Antarctic ice-discharge from SeaRISE response-functions

52 Pages·2013·4.61 MB·English

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©Author(s)2013.CCAttribution3.0License. Discussionsess sio 4,1117–1168,2013
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Thisdiscussionpaperis/hasbeenunderreviewforthejournalEarthSystem p
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ice-discharge from
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Projecting Antarctic ice discharge using D
response-functions
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response functions from SeaRISE s A.Levermannetal.
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ice-sheet models P
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A. Levermann , R. Winkelmann , S. Nowicki , J. L. Fastook , K. Frieler ,
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R. Greve , H. H. Hellmer , M. A. Martin , M. Mengel , A. J. Payne , D. Pollard ,
T. Sato5, R. Timmermann6, W. L. Wang3, and R. A. Bindschadler3 Dis Conclusions References
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1EarthSystemAnalysis,PotsdamInstituteforClimateImpactResearch,Potsdam,Germany ss Tables Figures
2InstituteofPhysics,PotsdamUniversity,Potsdam,Germany ion
3Code615,NASAGoddardSpaceFlightCenter,GreenbeltMD20771USA P J I
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4ComputerScience/QuaternaryInstitute,UniversityofMaine,Orono,ME04469USA pe
5InstituteofLowTemperatureScience,HokkaidoUniversity,Sapporo060-0819,Japan r J I
6Alfred-Wegener-Institute,Bremerhaven,Germany |
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7BristolGlaciologyCentre,UniversityofBristol,UniversityRoad,Clifton,BristolBS81SS,UK D
8EarthandEnvironmentalSystemsInstitute,PennsylvaniaStateUniversity,UniversityParkPA isc FullScreen/Esc
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Received:30November2013–Accepted:3December2013–Published:13December2013 D
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Correspondenceto:A.Levermann([email protected]) u
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PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion. io 4,1117–1168,2013
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ice-discharge from
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Abstract D
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The largest uncertainty in projections of future sea-level change results from the po- s
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tentially changing dynamical ice discharge from Antarctica. Basal ice-shelf melting in- io 4,1117–1168,2013
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duced by a warming ocean has been identified as a major cause for additional ice P
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5 flow across the grounding line. Here we derive dynamic ice-sheet response functions pe Antarctic
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for basal ice-shelf melting for four different Antarctic drainage regions using experi- ice-discharge from
mentsfromtheSea-levelResponsetoIceSheetEvolution(SeaRISE)intercomparison | SeaRISE
project with five different Antarctic ice-sheet models. Under the assumptions of linear- D response-functions
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responsetheoryweprojectfutureice-dischargeforeachmodel,eachregionandeach c
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10 ofthefourRepresentativeConcentrationPathways(RCP)usingoceanictemperatures ssio A.Levermannetal.
from19comprehensiveclimatemodelsoftheCoupledModelIntercomparisonProject, n
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CMIP-5, and two ocean models from the EU-project Ice2Sea. The uncertainty in the a
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climaticforcing,theoceanicresponseandtheice-modelresponseiscombinedintoan e TitlePage
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uncertainty range of future Antarctic ice-discharge induced from basal ice-shelf melt.
