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Addressing biases in Arctic–boreal carbon cycling in the Community Land Model Version 5 2021 Leah Birch.pdf

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.,14,3361–3382,2021
/gmd-14-3361-2021
©Author(s)
.
AddressingbiasesinArctic–borealcarboncyclinginthe
CommunityLandModelVersion5
LeahBirch1,,SueNatali1,DanicaLombardozzi2,GretchenKeppel-Aleks3,JenniferWatts1,
XinLin3,DonatellaZona4,WalterOechel4,TorstenSachs5,ThomasAndrewBlack6,
1WoodwellClimateResearchCenter,Falmouth,MA,USA
2NationalCenterforAtmosphericResearch,Boulder,CO,USA
3UniversityofMichigan,AnnArbor,MI,USA
4SanDiegoStateUniversity,SanDiego,CA,USA
5GFZGermanResearchCentreforGeosciences,Potsdam,Germany
6UniversityofBC,Vancouver,BC,Canada
Correspondence:LeahBirch(******@,birch.******@)
(******@)
Received:1November2020–Discussionstarted:14December2020
Revised:5April2021–Accepted:19April2021–Published:4June2021
–borealzone(ABZ)isexperiencingration(TER),-
amplifledwarming,activelychangingbiogeochemicalcy-sultssuggestthatalgorithmsdevelopedforlowerlatitudes
-to-atmospherefluxesandmoretemperateenvironmentscanbeinaccuratewhen
ofCO2intheABZhavethepotentialtoincreaseinmag-extrapolatedtotheABZ,andthatmanylandsurfacemodels
nitudeandfeedbacktotheclimatecausingadditionallarge-maynotaccuratelyrepresentcarboncyclinganditsrecent
-rapidchangesinhigh-latitudeecosystems,especiallywhen
bilityiscriticaltopreparationforawarmingworld,butEarthanalyzedbyindividualPFTs.
systemmodelshavebiasesthatmayhinderunderstandingof

circumpolarcarboncyclingrepresentedbytheCommunity
LandModel5()withafocusonseasonalgrosspri-1Introduction
maryproductivity(GPP)inplantfunctionaltypes(PFTs).
Webenchmarkmodelresultsusingdatafromsatellitere-AstheatmosphericconcentrationofCO2continuestorise,
–borealzone(ABZ)isexpectedtocontinueto
-warmmorerapidlythantherestoftheglobe(Serrezeand
straints:(1)theonsetofdeciduousplantproductivitytobeFrancis,2006;SerrezeandBarry,2011).Theimpactsof
late;(2)theoffsetofproductivitytolagandremainabnor-thisacceleratedwarmingaremanifestacrossallmajorcom-
mallyhighforallPFTsinfall;(3)ahighbiasofgrass,shrub,ponentsoftheABZ–thecryosphere,hydrosphere,and
andneedleleafevergreentreeproductivity;and(4)anunder-biosphere(Duncanetal.,2020).ThemultifacetedABZ

biases,wefocusonmodeldevelopmentofalternatephenol-(Jeongetal.,2018),permafrostthaw,intensiflcationofdis-
ogy,photosynthesisschemes,andcarbonallocationparam-turbanceregimes(AlexanderandMack,2016),changesin
-snowcoverandecosystemwateravailability(Callaghan
mentsarefocusedonproductivity,ourflnalmodelrecom-etal.,2011;Biancamariaetal.,2011),andshiftsinvege-
mendationresultsinothercomponentCO2fluxes,.,nettationstructureandcomposition(Becketal.,2011;Forkel
ecosystemexchange(NEE)andterrestrialecosystemrespi-etal.,2016;SearleandChen,2017).Thesechangesinthe
wholeABZterrestrialecosystemstructureandfunctionhave
:.
