orbit-track separation is also quite largecompared to the size of a large glacier, particularlyin southern Greenland and the AntarcticPeninsula where rapid changes are occurring,and elevation errors along individual orbittracks can be large (many tens of centimeters)over sloping ice. Progressive improvement inICESat data processing is reducing these errorsand, for both airborne and ICESat surveys,most errors are independent for each flight lineor orbit track, so that estimates of dS/dt averagedover large areas containing many surveytracks are affected most by systematic ranging,pointing, or platform-position errors, totalingprobably less than 5 cm. In Greenland, suchconditions typically apply at elevations above1,500–2,000 m. dS/dt errors decrease with increasingtime interval between surveys. Nearerthe coast there are large gaps in both ICESatand airborne coverage, requiring dS/dt valuesto be supplemented by degree-day estimates ofanomalous melting (Krabill et al., 2000, 2004).This supplementation increases overall errorsand almost certainly underestimates total lossesbecause it does not take full account of dynamicthinning of unsurveyed outlet glaciers.In summary, dS/dt errors cannot be preciselyquantified for either SRALT data, because ofthe broad radar beam, limitations with surfacetopography at the coast, and time-variablepenetration, or laser data, because of sparsecoverage. The SRALT limitations discussedabove will be difficult to resolve. Laser limitationsresult primarily from poor coverage andcan be partially resolved by increasing spatialresolution.All altimetry mass-balance estimates includeadditional uncertainties in:1. The density (rho) assumed to convertthickness changes to mass changes. If changesare caused by recent changes in snowfall, theappropriate density may be as low as 300 kilogramsper cubic meter (kg m –3 ); for long-termchanges, it may be as high as 900 kg m –3 . This isof most concern for high-elevation regions withsmall dS/dt, where the simplest assumption isrho = 600 ± 300 kg m –3 . For a 1-cm a –1 thicknesschange over the million square kilometers ofGreenland above 2,000 m, uncertainty wouldbe ± 3 Gt a –1 . Rapid, sustained changes, commonlyfound near the coast, are almost certainlycaused by changes in melt rates or glacier dynamics,and for which rho is ~900 kg m –3 .2. Possible changes in near-surface snowdensity. Densification rates are sensitive tosnow temperature and wetness. Warm conditionsfavor more rapid densification (Arthernand Wingham, 1998; Li and Zwally, 2004), andmelting is likely to be followed by refreezing asice. Consequently, recent Greenland warmingprobably caused surface lowering simply fromthis effect. Corrections are inferred from largelyunvalidated models and are typically