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Investigating therecentapparenthiatusin surfacetem peratureincreases:1.Construction oftwo30-m em berEarth System M odelensem blesStephen Outten1,PeterThorne1,2,Ingo Bethke3,and Øyvind Seland4
1Nansen Environm entaland Rem oteSensing Centre,BjerknesCentreforClim ateResearch,Bergen,Norway,2Departm ent
ofGeography,M aynooth University,M aynooth,Ireland,3UniResearch Clim ate,BjerknesCentreforClim ateResearch,
Bergen,Norway,4Norwegian M eteorologicalInstitute,Oslo,Norway
AbstractTherecentIntergovernm entalPanelonClim ateChangereport,alongwithnum erousstudiessince,hassuggestedthattheapparentglobalwarm inghiatusresultsfrom som ecom binationofnaturalvariabilityand
changesto externalforcings.Herein theexternalforcingsforgreenhousegases(GHGs),long-lived tracegases,
volcanicand troposphericaerosols,and solarirradiancehavebeen replaced in theNorwegian Earth System
M odelusing recentobservationalestim ates.Thepotentialim pactofthesealternativeforcings,and byresidual
theinternallygenerated variability,isexam ined through two30-m em berensem blescovering theperiod 1980
to 2012.TheReferenceensem bleusestheCoupled M odelIntercom parison Projectphase5historicalforcings
extended with theRepresentativeConcentrationPathway8.5(RCP8.5)scenario,whiletheSensitivityensem ble
usesthealternativeforcings.Overthehiatusperiod de ned herein as1998–2012,alloftheforcingsshow
som echangebetween theSensitivityand Referenceexperim entsand haveacom bined netforcingchangeof
0.03W m 2.TheGHG forcing is0.012W m 2 higherin theSensitivityforcings.Thealternativesolarforcing
differsfrom theReferenceforcingby 0.08W m 2,thesam easthealternativevolcanicforcingthatwasbased
on thelatestestim atesfrom NASA Goddard InstituteforSpaceStudies.Anthropogenicaerosolem issions
werereplaced using theEU-EclipseV4adatasetand produceam ean forcing changeof0.11W m 2 overthe
period.Part1detailsthecreationofthetwo30-m em berensem blesandtheircharacterizationforparam etersof
particularrelevanceto theexplanation ofthehiatus.A detailed investigation ofthetworesulting ensem bles
globalsurfacetem peraturebehaviorisgiven in Part2,along with com parisonsto observationaldatasets.
1.Introduction
The globalm ean surface tem perature hasincreased m ore slowly overthe past15yearsthan overthe last
50years.The trend has fallen from 0.11Kdecade 1 over1951–2012 to 0.04Kdecade 1 over1998–2012
[Flato etal.,2013],although updatesto observationalestim atessince [Cowtan and W ay,2014;Karletal.,
2015]change these num berssom ewhatforsom e observationalestim ates.Even so,in allcurrentobserva-
tionaldatasets,the period 1998–2012 exhibitsaslowerrate ofglobalm ean surface tem perature warm ing
thanthepreceding15years.W hilethisapparentpauseorhiatusintheglobalwarm ingispresentinthem ean
globalannualsurfacetem peraturetrends,ithasbeenshowntohavebothaseasonalandageographicaldis-
tribution,with thelargestdecreasein trendsfound overthecontinentsoftheNorthern Hem isphereduringthewinter[Cohen etal.,2012].Theglobalwarm ing hiatushasdrawn greatattention,in partbecauseithasnotbeen reproduced bythe vastm ajorityofglobalclim ate m odelrunsupon which future clim ate projec-
tions are based [Fyfe etal.,2013].The fth assessm entreport(AR5)ofthe Intergovernm entalPanelon
Clim ateChange(IPCC)statedthatthehiatusislikelyduetoacom binationofinternalvariabilityintheclim ate
system and areduced trend intheexternalforcings(m edium con denceand expertjudgm ent).W hilethese
ndingshavebeen m ostlycon rm ed byrecentstudies[e.g.,Hubberand Knutti,2014;Kosaka and Xie,2013;
Meehletal.,2014;Schm idtetal.,2014;MarotzkeandForster,2015],therenow existnum eroushypothesesin
the literature regarding the differentfactorscontributing to the globalwarm ing hiatus.Itisim portantto
stress thatseveralm odelruns by severaldifferent,independentm odelscapture notjustthe hiatusbut
potentially im portantaspectsofinternalvariability such asPacicOcean heatcontent[Meehletal.,2014]orElNiño–Southern Oscillation (ENSO)[Risbeyetal.,2014]which m aybeim portant.