| Abstract Introduction
The uncertainty range we derived for the Antarctic contribution to global sea-level rise
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from 1992 to 2011 is in full agreement with the observed contribution for this period if is Conclusions References
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weusethethreeice-sheetmodelswithanexplicitrepresentationofice-shelfdynamics u
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and account for the time delayed warming of the oceanic subsurface compared with s
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the surface air temperature. The median of the additional ice-loss for the 21st cen- n
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tury (Table 6) is 0.07m (66%-range: 0.02–0.14m; 90%-range: 0.0–0.23m) of global a
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sea-level equivalent for the low-emission RCP-2.6 scenario and 0.09m (66%-range: r J I
0.04–0.21m; 90%-range: 0.01–0.37m) for the strongest RCP-8.5 if models with ex-
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plicit ice-shelf representation are applied. These results were obtained using a time Back Close
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delaybetweenthesurfacewarmingsignalandthesubsurfaceoceanicwarmingasob- is
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served in the CMIP-5 models. Without this time delay the values increase to 0.09m u
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(66%-range: 0.04–0.17m; 90%-range: 0.02–0.25m) for RCP-2.6 and 0.15m (66%- io
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range: 0.07–0.28m; 90%-range: 0.04–0.43m) for RCP-8.5. Our results are scenario P
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dependentwhichismostvisibleintheupperpercentilesofthedistribution,i.e.highest p InteractiveDiscussion
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contributions to sea level rise. All probability distributions, as provided in Fig. 12, are D
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highlyskewedtowardshighvalues.Theappliedice-sheetmodelsarecoarse-resolution c ESDD
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with limitations in the representation of grounding-line motion. However, we find the ss
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main uncertainty to be introduced by the external forcing to the ice-sheets, i.e. the cli- n
5 matic and oceanic uncertainty dominate. The scaling coefficients for the four different Pa
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drainage basins provide valuable information for further assessments of future Antarc- e Antarctic
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1 Introduction isc
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Thefutureevolutionofglobalmeanandregionalsea-levelisimportantforcoastalplan- io
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10 ning and associated adaptation measures. The Fourth Assessment Report (AR4) of P
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theIntergovernmentalPanelonClimateChange(IPCC)providedsea-levelprojections p
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explicitly excluding changes in dynamic ice-discharge, i.e. additional ice flow across
the grounding line, from both Greenland and Antarctica (Solomon et al., 2007). These | Abstract Introduction
contributions might however be significant for the next century which would influence D
15 globalmean(VandenBroekeetal.,2011)aswellasregionalsealevelchanges(Mitro- isc Conclusions References
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vica et al., 2009), especially since contribution from the ice sheets are clearly relevant s Tables Figures
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on longer time scales (Levermann et al., 2013). While the part of the ice-sheet directly io
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susceptible to warming ocean waters on Greenland is limited, marine ice sheets in P J I
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WestAntarcticaalonehavethepotentialtoelevatesealevelgloballybyseveralmeters e
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(Bamber et al., 2009). Previous projections of the Antarctic ice-sheet mass-balance
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have used fully coupled climate-ice-sheet models (Huybrechts et al., 2011; Vizcaino
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etal.,2010,e.g.).Thesesimulationsincludefeedbacksbetweentheclimateandtheice D
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sheetandtherebyprovideveryvaluableinformationespeciallyonmulti-centennialtime c FullScreen/Esc
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scale. However, on shorter decadal to centennial time scales the direct climatic forc- s
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ing is likely to dominate the ice-sheet evolution. For 21st-century projections it is thus n Printer-friendlyVersion
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appropriate to apply the output of comprehensive climate models as external forcing a
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to the ice sheet, neglecting feedbacks while possibly improving on the accuracy of the D
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forcing anomalies. Here we follow this approach. c ESDD
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In order to meet the relatively high standards that are set by climate models for the s
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oceanic thermal expansion and glacier- and ice-cap models which use the full range n
5 of state-of-the-art climate projections, it is desirable to use a set of different ice-sheet Pa
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models to increase the robustness in the projections of Antarctica’s future sea-level e Antarctic
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contribution. While changes in basal lubrications, ice-softening from surface warming ice-discharge from
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and changes in surface elevation through altered precipitation can affect dynamic ice- SeaRISE
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discharge from Antarctica, changes in basal melt underneath the ice shelves will likely response-functions
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10 be the dominant driver of changes in dynamic ice loss. u
Here we combine the dynamic response of five different Antarctic ice-sheet mod- ss A.Levermannetal.
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els to changes in basal ice-shelf melt with the full uncertainty range of future cli- n
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mate change for each of the Representative Concentration Pathways (RCP, Moss a
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et al., 2010; Meinshausen et al., 2011a) using the current simulations from the Cou- r
pled Model Intercomparison Projection, CMIP-5 (Taylor et al., 2012). To this end we
15 | Abstract Introduction
derive response functions for the five ice-sheet models from a standardized melting
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experiment(M2)fromtheSea-levelResponsetoIceSheetEvolution(SeaRISE)inter- is Conclusions References
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comparison project (Bindschadler et al., 2013). This community effort gathers a broad u
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range of structurally different ice-sheet models to perform a climate-forcing sensitivity sio
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studyforbothAntarctica(Nowickietal.,2013a)andGreenland(Nowickietal.,2013b).
