Ice sheet observationsmodelling: status and requirements

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Ice sheet observations/modelling status and
requirements

• Ian Allison

Australian Antarctic Division and Antarctic
Climate and Ecosystems Cooperative Research
Centre, Australia
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Snow and ice on the earth Seasonal minimum and
maximum of snow, sea ice and seasonally frozen
ground, and the annual mean for the other
components.
361 km3 of ice is equivalent to 1 mm SLR
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Bentley, Thomas, Velicogna (2007) based on
material from K. Steffen
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Antarctic balance flow
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Past global sea level
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Recent global sea level
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AR4 Ice contribution to sea level change
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Influxoutflux classical traverse survey
Balance Fluxes
After Budd and Warner, 1996
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Satellite measurement of ISMB

• Three methods used to assess health of an ice
  sheet by satellite
• Direct measurement of
• change in elevation with time
• (using altimeters)
• Measurement of mass change
• with time (using GRACE)
• Estimation of mass fluxes
• (using SAR-derived velocities
• ice thickness from
• altimetry)

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Antarctic estimates from GRACE
Total monthly Antarctic mass change
Ramillien et al., 2006 -40 32 km3/yr 0.11
mm/yr
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Mass loss from flux estimates
Rignot et al. ( 2008)
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Recent Antarctic balance estimates
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Increased Greenland melt and runoff
Steffen and Huff
Hanna et al., 2008
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Greenland estimates from GRACE
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Recent Greenland balance estimates
IPCC warming-induced ice-sheet growth next
century
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Uncertainty in estimations

• Glacial isostatic adjustment separating ice
  change from total change
• Coarse spatial resolution of GRACE estimates.
• Bias between radar altimetry and laser
  altimetry.
• Errors in converting volume (elevation) change to
  mass change with changing firn density.
• Assumptions about column average ice velocity or
  density.

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Projected sea level rise
IPCC Third Assessment Report (TAR) 0.08 0.88 m
spread of SLR projection for a range of emission
scenarios
IPCC AR4 projection with allowance for ice
dynamic increase adds 0.1-0.2 m SLR and larger
values cannot be excluded
IPCC AR4 model projections (Chapter 10) 0.19
0.58 m SLR for the same range of emission
scenarios
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Uncertainty in projections

• Grounding line instability
• Ice shelf/ice tongue dissolution and accelerated
  discharge of grounded ice.
• Basal lubrication from surface melt drainage
• Dynamic impact of subglacial water

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Glacier speedup after ice shelf collapse
Glaciers lost ice shelf
Glacier lost ice shelf
Ice Velocity December, 2003 October,
2003 December, 2002 BREAKUP FEB. 2002 October,
2000 January, 1996
Glacier still has ice shelf
Rignot et al., 2004
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Greenlands supraglacial lakes
Fricker et al., unpub.
79N Glacier
NE Greenland Ice Stream
Photo Roger Braithwaite
20m drop in lake level
Background image of Mosaic of Greenland from NSIDC
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Antarctic subglacial water

• First lake discovered in the 1960s by radio
  echo sounding
• By 2005, 145 documented Antarctic subglacial
  lakes
• Until 2005, subglacial water flow was thought
  to be a steady trickle
• Recent discoveries (since 2005) have revealed
• major fluctuations in flow
• lakes under ice streams

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A limit to ice sheet dynamic response
5 m slr within 100 years from Greenland dynamics
alone requires outlet glacier speed of 72 -125
km/yr

• Present average is 1.2 km/yr
• Fastest ever is 12.6 km/yr

1.5 m slr contribution from West Antarctica (no
Ross or Filchner ice shelf collapse) requires
discharge speed of 54 km/yr
Likely max. SLR rise by 2100 is 0.8 m to 2 m
Pfeffer, Harper, ONeel (2008)
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Hellheim Glacier, Greenland. Nick, Vieli, Howat
and Joughin (2009)
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Improvements to ice sheet models 1

• A higher-order flow model, with a unified
  treatment of vertical shear stresses and
  horizontal-plane stresses.
• Improved models of basal sliding over hard and
  soft beds, including explicit treatment of
  surface, englacial, and subglacial hydrology
• Well-validated parameterization of melting
  and refreezing beneath ice shelves
• Accurate, semi-empirical law for calving
• Accurate, numerically robust treatment of
  grounding-line migration on a fixed grid

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Improvements to ice sheet models 2

• 5-km resolution
• polythermal ice
• crystal growth and anisotropy
• detailed bedrock topography and basal
  characteristics
• distribution of geothermal heat flux

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• Wilkins Ice Shelf Collapse
• 13,680 square km max thickness 200- 250 m
• initial calving of a long, thin iceberg (41 x
  2.5 km)
• runaway disintegration of 405 sq km
• now pinned only by a beam of ice 6 km wide
• much of the remaining 13,200 sq km may
  disintegrate in 2008-09?

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Amery Ice Shelf Craven, Allison, Fricker and
Warner, in press
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Conclusions

• Recent remote sensing provides the first
  realistic assessments that GIS and AIS are
  contributing to sea level rise.
• Errors in assessments will decrease as the period
  over which they are made increases.
• SLR 1993-2006 close to upper limit of IPCC
  assessments.
• IPCC projection do not account for ice sheet
  dynamic changes. Better ice sheet models are
  required.
• Inadequacies of projections are one-sided. Ice
  sheet instability can only increase SLR.
• With only slight further warming, GIS surface
  melt will increase more rapidly than
  precipitation and lead to irrevocable decay of
  the ice sheet over a few thousands of years.
  (More quickly with dynamic instability)