Ice Sheets

Ice Sheets

1. AIS and GrIS

   • AIS
     • volume: 27.10^6 km3
       • 58.3m SLR
     • 4km thickness
     • at/under SL
     • little precipitation
       • +ve mass balance
   • GrIS
     • volume: 3.10^6 km3
       • 7.36m SLR
     • above SL
       • centre below
     • accumlation at centre
       • strong melt at margins
       • -ve MB at some margins
   • ice velocity
     • slow at centre
       • high in ice stream
     • high at ice shelves

2. Ice Sheet Mass Balance - methods

   • Input/output
     • = accum - runoff - IV * IT * W
     • accum - runoff = SMB
     • modelling, radar imagery, airborne radar
   • Geodetic
     • dh/dt * area * denisty
     • changes in elevation
     • altimetry, radar, assumptions
   • Gravimetric
     • G - GIA
     • temporal variation of GRACE
     • modelling, uncertainties

3. Variation in Earth Orbit

   • effect how different parts of earth receive solar radiation
   • Obliquity (41 Kya cycle)
     • earth axis tilt variation
       • 2.4 degrees
     • higher obliquity = more extreme season
       • low summer melt important for ice sheet growth (low obliquity)
   • earth orbit elliptic
     • precession (22500 Kya cycle)
       • which hemisphere has strong seasonal cycle
       • when N. Hemis has weaker cycler (aphelion) = ice sheet growth
         • Aphelion = NH cold winter, SH mild summer
         • Perihelion = NH warm summer, SH mild winter
     • Eccentricity (100,000 Kya cycle)
       • control strength of precession
       • how elliptic orbit is
   • true 100,000 year cycle?
     • precission/eccentricity doesn't strongly affect annual energy received
     • before 1m.y.a. = 40,000 yr cycle
   • cycle asymmetric = slow build up, fast collapse
     • due to positive feedback mechanisms
       • regolith sediment = faster sliding
         • thinner/flatter
           • melt faster
         • weak,bed and strong-bed ice sheet - determines cycles
           • if ice sheet thick (strong-bed) can skip cycle
             • Laurentide Ice Sheet
             • Clark and Pollard (1998)

4. MB of ice sheet

   • flow of ice in ice sheet
     • own weight drives flow
       • driving stress at location
         • force that drives flow
         • friction from shearing balances driving stress
       • upstream weight pushing more than downward stream weight pushing back
   • flow of ice sheet
     • dependent on:
       • ice temp
       • sliding rate
       • thickness
   • thinner/gentle surface slope = less ice flow

5. Ice sheet termination

   • altitude-melt feedback
     • ^ melting = decrease elevation = ^ melting
       • elevation dependence on temp
         • meltwater pulse 1A = 3cm SLR
   • ice sheet drawdown
     • thermal oscillations
       • geothermal heating
       • melting at base
         • unconsolidated base = slip plane
           • loss of heat = freeze
             • but thinned