One ofthe prevailing hypothesesregarding internalvariability in the clim ate system wasputforward by
KosakaandXie[2013].Theyperform ed coupled m odelexperim entsinwhichtheyprescribed theseasurface
OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8575
PUBLICATIONSJournalofGeophysicalResearch:Atm ospheres
RESEARCH ARTICLE10.1002/2015JD023859
Thisarticleisacom panion to Thorneetal.[2015]doi:10.1002/2014JD022805.
Key Points:•Two setsofalternativeforcingshavebeen applied in NorESM overthehiatus
•Som eforcingshad largechangesbutwith littlenetchange(m axof0.1W m
2)
•Two 30-m em berensem bleswerecreated to coverperiod ofthehiatus
Supporting Inform ation:•TableS1
Correspondence to:S.Outten,[email protected]
Citation:Outten,S.,P.Thorne,I.Bethke,andØ.Seland (2015),Investigating therecentapparenthiatusin surfacetem peratureincreases:1.Constructionoftwo30-m em berEarthSystem M odelensem bles,J.Geophys.Res.Atm os.,120,8575–8596,doi:10.1002/2015JD023859.
Received 26 JUN 2015Accepted 28JUL2015Accepted articleonline30JUL2015Published online4 SEP 2015
©2015.TheAuthors.Thisisan open accessarticleundertheterm softheCreativeCom m onsAttribution-NonCom m ercial-NoDerivsLicense,which perm itsuseand distri-bution in anym edium ,provided theoriginalworkisproperlycited,theuseisnon-com m ercialand no m odicationsoradaptationsarem ade.
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tem perature(SST)in theEastern Pacicfrom historicalanom alies.Theresultshighlighted theroleofPacic
variabilityandENSO,suchasthestrongElNiñoeventin1998andtheLaNiña-likedecadalcooling,inexplain-
ing m uch ofthecooling thatisthecauseofthehiatus.Otherworkshavebuilton the ndingsofKosakaand
Xie[2013]byexam ining the clim ate m odelsbased on theirrepresentation ofENSO [Risbeyetal.,2014],by
exam ining changesin the windsthatdrive the cooling forPacic variability [England etal.,2014],and by
adjusting windsin a globalclim ate m odel(GCM )to reproduce the cooling effect[W atanabe etal.,2014].
Onecaveatto the ndingsofKosaka and Xie[2013]isthattheirm odelproduced strong warm ing overthe
wintertim e in Eurasia in contradiction to the strong cooling thathasbeen observed [Cohen etal.,2012]
and thatisapartofthehiatus.
TheobservedEurasianwintertim ecoolinghasbeenlinkedtothereductionofseaiceintheEurasiansectorof
theArctic,which causesstrong localwarm ing and theform ation ofatherm allow.Thisresultsin adecrease
ofthem eridionaltem peraturegradientbetweentheArcticandm idlatitudesandaweakeningofthewesterly
ow thattransportsheatandm oisturefrom theAtlantictothecontinent[OuttenandEsau,2012].Atthesam e
tim e,thelarge-scale ow ischanged,facilitating thetransportation ofcold airfrom theSiberian Arcticdown
to the m idlatitudesofEurasia [Petoukhovand Sem enov,2010;Morietal.,2014].Itrem ainsunclearto what
extentthisEurasian cooling isapredictablesecondaryresponseto anthropogenicforcing orisattributable
to naturalvariability.Norisitobviouswhethersuch cooling would continueinto thefutureundertransient
clim atechange.
W hile internalvariabilityislikelypartiallyresponsible fordeterm ining the globalwarm ing hiatus,the other
potentialpartoftheexplanation liesin changesto theexternalforcings,including greenhousegases(GHG
— including ozone-depleting substances),volcanic and anthropogenic aerosols,solarforcing,ozone,and
the effectofstratospheric watervapor.The inclusion ofthese forcings into the globalclim ate m odels
(GCM s)has,forthe past20years,been coordinated through the Coupled M odelIntercom parison Project
(CM IP),the fth phase ofwhich,CM IP5,hasform ed the basisofthe m odelrunsassessed within IPCC AR5.
In CM IP5,theobserved forcingsareapplied onlyuntiltheend ofthehistoricalexperim entsperiod in 2005.
Beyond this, the forcings are based on the Representative Concentration Pathway (RCP) scenarios.
Furtherm ore,duetom odelrun tim esand delaysinassim ilatingthelatestobservations,thefew yearsim m e-
diatelypreceding2005m ayalsohavequestionableforcings.Hence,theCM IP5m odelrunsonlyincludeaccu-
rate forcing estim atesforthe rst5 to 7yearsofthe hiatusperiod,de ned here as1998–2012.Num erous
individualstudieshave investigated how aslowdown in globalwarm ing could be the resultofchangesto
a single setofforcings,e.g.,non-CO2 GHGs [Hansen etal.,2000],ozone-depleting substances [Estrada
etal.,2013],volcanic[Ridleyetal.,2014;Solom onetal.,2011;Santeretal.,2014],and anthropogenicaerosols
[Neelyetal.,2013].
