Modeling Ice Sheets and Sea Level with the Community Earth System Model

William LipscombCESM Tutorial17 August 2017

Outline• Introduction

• IcesheetsinCESM1

• IcesheetsintheCESM2release

• IcesheetsbeyondCESM2

Sea-level rise over the past 25 years

Globalmeansea-levelrisefromsatellitealtimetrySLRrate=3.3± 0.4mm/yr,1993–2015

Nerem 2016

Growing ice sheet contribution to sea level

~1mm/yr fromGreenlandandAntarcticicesheets(~65%Greenland)~1mm/yr fromglaciersandicecaps~1mm/yr fromoceanthermalexpansion

Credit:ESA/NASA/PlanetaryVisions

AntarcticIceSheet• 60msea-levelequivalent

– 5minmarine-basedpartsofWestAntarctica

– 20minmarine-basedpartsofE.Antarctica

• Accumulationbalancedbyflowintofloatingiceshelves;littlesurfacemelting

• MasslossinrecentyearsfromWestAntarcticaandtheAntarcticPeninsula,triggeredbywarmoceanwaterreachingthebaseoficeshelves

Antarctic ice flow speed (Rignot et al. 2011)

GreenlandIceSheet

• 7msea-levelequivalent• Accumulationbalancedby

surfacerunoffandicebergcalving

• Growingmasslossinrecentyearsfromincreasedsurfacemeltingandrunoff,andfromthinningandaccelerationofoutletglaciers

Greenland surface ice speed (Greenland Ice Mapping Project)

IcesheetsinIPCCAR5• “UnderallRCPscenariostherateofsealevelrisewillvery

likelyexceedthatobservedduring1971–2010duetoincreasedoceanwarmingandincreasedlossofmassfromglaciersandicesheets.”

• Likelyrangeof21st centuryglobalmeansealevelrise:• 0.32to0.63m(RCP4.5,2081-2100)• 0.45to0.82m(RCP8.5,2081-2100)

• “Onlythecollapseofmarine-basedsectorsoftheAntarcticicesheet,ifinitiated,couldcauseglobalmeansealeveltorisesubstantiallyabovethelikelyrangeduringthe21stcentury….”

Marineicesheetinstability(MISI)

• IceinpartsofAntarctica,especiallytheAmundsen Searegion,isvulnerabletointrusionsofwarmCircumpolarDeepWater(possiblydrivenbychangesinwindforcing).

• Unbuttressed marineiceonareverse-slopingbedisunstable.• MISImayalreadybeactiveforPineIslandandThwaites glaciers.

Schematic of warm CDW reaching the grounding line (Jenkins et al. 2016)

Antarcticicesheetsensitivity

• LastInterglacial(125Kyearsago,CO2 =280ppm)– Globalmeansealevel6–9mhigherthantoday– Only~2mfromGreenland,0.4mfromoceanthermalexpansion,

henceanAntarcticcontributionof~5m• Pliocene (3Myearsago,CO2 =400ppm)

– Globalmeansealevel10–30mhigherthantoday– Maxof~7mfromGreenland,~5mfromW.Antarctica,so

probablyanE.Antarcticcontribution• EvenwithMISIincluded,itisdifficulttosimulatethismuch

retreatwithcurrentmodels.

– Arecriticalmechanismsmissing?

MechanismsforlargeAntarcticSLR?DeConto &Pollard(2016)suggestednewmechanisms:• Marineicecliffinstability(MICI)

– Icecliffsattheedgeofoutletglacierscanbe~1kmthick– Whenmorethan~90moficesitsoutsidethewater,atypicalicecliffwillcollapse(stress>1Mpa)

– Cliffretreatonareverse-slopingbedisdynamicallyunstable(similartoMISI).

• Hydrofracture– Increasedmeltingoniceshelvescouldgrowcrevassesandincreasecalving

– Loseofbuttressingshelvescouldtriggercliffinstability(e.g.,Jakobshavn inGreenland)

SimulatedAntarcticretreatDeConto &Pollard(2016)

• RCP8.5:77cmby2100,12mby2500– WAIScollapseby2250– MajorretreatinWilkes

andAuroraBasins• Atmosphericwarmingis

themaindriver,butoceanthermalmemoryinhibitsrecovery.