.:
importantimplicationsforglobalclimate,giventheregion’
strongbiophysicalcoupling(Bonanetal.,1992;Balaetal.,fluxesarerequiredformechanisticunderstandingbutareof-
2007;Rogersetal.,2013,2015)andlarge,andpotentiallytenlimitedacrosstimeandspace,especiallyinlargeand
vulnerable,reservoirsofbelowandabovegroundcarbon,es-remoteregionswithextremetemperatures,liketheABZ
peciallyinthepermafrostzone(Shaveretal.,1992;McGuire(Virkkalaetal.,2018,2019).Forexample,respirationdur-
etal.,2009,2010;Kovenetal.,2015;Parazooetal.,2018b;ingthewinterhaslongbeenassumedtobeeffectivelyzero,
Natalietal.,2019;McGuireetal.,2018).butbettertechnologyhasslowlyallowedtheseasonalcycle
TheresponsesofcarboncyclingintheABZtochangesstorytogrow(Natalietal.,2019).Satelliteobservationspro-
inglobalclimatearecomplex,interconnected,andmayvidenear-completecoverageinspaceandtimebutarein-
havecompensatingeffects(Welpetal.,2016).Forexam-directobservationsofecosystemproperties,arechallenging
ple,airandsoilwarming,inconjunctionwithalengthen-intheABZduetolowinsolationinthewintermonthsand
ingoftheannualnon-frozenperiodacrosstheABZ(Kimextremesnowstorms,andcontainavarietyofuncertainties
etal.,2012),stimulateplantproductivitydirectlyandin-relatedtosensorproperties,atmosphericcontamination,and
directlythroughincreasednutrientandwateravailabilityprocessing(Duncanetal.,2020).Thebrevityofthegrowing
(Natalietal.,2014;Salmonetal.,2016).WarmingandseasonandlackoflightintheABZthroughouttheyearalso
CO2fertilizationhavecontributedtowidespread“greening”contributestobiasesinsatellitemeasurements(Randerson
acrosstheABZ,includingshrubiflcation(Myers-Smithetal.,etal.,1997).Process-basedmodels,orterrestrialbiosphere
2011,2015)andnorthwardtreelineexpansion(Lloydandmodels(TBMs),areaparticularlyinvaluableresourcefor
Fastie,2003;Chapinetal.,2005),.,theencroachmentofexaminingmechanismsacrossspatialandtemporalscales,
,rapidwarmingevenprojectingcarboncyclefeedbacksinthefutureunder
acrossmuchoftheABZisalsoacceleratingdecomposition,varyingsocioeconomicscenarios(Tayloretal.,2012;Eyring
causingdroughtstressinwarmeranddrierlandscapes(Car-etal.,2016,CMIP5and6).However,duetodifferentfor-
rolletal.,2011;WalkerandJohnstone,2014;Walkeretal.,mulations,assumptions,mechanisms,modelinputs,andpa-
2015;CarrollandLoboda,2017)andintensifyingdistur-rameterizations,TBMsdisplayawiderangeofCO2source–
banceregimessuchaswildflreandinsectoutbreaks(Turet-sinkdynamicsintheABZ(Fisheretal.,2014;Huntzinger
skyetal.,2011;Kasischkeetal.,2010;Rogersetal.,2018;etal.,2013)andbiasescomparedtoobservations(Schwalm
Hanesetal.,2019),allofwhichcontributetotheincreas-etal.,2010;Schaeferetal.,2012).Giventhecriticalityof
inglyobservedpatternsof“browning”intheABZ(Verbyla,theABZtofutureglobalcarbonbalanceandtheheterogene-
2011;Elmendorfetal.,2012;PhoenixandBjerke,2016).ityoflandscaperesponsestowarming,itisahighpriority
AsanemergentpropertyofglobalchangedriversinthetounderstandandaddressthecurrentbiasesinTBMcarbon
ABZ,theseasonalcycleofCO2exchangeacrosstheABZcycling.
