Modeling past and future variations of the Antarctic Ice Sheet

37 Pages·2016·4.66 MB·English

by  David Pollard2

Modeling past and future variations of the
Antarctic Ice Sheet

David Pollard1 and Robert DeConto2

1Pennsylvania State Univ. 2Univ. of Massachusetts
NCAR CESM/LIWG meeting, June 22, 2016
Outline
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?????? ??????
time
~3 Ma +5 kyr
modern
20 ka (~LGM)
(mid Pliocene) (future)

(1) Calibration vs. last 20,000 years with Large Ensembles

(2) Add drastic warm-climate mechanisms to capture Pliocene sea-level rise

(3) Apply to future 5000 years, for RCP 2.6 to 8.5 scenarios
Introduction: Steps in Large-Ensemble modeling
Statistical LE
Run model from RRReeepppeeeaaattt fffooorrr 666222555 rrruuunnnsss,,, AAAggggggrrreeegggaaattteee ssscccooorrreee fffooorrr
Staatnisatliycsails
30 kyr BP to sssyyysssttteeemmmaaatttiiicccaaallllllyyy vvvaaarrryyyiiinnnggg eeeaaaccchhh rrruuunnn vvvsss... mmmooodddeeerrrnnn
present mmmooodddeeelll pppaaarrraaammmeeettteeerrrsss aaannnddd gggeeeooolllooogggiiiccc dddaaatttaaa L•EP raonbaabliylitsy is
densities of model
parameter values
• Envelopes of
future ice retreat
and sea-level rise
Aggregate score:
S = (S S S S S S S ) 1/7
1 2 3 4 5 6 7
× 625 S1. Modern area of grounded ice
S . Modern grounded-ice
2
thickness
S . Past 1-D grounding-line
3
distance along PIG, Ross,
Ronne centerlines Reconstructions of grounding lines
by RAISED Consortium (QSR, 2014)
S . Past 2-D grounding line
4
positions
Ice sheet model:
S . Past Relative Sea Level
(Pollard et al., 2012) 5
records
• 20 km grid.
• SSA+SIA dynamics, grounding S6. Past elevation-age data
line flux (Schoof, 2007).
S . Modern uplift rates
• Atmospheric forcing: uniform 7
shifts.of modern climatology Data compilations by Briggs and
• Oceanic forcing: Liu et al., Tarasov (2013), Whitehouse et al.
Science, 2009, CSM 22 kyrs. (2012).
Equivalent sea-level envelopes Probability density maps
Probability of grounded ice (0 to 1)
Showing all 625 runs (grey: score S = zero)
= Σ(S with grounded ice at x,y,t) / Σ(S, i =1 to 625)
i i
20 ka 15 ka 14 ka
Score-weighted mean 13 ka 12 ka 11 ka
and1-σ deviations
10 ka 5 ka 0 ka
Pollard et al., GMD, 2016.
Other recent Antarctic ~LE modeling:
Whitehouse et al., 2012a,b.
Briggs and Tarasov, 2013-14.
Golledge et al., 2014.
Maris et al., 2014.
Black lines: grounding line reconstructions,
RAISED Consortium, QSR, 2014
4 model parameters calibrated
Large Ensemble, 5 values for each of 4 parameters (625 runs)

CALV
ice
OCFAC
TAUAST ocean
bed
OCFAC:
CSHELF:
Pollard et al., GMD,
Chang et al., J. Amer. Stat. Assoc., 2016.
2016. Simple score-
State-of-the-art Bayesian approach:
averaging.
emulation, likelihood functions, MCMC.
TAUAST:
CALV:
Outline
??????
?????? ??????
time
~3 Ma
20 ka (~LGM) modern +5 kyr
(mid Pliocene)
(future)

(1) Calibration vs. last 20,000 years with Large Ensembles

(2) Add drastic warm-climate mechanisms to capture Pliocene sea-level rise

(3) Apply to future 5000 years, for RCP 2.6 to 8.5 scenarios
Adding hydrofracture & cliff failure, to produce large Pliocene sea-level rise
Rovere et al., EPSL, 2014: mid-Pliocene shore-line
elevations > ~20 m. But note uncertainty due to GIA and
tectonic uplift (Austermann et al., Geol., 2015)
••
GGeeoollooggiicc eevviiddeennccee ooff PPlliioocceennee,, MMiioocceennee eeppiissooddeess uupp ttoo ~~2200mm SSLLRR
-- MMaayy bbee lleessss,, dduuee ttoo GGIIAA aanndd ddyynnaammiicc ttooppooggrraapphhyy eeffffeeccttss ((llaatteerr))

••
EEaarrlliieerr mmooddeellss ssiimmuullaattee MMaarriinnee IIccee SShheeeett IInnssttaabbiilliittyy oonnllyy iinn WWeesstt
AAnnttaarrccttiiccaa dduurriinngg wwaarrmm cclliimmaatteess,, <~ 5~ 5m m e qeuqivu.i vs.e sae-ale-vleevl erils reis e

••
22 ddrraassttiicc rreettrreeaatt mmeecchhaanniissmmss aaddddeedd ttoo tthhiiss mmooddeell
-- pprroodduucceess mmiidd--PPlliioocceennee rreettrreeaatt iinn EEaasstt AAnnttaarrccttiicc bbaassiinnss,, ++1177 mm SSLLRR

old model +3m ESL new model +17m SLR
Adding hydrofracture & cliff failure, to produce large Pliocene sea-level rise
Two mechanisms to produce drastic EAIS basin retreat in warm Pliocene climates:
(1) Surface melt and hydrofracture, (2) Large tidewater cliff failure
LLaarrsseenn BB bbrreeaakkuupp,, 22000022
~1000 km ((SSccaammbbooss eett aall.. 22000033))
(1)
Hydro-
fracture melt-enhanced calving
> 1000 m
~
(2) Terminus of Helheim Glacier, E. Greenland.
Cliff height above waterline is nearly ~ 100m.
Cliff
Photo: Knut Christianson, U. Washington.
~100 m
failure
<100 m
(3)
Maximum
retreat
Adding hydrofracture & cliff failure, to produce large Pliocene sea-level rise
old model +3m ESL new model +17m SLR
Pollard et al., EPSL, 2015
Outline
??????
?????? ??????
time
~3 Ma
20 ka (~LGM) modern +5 kyr
(mid Pliocene)
(future)

(1) Calibration vs. last 20,000 years with Large Ensembles

(2) Add drastic warm-climate mechanisms to capture Pliocene sea-level rise

(3) Apply to future 5000 years, for RCP 2.6 to 8.5 scenarios