• Mechanismsareplausible,butratesarehighlyuncertain.o Coarse-resolutionicesheetmodel(10km)with

parameterizedfluxesatthegroundinglineo Prescribedoceanmeltrateswithnocoupling

DeConto &Pollard2016

Questions• Whatrangeofsea-levelriseshouldplannersand

policymakersassume?o Canwebeconfidentthat21st centurysea-levelrisewillnotexceed1m?

• CanCESMandotherEarthsystemmodelsprovideactionablesea-levelscience?

o SurfacemassbalanceprojectionsfromEarthsystemmodelsareincreasinglycredible.

o Butitremainsdifficulttoputanupperboundonthesea-levelcontributionfromAntarctica.

IcesheetsinCESM1• Formanyyears,globalclimatemodelsdidnotincludedynamic

icesheets.Icesheetsweretreatedasbigbrightrocks.o Thetraditionalviewwasthaticesheetsevolvedonmulti-

centuryandlongertimescales.Thisviewchangedwithobservationsoficesheetmasslossinthe1990sand2000s.

• In2009,theCESMLandIceWorkingGroupformedwiththegoalsof1) integratingawellvalidated,fullydynamicalicesheetmodel

inCESM2) determiningthelikelyrangeofdecade-to-century-scale

sea-levelriseassociatedwiththelossoflandice• CESMv1.0wasreleasedin2010withapreliminary

implementationofdynamicicesheets.

IcesheetsinCESM1CESM1includedtheGlimmerCommunityIceSheetModel(Glimmer-CISM).

• DynamicGreenlandicesheetonastructured5kmgrid

• Serialcode;Glide shallow-icedynamics(validforslowinteriorflows,butnotfastflowinicestreamsandiceshelves)

CESM1alsoaddedasurface-mass-balanceschemeforlandice.• Thesurfacemassbalance(SMB)forglaciatedregionsiscomputedbytheCommunityLandModelinmultipleelevationclasses,thensenttothecoupleranddownscaledtothelocalicesheetgrid.

• AdvantagestocomputingSMBinthelandmodel:o Coupleicealbedotoatmosphereonhourlytimescaleso Avoidduplicationofsnowphysicso Computationalsavings(landgridcoarserthanicesheetgrid)

IcesheetsinCESM1Land -> Ice sheet (10 classes)n Surface mass balancen Surface elevationn Surface temperature

Coupler

Atmosphere

Ocean

Sea Ice

Land surface(Ice sheet surface

mass balance)

Ice sheet(Dynamics)

Ice sheet -> Land

n No fields passed; placeholders only

GreenlandSMB:ComparisonwithRACMO2CESM RACMO2

SMB(Gt/yr) 359± 120 376 ± 117

• Goodmatchinablationzones• Accumulationisoverestimatedintheinteriorandunderestimatedin

thesoutheast(smootherorographyinCESM)

Courtesy of M. Vizcaíno!"#$%&'&()& *+!$,-%&.//&()&

Greenland ice sheet SMB (kg m-2 yr-1), 1960-2005

Red = net accumulationPurple = net melting

GreenlandSMB:20th v.21st century20th-century (1980-1999) RCP8.5 (2080-2099)

SMB (Gt/yr) 372± 100 -78± 143

Greenland ice sheet SMB (kg m-2 yr-1)

• ForRCP8.5,precipitationincreases,butmeltandrunoffincreasemore.• Warmingisgreatestinnorth(lessseaice),leastinsoutheast(weakerMOC).• AverageSMBisnegativeby2100,implyinglong-termdecayoficesheet.

Red = net accumulationPurple = net melting

Courtesy of M. Vizcaíno !"#$%""& '$#$%""&

Glimmer-CISMinCESM1• CapturesbroadpatternsofGreenlandiceflow,butoutlet

glacierstendtobeslowanddiffuse• Icesheettooextensiveinnorthandeast(positiveSMBbias)

(a) Greenland balance velocities based on observed thickness and the SMB of Ettema et al. (2009)

(b) Vertically averaged Greenland velocities simulated by Glimmer-CISM in CESM (top-ranking ensemble member, Lipscomb et al. 2013).