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different types of climate input, namely enhanced sub-shelf melting, enhanced sliding r J I
and surface temperature increase combined with enhanced net accumulation. |
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The spread in the response of the participating models to these experiments origi-
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nates from differences in the stress-balance approximations, the treatment of ground- is
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ing line motion, the implementation of ice-shelf dynamics, the computation of the s
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surface-mass balance, and in the computational demand which sets strong limits on io
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the spin-up procedure. Our approach allows to identify the sensitivity of the response P
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of coarse-resolution ice-sheet models to changes in different types of climate-related p InteractiveDiscussion
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boundary conditions. An interpolation analysis of the results is performed in (Bind- D
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schadler et al., 2013) in order to provide a best-guess estimate of the future sea-level c ESDD
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contribution from the ice sheets. The framework of linear response theory has been s
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usedbefore,forexampletogeneralizeclimaticresponsetogreenhousegasemissions n
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2 Brief description of the ice-sheet and ocean models |
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All ice-sheet models are described in detail by Bindschadler et al. (2013) (Table 2). is
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Here we provide a brief summary referring to relevant publications from which more s A.Levermannetal.
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detailed descriptions can be obtained. All model’s applied are continental ice-sheet io
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models and coarse in resolution. As a consequence these models have deficiencies P
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in the representation of the motion of the grounding line which is a possible response p
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to enhanced ice flux and upstream thinning. This is documented in the MISMIP and
MISMIP-3d intercomparison projects (Pattyn et al., 2012, 2013). | Abstract Introduction
AIF The Anisotropic Ice-Flow model is a 3-D ice-sheet model incorporating D
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15 anisotropic ice flow and fully coupling dynamics and thermodynamics (Wang et al., cu
2012). It is a higher-order model with longitudinal and vertical shear stresses but cur- s Tables Figures
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rentlywithoutanexplicitrepresentationoficeshelves.Themodelusesthefinitediffer- ion
ence method to calculate ice-sheet geometry including isostatic bedrock adjustment, P J I
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3-D distributions of shear and longitudinal strain rates, enhancement factors which ac- e
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count for the effect of ice anisotropy, temperatures, horizontal and vertical velocities,
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shear and longitudinal stresses. The basal sliding is determined by Weertman’s slid-
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ing law based on a cubic power relation of the basal shear stress. As the model lacks D
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of ice shelves, the prescribed melt rates are applied to the ice-sheet perimeter grid- c FullScreen/Esc
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points only with a bed below sea level. The ice-sheet margin, which is equivalent to s
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the grounding line in this model, moves freely within the model grid-points and the n Printer-friendlyVersion
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groundinglineisdetectedbyhydrostaticequilibrium(i.e.thefloatingcondition)without a
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sub-grid interpolation. e
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Penn-State-3-D The Penn State 3-D ice sheet model uses a hybrid combination of D
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thescaledShallowIce(SIA)andShallowShelf(SSA)equationsforshearingandlongi- c ESDD
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tudinalstretchingflowrespectively.Thelocationofthegroundinglineisdeterminedby s
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simpleflotation,withsub-gridinterpolationasin(Gladstoneetal.,2010).Aparameter- n
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5 ization relating ice velocity across the grounding line to local ice thickness is imposed a
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as an internal boundary-layer condition, so that grounding-line migration is simulated e Antarctic
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reasonably well without the need for very high, i.e. of the order of 100m, resolution ice-discharge from
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(Schoof, 2007). Ocean melting below ice shelves and ice-shelf calving use simple pa- SeaRISE
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rameterizations,alongwithasub-gridparameterizationatthefloating-iceedge(Pollard response-functions
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10 and Deconto, 2009, 2012). The Penn-State-3-D model shows the best performance of u
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grounding line motion within the MISMIP intercomparison compared to the other mod- s
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PISM TheParallelIceSheetModel(www.pism-docs.org)usedhereisbasedonver- a
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sion stable 0.4, which incorporates the Potsdam Parallel Ice Sheet Model (PISM-PIK) r
(Winkelmann et al., 2011; Martin et al., 2011). Ice flow is approximated by a hybrid
15 | Abstract Introduction
schemeincorporatingboththeSIAandSSAapproximations(BuelerandBrown,2009).