Atleasttwo preceding studieshave considered the sensitivityto forcing m isspecication in clim ate m odel
sim ulationsspecically[Schm idtetal.,2014;Santeretal.,2014].Santeretal.[2014]showed thatbetterspeci-
cationofvolcaniceruptionsproduced a15% decreaseinthedifferencesbetweenobservationsand sim ula-
tionsofsurfacetem perature.W hatism issingtodateisanaggregatedapproachinwhichtheim pactofm ost
ifnotalloftheindividualexternalforcingsareincludedsim ultaneouslyinfullycoupledruns.Aninvestigation
wasm adeinthisveinbyutilizingapproxim ationsofhow theforcingshavechanged,i.e.,linearinterpolations
forGHGsand ozone-depleting substances,estim ated aerosolim pacts,estim atesofvolcanicaerosols,etc.,
and adjusting the CM IP5 ensem ble with acorrection based on asim ple im pulse/response m odel[Schm idt
etal.,2014].
Aspartofan assessm entofhow changesin externalforcingshavecontributed totheobserved warm ing
hiatus,thisstudydetailstheunderlying experim entaldesign,and thealternativeforcingsused to drive
the Norwegian Earth System M odel(NorESM )to bring them in line with in each case one realization of
recentobservations.These forcingsare then used to drive two 30-m em berensem bles— a Reference
ensem ble using CM IP5-prescribed forcings and a Sensitivity ensem ble using the alternative forcings
discussed here. Section 2 outlines the broad experim ental design and its underlying rationale.
Section 3 describesthe alternative forcingsand com paresthem to the originalforcingsused.Section 4
describesthe NorESM m odeland how the two 30-m em berensem bleswere created.Section 5 outlines
som e principalcharacteristicsand salientfeaturesofthe ensem bles.Section 6 providesa discussion of
caveats,and section 7sum m arizeswith ahand-offto thesecond paper.Theaccom panying Part2paper
JournalofGeophysicalResearch:Atm ospheres 10.1002/2015JD023859
OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8576
[Thorneetal.,2015]then com paresthe resulting ensem blesto the available observationalestim atesof
surface tem peratures.
2.Experim entalDesign
Asoutlined in section 1,there existm ultiple published hypothesesforthe underlying causesofthe recent
warm ing hiatusthatinvolveforcingsand/orvariability.Them ostam enabletoolsto investigatetheseissues
furtherareclim atem odels,whichcanberunwithdifferentexternalforcingsandbeused toexplorepossible
internalclim ate system variabilityasitisdiagnosed bythe m odel.Butclim ate m odelsare com putationally
expensiveto run,m eaning thatcarefulforethoughtin experim entaldesign isnecessaryand thereareinevi-
tabletrade-offsrequired.Sexton etal.[2003]discussed thisforatm osphereonlyruns,butto ourknowledge
no sim ilaranalysishasbeen applied to such an estim ation approach in Earth System M odels(ESM s)such as
NorESM .Furtherm ore,itisnotclearhow experim entaldesign considerationsdepend upon the spatiotem -
poralscaleofthequestionbeingassessed.Certainly,lookingatdecadalversusm ultidecadalorglobalversus
regionalm ay require very distinctm odeling strategies.Forthe hiatus,ourinterestisin a relatively short
period and understanding theregionalsignatures.
Ouroverriding concern wasto attem ptascom prehensivelyaspossibleto testthevariousexisting hypoth-
eses.Thereisno singlerightwayto dothis,especiallygiven niteavailablecom puterresources.Theconun-
drum wasto ndabalancebetweenexploringforcinguncertaintyeffectsandinternalvariabilityeffectsinan
ESM with no obviousaprioribasisupon which to decide,in an inform ed m anner,how to weightthesetwo
aspects.W eknow them odeltreatstheprescribed forcingsasentirelydeterm inistic.Given thenotionalcom -
putationalresourceavailablefor60runs,wecould haverun 60runseach with som ewhatdiffering forcings,
twosetsof30runswithsolelytwodifferentforcingscenarios,oranythingbetween.Theonlywayinwhichto
exploreforcinguncertaintyistorunthem odelm ultipletim eswithdistinctforcingancillaries.Onthe ipside,
cleanlyassessing the role ofvariability alone requiresconsideration ofa seriesofrunswith the sam e pre-
scribed forcings,i.e.,an ensem ble.
