Polar%20Ice%20Sheets%20and%20Ice%20Shelves:%20Mass%20Balance,%20Uncertainties,%20and%20Potential%20Improvements%20Robert%20H%20Thomas

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Polar Ice Sheets and Ice ShelvesMass Balance,
Uncertainties, and Potential ImprovementsRobert
H Thomasetc
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Estimating ice-sheet mass balancetechniques

• 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

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Mass budget uncertainties
Antarctica Greenland
(Gt a-1) (Gt a-1)
Snowfall 130 (7) 25 (5)
Ice flow 93 (5) 25 (5)
Melt runoff Very small 30 (10)
Mass balance (mm/yr SLE) 160 ( 0.5) 46 ( 0.15)

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Altimetry uncertainties
SRALT ICESat ATM
Topography effects Laser pointing, scattering, saturation
Changing dielectrics Spatial/temporal cover (clouds) Spatial/temporal cover (clouds)
Basal uplift Basal uplift Basal uplift
Changing snow-densification rates Changing snow-densification rates Changing snow-densification rates

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GRACE uncertainties
Measurement errors Spill-over effects Atmosphere Basal uplift
Greenland Small (few Gt a-1) Small (few Gt a-1) Very Small 12 Gt a-1
Antarctica V W R et al 13 Gt a-1 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 Low-resolution results refer to entire ice sheets, but are seriously limited by short temporal coverage Low-resolution results refer to entire ice sheets, but are seriously limited by short temporal coverage Low-resolution results refer to entire ice sheets, but are seriously limited by short temporal coverage Low-resolution results refer to entire ice sheets, but are seriously limited by short temporal coverage

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Greenland Ice Sheet rates of surface-elevation
change (dS/dt)Above (red), and below (blue)
2000-m elevation
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Greenland Mass Balance
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ERS wavefront over Jakobshavn
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Antarctic Surface Elevation Change
LIS
FRIS
AP
PIG
RIS
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Crane Glacier after Larsen breakup

• airborne lidar profiles measured in 2002 2004

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Recent mass-balance of Greenland and Antarctica.
Greenland Antarctica
Area (M km2) 1.7 12.3
Volume (M km3) 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 Balance Since 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 gt 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.
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Causeso post-glacial memoryo variability/tren
ds in snowfall/meltingo changes in glacier
velocities - ice shelves - basal
lubrication - ????

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Sea-level change and the cryosphere, June 2006
QUESTION ACTION
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