()isthe
-
fluxes,(,2020).CLMisoneofthemostwidely
beenmeasuredtobeincreasingbetween30%–60%duringusedlandsurfacemodelsandcontributestomanyglobalin-
thelast60years(Keelingetal.,1996;Randersonetal.,1999;tercomparisons(ZhaoandZeng,2014;Pengetal.,2015;
Gravenetal.,2013;Liptaketal.,2017;Jeongetal.,2018).Itoetal.,2016)andfutureclimateprojectionsrelevantfor
OurcurrentknowledgeoftheABZseasonalcycleofCO2scientistsandpolicymakers(Piaoetal.,2013,.,IPCC).
suggeststhatmuchoftheobservedchangeinseasonalam-Thecurrentstate-of-the-artreleaseoftheCommunityLand
plitudeisduetoincreasedvegetationproductivityduringtheModel(Lawrenceetal.,2019,CLM)incorporatesseveral
growingseason,aresultofCO2fertilizationandwarmingimprovementstoclimaticfluxesandbiogeochemistryrel-
(Forkeletal.,2016;Itoetal.,2016;Zhaoetal.,2016).
thesametime,
portionoftheannualCO2budget(Euskirchenetal.,2014;(.,).However,ahighbiasinphotosyn-
Natalietal.,2019)andhavebeenincreasingwithclimatethesisorgrossprimaryproductivity(GPP)athighlatitudes
change(Belsheetal.,2012;Piaoetal.,2008),makingtheremainsawell-documentedissue(Wiederetal.,2019)in
implicationsfornetsink–sourcedynamicsuncertain(,weexplorethesimulationofGPPalongwith
etal.,1995;McGuireetal.,2018).Hence,thecurrentandthenetecosystemexchange(NEE)andterrestrialecosystem
anticipatedstateofcarbonsource–sinkdynamicsremainsanrespiration(TER)inordertoidentifybiasesinthesimulation
openquestioninpartduetotheuncertaintyinthedomi-oftheseasonalcarbonbalance.
nantmechanismsanddifferentialresponsesgoverningcar-
,iden-
Groundobservations,satelliteproducts,andprocess-basedtiflesdeflcienciesinthesimulationofABZcarbonfluxes,
climatemodelsareallusedtounderstandinteractionsandandprovidesamodelrecommendationforapplicationin
-by-stepdiagnosisofthema-
.,14,3361–3382,2021/gmd-14-3361-2021:.
.:
jorfactorscontributingtobiasesinthesimulationofthediation,downwardlongwaveradiation,10mwindspeed,and
-cloudcoverfraction)fromtheGlobalSoilWetnessProject
COM(agriddedproductbasedonmachinelearning;Jung(GSWP3v1,-/,lastac-
etal.,2017,2020)andtheInternationalLandModelBench-cess:28May2021),whichisastandardforcingdatasetinthe
markingProject(ILAMB;Collieretal.,2018)toassessLandSurface,SnowandSoilMoistureModelIntercompari-
modelresults,andinsitudatafromFluxNet((VandenHurketal.,2016,LS3MIP).GSWP3v1
,lastaccess:28May2021)andAmeriflux(https:hasbeenshowntobeappropriateandistheleastbiasedforc-
///,lastaccess:28May2021).(Lawrence
developmentonthesimulationofCO2fluxesforeachABZetal.,2019).
,representingthetundraandbo-in1850andrunthrough2014includingdefaulttimeseries
-basedsimulationsateddycovari-inputsofCO2,aerosoldeposition,nitrogendeposition,and
ance(.,ECorflux)towersitestoinformthefailureorsuc-landusechange(Lamarqueetal.,2010;Lawrenceetal.,
cessofeachmodeldevelopmenttestofthephenologyand2016),whichareavailableonNCAR’sCheyennesystem
-(ComputationalandInformationSystemsLaboratory,2017).

(withheldfromtheinitialmodeldevelopment)beforemak-40Nacrossbothhemispheres,allowingustofocusex-
,weiden-
tifyandresolvemanyoftheknownbiasesintherepresenta-(Lawrence
tionofphenology(Richardsonetal.,2012),photosynthesisetal.,2019)
(Lawrenceetal.,2019),,thesameinputdatasets.
,weuseanavailable
’sCheyennesystem
(ComputationalandInformationSystemsLaboratory,2017)
withspun-,we
2Methodsagainspinthemodelusingthisinitialdataset,andweflnd
GPPtoequilibratequickly,within20years(
WeinvestigatetheseasonalcycleofABZCO2fluxeswithSupplement).Tobeconservative,wespinupthemodelwith
-ourrecommendedmodeldevelopmentversionsfor100years
-
cessesincludesnowphysicsrelatedtosnowageandden-usethisequilibratedstateasinitial