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Response of the Antarctic ice sheet to increased ice-shelf oceanic melting

Response of the Antarctic ice sheet to increased ice-shelf oceanic melting. David Pollard Pennsylvania State University Robert DeConto University of Massachusetts. GFDL Ocean Climate Model Development Meeting, October 28-30, 2009. Ice velocities across the grounding line…

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Response of the Antarctic ice sheet to increased ice-shelf oceanic melting

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  1. Response of the Antarctic ice sheet to increased ice-shelf oceanic melting David Pollard Pennsylvania State University Robert DeConto University of Massachusetts GFDL Ocean Climate Model Development Meeting, October 28-30, 2009

  2. Ice velocities across the grounding line… - are slowed by ice-shelf buttressing - increase strongly with g.l. depth • So, if ice-shelf buttressing is lost, and the bed deepens upstream...then possibility of runaway retreat ! ice interior grounding line retreat Weertman (1976, Nature); Mercer (1978, Nature); Schoof (2007, JGR) B A ? B in 100 years? A B in 1000+ years? B Grounding-Line Retreat Instability Ronne Ice Shelf Pine Island & Thwaites glaciers Ross Ice Shelf

  3. Outline 1. 3-D ice sheet-shelf model 2. Last 5 Myr, vs. ANDRILL record 3. Same model run into future O(103) years -Prescribed sub-ice-shelf oceanic melting WAIS retreat? - Other variations added

  4. Features in ice sheet-shelf model qg qg ice ocean bed Predicts ice thickness, temperature, bedrock elevation. 40 km grid size. Follows standard model lineage… PLUS: • Hybrid combination of the 2 scaled equations for shearing (grounded interior) and stretching (floating/stream) ice flow • C. Schoof’s (2007,JGR) parameterization of flux across grounding lines (qg). Allows realistic grounding-line migration and ice-shelf buttressing • Simple parameterizations of forcings: - sea level - surface mass balance and temperature - sub-ice-shelf oceanic melt rate

  5. 2. Last 5 million years

  6. ANtarctic geological DRILLing program • WAIS has been dynamic, advancing and retreating on ~40 kyr obliquity cycles • Long-term trends: • - Modern glacials, 1 to 0 Ma • - Cooling transition, 3 to 1.5 Ma • - Warmest in early Pliocene, • extended interglacials, 5 to • 3.5 Ma Main Results Depth (mbsf) Approx. Age (Ma) 0 1 2 ANDRILL MIS 3 4 5 MIS core, upper 600m (last ~5 Myr) Naish et al., Nature, 2009 photo: C. Millan Yellow = diatomite (open ocean) Green = glacial till (grounded ice) Grey = mud/silt/sand (ice nearby) Orange = volcanic

  7. FORCING: benthic 18O Glacial maxima  Lisiecki and Raymo, Paleoceanogr., 2005 MODEL OUTPUT: total Antarctic ice volume pink = floating ice shelves ~Modern MODEL OUTPUT: total Antarctic ice volume  Pollard and DeConto, Nature, 2009 Pleistocene WAIS collapses Super-Interglacials Naish et al., Nature, 2009 ANDRILL MIS core:  Closest model grid point and ANDRILL core agree: • ~5 to 3 Ma: Long periods with open ocean • ~3 to 1 Ma: Cooling trend • ~1 to 0 Ma: Current glacial cycles Model Antarctic ice volume, last 5 million years

  8. 3. Next few thousand years

  9. Sub-ice-shelf oceanic melt – most important future forcing Sources of sub-ice-shelf water depend on larger-scale circulation –(Circumpolar Deep Water CDW, and High Salinity Shelf Water HSSW) Circulation under ice shelves is an active research area with Regional Ocean Models – (Dinniman, Galton-Fenzi, Hollands, Jenkins, Makinson, Little, Walker, et al!) Beckmann and Goosse, Ocn. Model., 2003

  10.  2 m y-1 x 0.4 m y-1/ oC A-OGCM future warming x “empirical” coefficient: Modern ocean melt rates = [.1,5,5], where .1 = protected shelf (most important) 5 = exposed shelf 5 = deep ocean areas sub-ice melt increase per nearby ocean T, for large shelves (Beckmann and Goose, 2003) Overpeck et al., Science, 2006. Ocean T, 100m and 200 m, 2100 and 2130 AD For now, we simply prescribe sudden increases in oceanic melt rates, from modern [.1,5,5] m/yr …but to what?

  11. Nested domain over West Antarctica Continental Antarctica, 20 km grid Nested WAIS, 10 km grid

  12. 0 yr 300 or 3000 yr? • Given plausible future increases in sub-ice-shelf ocean melting, central WAIS is likely to collapse • Time scale of collapse depends on magnitude of sub-ice ocean melt: - 2 m/yr  ~3000 years,  m/yr  ~300 years - What future melt rates for the major ice shelves? … Will need Regional Ocean Models and GCMs for projections. … Different rates for different embayments? ~40 m/yr for PIG-THG! • Test with next generation higher-order higher-resolution ice models! Large melt rates are already observed under smaller shelves! Rignot and Jacobs, Science, 2002 Future WAIS summary

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