LandicegoalsforCESM2

• Parallelicesheetmodelwith“higher-order”flow;validinallpartsoftheicesheet

• Land-iceSMBcalculationssupportedbydefault;notjustforcustomLIWGsimulations

• Improvedtreatmentofsnowphysics(deepfirnlayers,insteadofsnowdepthcappedat1m)

• Supportfortwo-waycoupling:Changesinicesheetelevationandextentcanfeedbackontheclimate

HierarchyofStokesapproximations

• Previousgenerationoficesheetmodelsmostlyusedshallow-iceorshallow-shelfapproximations

• Newermodels(BISICLES,Elmer-Ice,ISSM,PISM,PSU,etc.)haveoneormorehigher-ordervelocitysolvers

• PrimarygoalforCISM2wasarobust,parallelhigher-ordersolver

Stokes3Dsolveforu,v,w,p

Higher-order(Blatter-Pattyn)3Dsolveforu,v

Depth-integratedhigher-order2Dsolveforu,v

Shallowiceapproximation

(Verticalshearstresses)

Shallowshelfapproximation(Membranestresses)

CISM2CISM2 wasreleasedinOct.2014:

• Codeathttp://oceans11.lanl.gov/cism/;git repoat https://github.com/cism

• Paralleldynamicalcore(Glissade)withsuiteofhigher-ordervelocitysolvers

• Incrementalremappingformassandtemperaturetransport

• Newtestcases(shallow-ice,higher-order)withPythontools

• Improvedcouplinginterface• PortedtoCESMbyBillSacks

SimulatedCISM2velocities.Top:GreenlandicesheetBottom:RossIceShelf

CISM2: ISMIP-HOM tests• Compared higher-order model results to community benchmarks

(Pattyn et al. 2008) for problems with small-scale variations in topography and basal traction

• Glissade’s higher-order solvers agree well with benchmarks

ISMIP-HOM Test A: Sinusoidal pattern in basal topography at 6 grid scales (Glissade output shown by black lines)

CISM2:Greenlandthickness

Observedicethickness(m)

CISMicethickness(m)after50ka spin-up

Modelminusobservedthickness(m)

• CISM2isrobustandefficientforlongGreenlandspin-ups(~1200yr/wallclockhouron4kmgrid).

• Aftera50kyr spin-upwithSMBforcingfromRACMO2,modelthicknessisclosetoobservations(abitthininnorthandwest,thickinsoutheast).

CISM2: Greenland velocities• Usingadepth-integratedHOsolver(DIVA)andpseudo-plasticsliding

law,CISMvelocitiesareingoodagreementwithobservations.

Observedsurfacespeed(m/yr,logscale)

ModeledsurfacespeedinCISM(m/yr,logscale)

CISM2: Greenland basal state• CISM’sdistributionoffrozenandthawedregionsissimilartoestimates

basedonobservationsandothermodels.

SynthesisofGreenland’sbasalthermalstatefromMacGregoretal.(2016)

Basalwaterdepth(m)inCISM;purple =frozen(nobasalwater),red =thawed(waterpresent).

CESM1.0 CESM2.0

Serial,shallowiceapproximation Parallel,higher-order approximation

One-waycoupling(CLMà CISM) Two-waycoupling(CLMßà CISM)with dynamic landunits

DownscalinginCISM/Glint,withSMBnotconserved

Downscaling inthecoupler,withSMBconserved

1-msnowpackinCLM Firn modelinCLM(10-msnowpack,improvedsnowdensity)

SMB computedonlyinrunsdonebyLIWG SMB computedinallruns

LandiceprogressinCESM

IcesheetsinCESM1Land -> Ice sheet (10 classes)n Surface mass balancen Surface elevationn Surface temperature

Coupler

Atmosphere

Ocean

Sea Ice

Land surface(Ice sheet surface

mass balance)

Ice sheet(Dynamics)

Ice sheet -> Land

n No fields passed; placeholders only

IcesheetsinCESM2Land -> Ice sheet (10 classes + bare land )n Surface mass balancen Surface elevationn Surface temperature

Coupler

Atmosphere

Ocean

Sea Ice

Land surface(Ice sheet surface

mass balance)

Ice sheet(Dynamics)

Ice sheet -> Landn Ice extentn Ice surface elevationn SMB mask

Ice sheet -> Oceann Solid and liquid fluxes

Ice sheet -> Atmosphere (offline)n Surface topography

Greenland surface mass balance in CESM2Overall Greenland climate is very good.

