1. Polar Ice Sheets and Ice Shelves: Mass Balance, Uncertainties, and Potential Improvements Robert H Thomas…etc

3. Estimating ice-sheet mass balance: techniques Mass-budget, compares total snow accumulation with losses by ice discharge and melt runoff Repeated altimetry, to estimate volume changes Temporal changes in gravity, to infer mass changes

4. Mass budget: uncertainties AntarcticaGreenland (Gt a -1 ) Snowfall+ 130 (7%)+ 25 (5%) Ice flow+ 93 (5%)+ 25 (5%) Melt runoffVery small+ 30 (10%) Mass balance (mm/yr SLE) + 160 (+ 0.5) + 46 (+ 0.15)

5. Altimetry: uncertainties SRALTICESatATM Topography effects Laser pointing, scattering, saturation Changing dielectrics Spatial/temporal cover (clouds) Basal uplift Changing snow-densification rates

6. GRACE: uncertainties Measurement errors Spill-over effects AtmosphereBasal uplift GreenlandSmall (few Gt a -1 ) Very Small+ ~12 Gt a -1 Antarctica *V & W # R et al + ~13 Gt a -1 + ~9 Gt a -1 + ~72 Gt a -1* + ~25 Gt a -1 # Low-resolution results refer to entire ice sheets, but are seriously limited by short temporal coverage

8. Greenland Ice Sheet: rates of surface-elevation change (dS/dt) Above (red), and below (blue) ~ 2000-m elevation

9. Greenland Mass Balance

12. ERS wavefront over Jakobshavn

17. Antarctic Surface Elevation Change PIG AP LIS RIS FRIS

18. Crane Glacier after Larsen breakup airborne lidar profiles measured in 2002 & 2004

20. Recent mass-balance of Greenland and Antarctica. GreenlandAntarctica Area (M km 2 ) 1.712.3 Volume (M km 3 )* 2.9 (7 m SLE) 24.7 (57 m SLE) Total accumuln. (Gt a -1 ) # 500 (1.4 mm SLE) 1850 (5.1 mm SLE) Mass BalanceSince ~1990: Thickening above 2000 m, at an accelerating rate; thinning at lower elevations, also accelerating to cause a net loss from the ice sheet of perhaps > 100 Gt a -1 after 2000. Since early 1990s: slow thickening in central regions and southern Antarctic Peninsula; localized thinning at accelerating rates of glaciers in Antarctic Peninsula and Amundsen Sea region. Probable net loss, but close to balance.

21. Causes opost-glacial memory ovariability/trends in snowfall/melting ochanges in glacier velocities -ice shelves -basal lubrication -????

23. Sea-level change and the cryosphere, June 2006 QUESTIONACTION -How long will glacier acceleration continue, resulting from existing perturbations? - Extend time series of observations - Model development and comparison with truth - Relative importance of ice-shelf weakening and melt-water lubrication? - As above plus field measurements - East Antarctica: is this also vulnerable to “perturbation weakening”? - Remote-sensing surveys, and time series to identify current status - Why are ice shelves thinning? - Ocean measurements near/beneath ice shelves - Model development and comparison with truth - Possible causes for ice-shelf breakup?- Field observations and modeling - Importance of oscillations and trends in surface accumulation? - Ice cores, accumulation radar, and modeling - Quantify the impact of changes in summer melt rates? - AWS, field work including percolation, and modeling - Relationship to the “Big Picture”: the link to prescribed scenarios of future climate change - Climate-warming scenarios must include parameters important to the ice sheets: accumulation; summer temperatures; ocean conditions near ice sheets and beneath ice shelves