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An enthalpy formulation (Aschwanden et al., 2012) is used for thermodynamics, and is Conclusions References
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the model employs a physical stress-boundary condition to the shelfy-stream approxi- u
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mation at ice fronts, in combination with a sub-grid interpolation (Albrecht et al., 2011) io
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and a kinematic first-order calving law (Levermann et al., 2012) at ice-shelf fronts. In
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PISM-PIK, the grounding line is not subject to any boundary conditions or flux correc- p
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tions.Itspositionisdeterminedfromiceandbedrocktopographiesineachtimestepvia r J I
the floatation criterion. The grounding line motion is thus influenced only indirectly by |
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thevelocitiesthroughtheicethicknessevolution.SincetheSSA(shallowshelfapprox-
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imation) velocities are computed non-locally and simultaneously for the shelf and for is
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thesheet,acontinuoussolutionoverthegroundinglinewithoutsingularitiesisensured s
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and buttressing effects are accounted for. The PISM model shows good performance io
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of the grounding line motion within the MISMIP intercomparisons only at significantly P
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SICOPOLIS The SImulation COde for POLythermal Ice Sheets is a three- D
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dimensional, polythermal ice sheet model that was originally created by (Greve, 1995, c ESDD
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1997) in a version for the Greenland ice sheet, and has been developed continu- s
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ouslysincethen(SatoandGreve,2012)(sicopolis.greveweb.net).Itisbasedonfinite- n
5 difference solutions of the shallow ice approximation for grounded ice (Hutter, 1983; Pa
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Morland, 1984) and the shallow shelf approximation for floating ice (Morland, 1987; e Antarctic
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MacAyeal, 1989). Special attention is paid to basal temperate layers (that is, regions ice-discharge from
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with a temperature at the pressure melting point), which are positioned by fulfilling SeaRISE
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a Stefan-type jump condition at the interface to the cold ice regions. Basal sliding is response-functions
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10 parameterized by a Weertman-type sliding law with sub-melt sliding (that allows for u
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a gradual onset of sliding as the basal temperature approaches the pressure melting s
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pointGreve,2005),andglacialisostasyisdescribedbytheelasticlithosphere/relaxing n
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asthenosphere (ELRA) approach (Le Meur and Huybrechts, 1996). The position and a
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evolution of the grounding line is determined by the floating condition. Between neigh- r
bouring grounded and floating grid points, the ice thickness is interpolated linearly,
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and the half-integer auxiliary grid point in between (on which the horizontal velocity is
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defined, Arakawa C grid) is considered as either grounded or floating depending on is Conclusions References
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whethertheinterpolatedthicknessleadstoapositivethicknessabovefloatationornot. u
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SICOPOLIS has not participated in the MISMIP experiments (Pattyn et al., 2012). The io
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performance of the ice-shelf solver was tested against the analytical solution for an
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ice-shelf ramp (Greve and Blatter, 2009, Sect. 6.4) and showed very good agreement p
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of the horizontal velocity field already at low resolution, as discussed by Sato (2012). r J I
The grounding line motion of the model has however not been systematically tested |
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yet.