• SW and LW biases reduced compared to CESM1

• Recent tuning has improved winds and precipitation

• Model captures the western ablation zone

• Too little ablation in north; cold climate bias

GreenlandSMBinCESM2onCLMgrid(left)anddownscaledtoCISMgrid(right),comparedtoRACMO2(center).Red =accumulation,blue =ablation

Antarctic surface mass balance in CESM2

• Excellent Antarctic climate and SMB

• Improvements in snow physics have reduced spurious melting

• Good baseline for warmer climates with increased ablation AntarcticsurfacemassbalanceinrecentCESM2

tuningruncomparedtoRACMO2.Red =accumulation, blue =ablation

LandicegoalsforCESM2+• Supportmultipleicesheets(notjustGreenland)withappropriate

physicsandnumericso Antarcticao Paleo icesheets(e.g.,Laurentide)

• Worktowarddynamiccouplingofmarineicesheets

o Long-term:Interactivecouplingwithoceancirculationinsub-shelfcavitiesandadynamicice/oceaninterface

o Nearterm:UsesimplemodelsandparameterizationstolinkoceanGCMoutput(atvariousresolutions,withorwithoutcavities)tosub-shelfmelting

• ParticipateintheIceSheetModelIntercomparisonProjectforCMIP6

o ExperimentsforstandaloneISMsandcoupledAOGCM/ISMs

CurrentstateofCISM• User-friendly• Portable• Maintainableandextensible• Robust• Verified• Versatile

• SuiteofStokesparameterizations(BP,DIVA,SSA,SIA)• Butsomephysicsparameterizationsneedmoreresearch

• Efficient• NativeFortransolverisfastformoderateresolution

Greenland(~4km),adequateforfineresolution(~1–2km)• Solverimprovementswouldfacilitatewhole-icesheet

simulationsforAntarctica

NewCISMphysicsandnumerics• Newphysicsformarineicesheets

o Grounding-lineparameterization(complete)o Damage-basedcalving(inprogress)o Sub-shelfplumemodel(inprogress)o Marineicecliffinstabilityandhydrofracture

• Otherphysicsimprovementso Evolutionarybasalhydrologyo Fullyparallelisostasymodel

• Improvedefficiency(for1-2kmresolution)o Fastermatrixsolver(betterpreconditioningforiceshelves)o Betterloadbalancing(removeice-freeoceancells)

CISMAntarcticicesheetsimulations• CISM’s higher-order velocity solvers can handle Antarctica

out of the box at resolutions down to 1 km.• Challenge is to simulate realistic long-term evolution.

ObservedAntarcticsurfacevelocities(m/yr,logscale)

ModeledAntarcticsurfacevelocities(m/yr,logscale)

MarineIceSheetModelIntercomparisonProject

• MISMIP3d(Pattyn etal.2013)• Perturbedbasalslidingparametersgivelateralvariationandbuttressing,withcurvedgroundinglines

• CISMusesagroundinglineparameterization(GLP)toresolvesubgrid variationsinbasaltractions

• Thisallowsustomodelgroundinglinesaccuratelyatpracticalresolutions(~1–2km)

• MISMIPconsistsofidealizedexperimentsthattestamodel’sabilitytotrackgrounding-lineadvanceandretreat.DoestheGLreturntoitsstablestartingposition?

