Book 2.indb - US Climate Change Science Program

The U.S. Climate Change Science Program Chapter 23.1.3 Temporal Variationsin Earth’s GravitySince 2002, the GRACE satellite has measuredEarth’s gravity field and its temporal variability.After removing the effects of tides, atmosphericloading, spatial and temporal changes in oceanmass, etc., high-latitude data contain informationon temporal changes in the mass distributionof the ice sheets and underlying rock.Because of its high altitude, GRACE makescoarse-resolution measurements of the gravityfield and its changes with time. Consequently,resulting mass-balance estimates are also atcoarse resolution—several hundred kilometers.But this has the advantage of covering entire icesheets, which is extremely difficult using othertechniques. Consequently, GRACE estimatesinclude mass changes on the many small icecaps and isolated glaciers that surround thebig ice sheets; the former may be quite large,being strongly affected by changes in thecoastal climate. Employing a surface massconcentration (mascon) solution technique,Luthcke et al. (2006) computed multiyeartime series of GRACE-derived surface massflux for Greenland and Antarctica coastal andinterior ice sheet sub-drainage systems as wellas the Alaskan glacier systems. These masconsolutions provide important observations ofthe seasonal and interannual evolution of theEarth’s land ice.Error sources include measurement uncertainty,leakage of gravity signal from regions surroundingthe ice sheets, interannual variabilityin snowfall, melt and ice dynamics, and causesof gravity changes other than ice-sheet changes.Of these, the most serious are the gravitychanges associated with vertical bedrock motion.Velicogna and Wahr (2005) estimateda mass-balance correction of 5 ± 17 Gt a –1 forbedrock motion in Greenland, and a correctionof 173 ± 71 Gt a –1 for Antarctica (Velicognaand Wahr, 2006a), which may be underestimated(Horwath and Dietrich, 2006) or quitereasonable (Barletta et al., 2008). Althoughother geodetic data (variations in length of day,polar wander, etc.) provide constraints on masschanges at high latitudes, unique solutions arenot yet possible from these techniques. One possibleway to reduce uncertainties significantly,however, is to combine time series of gravitymeasurements with time series of elevationchanges, records of rock uplift from GPSreceivers, and records of snow accumulationfrom ice cores. Yet, this combination requiresyears to decades of data to provide a significantreduction in uncertainty (see point 4 above).3.2 Mass Balance of the Greenland andAntarctic Ice SheetsIce locked within the Greenland and Antarcticice sheets (Table 2.1) has long been consideredcomparatively immune to change, protectedby the extreme cold of the polar regions. Mostmodel results suggested that climate warmingwould result primarily in increased meltingfrom coastal regions and an overall increase insnowfall, with net 21st-century effects probablya small mass loss from Greenland and a smallgain in Antarctica, and little combined impacton sea level (Church et al., 2001). Observationsgenerally confirmed this view, althoughGreenland measurements during the 1990s(Krabill et al., 2000; Abdalati et al., 2001)began to suggest that there might also be aTable 2.1. Summary of the recent mass balance of Greenland and Antarctica. (*) 1 km 3 of ice =~0.92 Gt; ( # ) Excluding ice shelves; SLE = sea level equivalent.GreenlandAntarcticaArea (10 6 km 2 ) 1.7 12.3Volume (10 6 km 3 )* 2.9 (7.3 m SLE) 24.7 (56.6 m SLE)Total accumulation (Gt a –1 ) # 500 (1.4 mm SLE) 1850 (5.1 mm SLE)Mass balanceSince ~1990: Thickening above2,000 m, at an accelerating rate;thinning at lower elevations alsoaccelerating to cause a net lossfrom the ice sheet of perhaps>100 Gt a –1 after 2000.Since early 1990s: Slow thickeningin central regions andsouthern Antarctic Peninsula;localized thinning at acceleratingrates of glaciers in Antarctic Peninsulaand Amundsen Sea region.Probable net loss, but close tobalance.44