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UMISM TheUniversityofMaineIceSheetModelconsistsofatime-dependentfinite- is
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element solution of the coupled mass, momentum, and energy conservation equa- s
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tions using the SIA (Fastook, 1990, 1993; Fastook and Chapman, 1989; Fastook and io
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Hughes, 1990; Fastook and Prentice, 1994) with a broad range of applications (for ex- P
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ampleFastooketal.,2012,2011)The3-Dtemperaturefield,onwhichtheflowlawice p InteractiveDiscussion
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hardnessdepends,isobtainedfroma1-Dfinite-elementsolutionoftheenergyconser- D
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vationequationateachnodewithoutdirectrepresentationofhorizontalheatadvection. c ESDD
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This thermodynamic calculation includes vertical diffusion and advection, but neglects ss
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horizontal movement of heat. Also included is internal heat generation produced by n
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5 shear with depth and sliding at the bed. Boundary conditions consist of specified sur- a
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face temperature and basal geothermal gradient. If the calculated basal temperature e Antarctic
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exceedsthepressuremeltingpoint,thebasalboundaryconditionischangedtoaspec- ice-discharge from
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ified temperature, and a basal melt rate is calculated from the amount of latent heat of SeaRISE
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fusion that must be absorbed to maintain this specified temperature. Conversely, if response-functions
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present at the bed, a similar treatment allows for the calculation of a rate of basal s
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freezing. A map-plane solution for conservation of water at the bed, whose source is n
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thebasalmeltorfreeze-onrateprovidedbythetemperaturesolution,allowsformove- a
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mentofthebasalwaterdownthehydrostaticpressuregradient(JohnsonandFastook, r
2002).Areasofbasalslidingcanbespecifiedifknown,ordeterminedinternallybythe
15 | Abstract Introduction
model as regions where lubricating basal water is present, produced either by melting
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inthethermodynamiccalculationorbymovementofwaterbeneaththeicesheetdown is Conclusions References
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the hydrostatic gradient. Ice shelves are not modeled explicitly in UMISM. However, u
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a thinning rate at the grounding line produced by longitudinal stresses is calculated io
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from a parameterization of the thinning of a floating slab (Weertman, 1957). No sub-
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grid grounding line interpolation is applied. p
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subsurface temperature projections from two different ocean models, namely the Bre- |
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merhaven Regional Ice Ocean Simulations (BRIOS) model and the Finite-Element
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Southern Ocean Model (FESOM). is
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BRIOS is a coupled ice–ocean model which resolves the Southern Ocean south s
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of 50◦S zonally at 1.5◦ and meridionally at 1.5◦×cosφ. The water column is vari- io
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ably divided into 24 terrain-following layers. The sea-ice component is a dynamic- P
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thermodynamic snow/ice model with heat budgets for the upper and lower surface p InteractiveDiscussion
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layers (Parkinson and Washington, 1979) and a viscous-plastic rheology (Hibler III, D
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1979). BRIOS considers the ocean-ice shelf interaction underneath ten Antarctic ice c ESDD
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shelves (Beckmann et al., 1999; Hellmer, 2004) with time-invariant thicknesses, as- s
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suming flux divergence and mass balance to be in dynamical equilibrium. The model n
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5 has been successfully validated by the comparison with mooring and buoy observa- a
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tions regarding, e.g., Weddell gyre transport (Beckmann et al., 1999), sea ice thick- e Antarctic
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nessdistributionanddriftinWeddellandAmundsenseas(Timmermannetal.,2002a; ice-discharge from
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Assmann et al., 2005) and sub-ice-shelf circulation (Timmermann et al., 2002b). SeaRISE
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FESOM is a hydrostatic, primitive-equation ocean model with an unstructured grid response-functions
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10 that consists of triangles at the surface and tetrahedra in the ocean interior. It is based u
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on the Finite Element model of the North Atlantic (Danilov et al., 2004, 2005) cou- s
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pled to a dynamic-thermodynamic sea-ice model with a viscous-plastic rheology and n
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evaluated in a global setup (Timmermann et al., 2009; Sidorenko et al., 2011). An ice- a
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shelf component with a three-equation system for the computation of temperature and r
salinity in the boundary layer between ice and ocean and the melt rate at the ice shelf
15 | Abstract Introduction
base (Hellmer et al., 1998) has been implemented. Turbulent fluxes of heat and salt
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arecomputedwithcoefficientsdependingonthefrictionvelocityfollowingHollandand is Conclusions References
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Jenkins(1999).Thepresentsetupusesahybridverticalcoordinateandaglobalmesh u
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with a horizontal resolution between 30 and 40km in the offshore Southern Ocean, sio
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whichisrefinedto10kmalongtheAntarcticcoast,7kmunderthelargericeshelvesin
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theRossandWeddellSeas,andto4kmunderthesmalliceshelvesintheAmundsen p
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Outside the Southern Ocean, resolution decreases to 50km along the coasts and |
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about 250–300km in the vast basins of the Atlantic and Pacific Oceans, while on the
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other hand some of the narrow straits that are important to the global thermohaline is
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circulation(e.g.,Fram-andDenmarkStraits,andtheregionbetweenIcelandandScot- s
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land) are represented with high resolution (Timmermann et al., 2012). Ice shelf draft, io
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cavity geometry, and global ocean bathymetry have been derived from the RTopo-1 P
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