MISMIP3d: Applied basal perturbation

SSA withGLP(1km):GLreturnstostartposition(598km),closetoanalyticsolution(612km)

Black =startingposition; red =advance;lt.blue=return

592 594 596 598 600 602 604 606 608 6100

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GL position throughout experiments

StndP75SP75R

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GL position throughout experiments

StndP75SP75R

SSA withoutGLP(1km):GLtoofarretreatedatstart(504km)

andfailstoreturn

Startfromsteadystate;runfor100yearswithbasalperturbation;turnoffperturbationandreturntosteadystate.

MISMIP3d: Applied basal perturbation

SSA withGLP:GLreturnstostartposition(598km),closetoanalyticsolution(612km)

Black =startingposition; red =advance;lightblue=return

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DIVAwithGLP:GLreturnstostartposition(558km);iceissofterwithverticalshearstresses

554 556 558 560 562 564 566 568 570 572 5740

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• SSAgroundinglineisclosetoanalytic1Dsolution.• DIVAandBPresultsareclosetobenchmarkStokessolution.

MISMIP summary

• Veryhighresolution(~200m)doesnotseemnecessaryforaccurategrounding-lineresolution.WithaGLP,aresolutionof~1kmisadequate,evenwithasharptransitionzone.

• Depth-integratedviscosityapproximation(DIVA)appearstobeanaccurateandefficientcompromisebetweenBlatter-PattynandSSA.

• Thisisgoodnewsformodels!At1kmwithadepth-integratedsolver,itisfeasibletomodelentireicesheets,evenonCISM’sstructuredgrid.

Calving• CalvingaccountsforabouthalfofmasslossfromAntarctica

andGreenland.• CISMhasseveralsimplecalvingschemesbasedonice

geometryorlocation:o Calveallfloatingice.o Calveicewherethebedisdeeperthanacriticaldepth.o Calvefloatingicewhenthinnerthanacriticalthickness.

• Theseschemesdonotaccountforicemechanics.Icebergscalvewhentheiceissufficientlydamaged.

Schematicoffloatingicetonguewithcrevassesleadingtocalving.CourtesyofM.Whitcomb.

Damage-basedcalving• DOEcollaborationtodevelopadamage-basedcalvingmodel:U.

Michigan(J.Bassis,M.Whitcomb),LANL• Iceisdescribedbyadamagetracerrepresentingcrevassedepth.

CalvingoccurswhenD=1.• Tensilestressandbasalmeltingopencrevasses;gravitationalforces

closecrevasses.• Modelappliedtoobservedicestreamsandicetongues:Erebus,

Drygalski,PineIsland,Petermann

ModeleddamageofErebusIceTongue,comparedtoobservedthicknessprofile.CourtesyofM.Whitcomb.

Sub-shelfmelting

• Sub-shelfmeltratesaresometimesparameterizedasafunctionofdepth,failingtocapturethespatialstructureofmelting.

• ItisexpensivetorunoceanGCMsbeneathiceshelves(requiredgridresolution~2km),andnotalloceanmodelshavethiscapability.o POPgridhasaverticalwallattheshelfedge.o MOM6hasbeenrunwithoceancavities,butnotyet

operationalforglobalsimulations.

• Canweestimatesub-shelfmeltrateswithamodelofintermediatecomplexity?

Plumemodel• Holland,Jenkins&Holland(2008)modeledoceanflowinthecavity

beneathastaticiceshelf.TheysuggestedthatoceanGCMresultscanbeexplainedintermsofasteady-stateplumemodel.

• Theplumeisawell-mixed,buoyantlayerattheiceshelfbase,characterizedbythicknessD,temperature T,salinityS,andvelocityu=(u,v).

• Wearegiventhecavitygeometry(shelfbase,bedtopography) andtheambienttemperatureandsalinityTa andSa.

• TheplumemodelisbeingappliedtotheISOMIP+experimentsforsub-shelfoceanmodels(Asay-Davisetal.2016).

• T0 =-1.9oC,Tbot =1.0oC

• S0 =33.8psu,Sbot =34.7psu

Plumethicknessandvelocity

• Dominantflowisupslope(eastward),limitedbydraganddivertednorthwardbytheCoriolis force.

• Theplumethicknessis5to20minmostofthedomain.• Waterpilesuponthenorthcavityedge,drivinganeastwardjet.

Basalmeltrate

Plumemodel

• PreliminaryISOMIP+resultsfromPOP2X andMPAS-Ocean,courtesyofX.Asay-Davis

• LargestmeltratestendtobeinNWandSWcornersofthedeepcavity

• PlumemodelandMPAS-OceanarelessnoisythanPOP2x(noiseassociatedwithz-gridstair-stepping)

Plumesummary

• PlumevelocitiesandmeltratesarecomparabletotheresultsofoceanGCMs.o Thereissignificantvariabilityamongoceanmodels.ForISOMIP+

theplumemodelisdifferent,butnotclearlybetterorworse.

• AlthoughlessphysicallyrealisticthanoceanGCMs,theplumemodelissimpler,cheapertorun,andeasiertocouple.o Couldbeforcedbyanoceanmodelwithoutcavities;justrequires

verticalprofilesofTa andSa neartheshelfedge

• Nextsteps:o ApplytoMISOMIP1testcase(coupledoceanandicesheet).

o TestinmorerealisticGreenlandandAntarcticageometries.

IceSheetModelIntercomparisonProjectforCMIP6(ISMIP6)

• ISMIP6isanewtargetedactivityoftheClimateandCryosphere (CliC)projectoftheWorldClimateResearchProgram.

• Primarygoal:ToestimatepastandfuturesealevelcontributionsfromtheGreenlandandAntarcticicesheets,alongwithassociateduncertainty

• Secondarygoal:Toinvestigatefeedbacksduetodynamiccouplingbetweenicesheetandclimatemodels,andimpactsoficesheetsontheEarthsystem

• TheLIWGplanstorunISMIP6standaloneexperimentswithCISM(forbothicesheets)andcoupledexperimentswithCESM(forGreenland).

ExperimentaldesignforISMIP6

1. ExistingCMIP6experimentstobeanalyzed intermsoficesheetforcing

2.StandaloneicesheetexperimentsbasedonCMIP6modeloutputtoestimatepastandfuturesealevelrise,andexploreuncertaintyduetoicesheets

3.CoupledAOGCM-ISMexperimentstoexploreimpactsandfeedbacksduetoicesheets

CMIP6exptobeusedbyISMIP6(allAOGCM)

- Pre-industrial control- AMIP- 1%peryrCO2 to4xCO2- Abrupt4xCO2- CMIP6Historical Simulation- ScenarioMIP RCP8.5/SSP5x(uptoyear2300)- LastInterglacial PMIP

Standalone ISMIP6exp (ISMonly)

- ISMcontrol- ISMfor lastfewdecades(AMIP)- ISMforthehistoricalperiod- ISMforcedby1%peryrCO2 to4xCO2- ISMfor21st /23rd century(RCP8.5/SSP5x)- ISMforLastInterglacial- ISMspecificexperimentstoexploreuncertainty

Newproposed ISMIP6exp (coupledAOGCM-ISM)

- Pre-industrial control- 1%peryrCO2 to4xCO2- ScenarioRCP8.5/SSP5x(toyear2300)

Summary• Sea-levelriseisstillawide-openscientificproblem,largely

becauseofuncertaintiesinthedynamicsofmarineicesheets.

• TheCESMcommunity—withitsinterdisciplinaryworkinggroupstructure,linkstotheacademiccommunity,andexperienceindevelopingCESM1andCESM2—iswellequippedtotacklethescience.

• CISMisaccurate,efficientandversatileenoughtosupportsimulationsoftheGreenland,Antarcticandpaleo icesheetsoverthenextseveralyears.

• CISMcanreadilyincorporatenewdynamicsandphysicstosupportemergingscience.

LandIceWorkingGroupinfo

Webpage:http://www.cesm.ucar.edu/working_groups/Land+Ice/

Liaisons:GunterLeguy (gunterl@ucar.edu) andBillSacks(sacks@ucar.edu)

Co-chairs:JanLenaerts(Jan.Lenaerts@colorado.edu) andBillLipscomb(lipscomb@ucar.edu)

Upcomingmeetings:• WinterLIWGmeeting,Boulder,10-11January2018• 23rdannualCESMworkshop,Colorado,18-21June2018