Running 60 distinctly forced runswould m ake a clean distinction between internalvariability and forced
response im possible,while running two 30-m em berensem blesyieldsonly 2 degreesoffreedom in the
forced response dim ension.Given (i)readily apparentdecadaltim e scale intraensem ble spread in global
m eansurfacetem peraturesinthoseCM IP5subm issionsthatundertookm ultipleensem blesand(ii)precursor
analysesofe.g.,HawkinsandSutton[2009]showing thelikelydom inantroleofinternalvariabilityin decadal
predictability[seeThorneetal.,2015,Figure1],weconsciouslychoseto undertaketwo ensem blesofequal
size.W ereasoned thatitm ayrequireafarlargernum berofrunstoelucidatevariabilityonhiatustim escales
than theeffectsofanydistinction in forcings.
In sum m ary,theuseoftwo 30-m em berensem bleswasfeltattheoutsetoftheprojectto bepreferablein
elucidatingthepotentialrolesofforcingand internalvariabilitycleanly.Caveatsandlim itationsassociated
with thischoice,m any ofwhich naturally only becam e apparentatprojectcom pletion,are returned to
in section 6.
3.Clim ate Forcings
This section describes which forcings were considered in creating a new setofancillaries which takes
advantage ofthe latest(attim e ofprojectinception in early 2014)available observationalestim ates of
changesthrough atleast2012.Herein,the term forcing refersto “externalforcing”asde ned in the IPCC
Assessm entReport5glossary[Planton,2013],thus,
Externalforcing refersto a forcing agentoutside the clim ate system causing a change in the clim ate
system .Volcanic eruptions,solar variations and anthropogenic changes in the com position of the
atm osphereand land usechangeareexternalforcings.Orbitalforcing isalso an externalforcing asthe
insolation changeswith orbitalparam eterseccentricity,tiltand precession oftheequinox.
In each case,the revised forcingsare com pared to those used in our30-m em berReference ensem ble,
which were identicalto the forcingsused in the version ofNorESM extended historicalrunssubm itted
JournalofGeophysicalResearch:Atm ospheres 10.1002/2015JD023859
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to the CM IP5 archive.ForNorESM ,the extended historicalruns used RCP8.5 forcings post-2005 and
the sam e choice is m ade in the Reference ensem ble.The changes to the forcings include new
m easurem ents taken since 2005,as wellas m odications m ade to the forcings priorto 2005.These
m odi cationsm aycom efrom inclusion ofnew dataordueto changesin theinterpretation ofpreexist-
ing m easurem ents.
Asoutlined in section 2,the forcingsare treated entirelydeterm inistically within the m odel.Therefore,by
de nition,thereisabsolutelyno uncertaintyin theapplied forcings.Rather,theuncertaintyarisesin which
ofthe (form any forcing factors)severalpossible forcing realizationsto selectand presentto the m odel.
Uncertainty in the selection offorcing historiesto use and potentialim plicationsforthe analysisin Part2
isreturned to in depth in section 6.Forthe rem ainderofthissection,we concentrate upon describing the
applied Sensitivityforcings,discussing where we sourced them to provide fulldata provenance,justifying
where forcingswere notchanged between the two ensem bles,and, nally,characterizing the differences
in applied RadiativeForcing (RF)between theensem bles.
3.1.Greenhouse Gases
Theprim arygreenhousegas(GHG)concentrationsusedinNorESM consistofcarbondioxide(CO2),m ethane
(CH4),nitrous oxide (N2O),CFC-11 (including additionalCFCs and hydrochloro uorocarbons (HCFCs)via
nudging),and CFC-12.Forthe historicalrunsin the CM IP5 archive,observationsofprim aryGHGsare used
where available forthe period of1850 to 2005,although such observationsonlybegan during the lastsix
decadesand forseveralofthesegases,reliableobservationsonlystarted m uch m orerecently.Theconcen-
trationsused in the Reference runsofthisstudywere an average ofthe annualsurface globalm ean m ole
fractionsheld bythe NationalOceanicand Atm osphericAdm inistration (NOAA)and the Advanced Global
Atm ospheric Gases Experim ent(AGAGE)[Prinn etal.,2000].In the CM IP5 archive,the concentrationsin
the earlierpartofthe historicalrunscom e from reconstructionsbased on airtrapped in polarice cores
[Myhreetal.,2013].ForCO2,forexam ple,observationsatM auna Loa,Hawaii,and atthe South Pole start
in the late 1950s,with earliervaluesbased on airextracted from ice coresand rn.From 2005 until2012,
the concentrationsin the Reference ensem ble are taken from the Representative Concentration Pathways
8.5 scenario (RCP8.5).Thisisthe sam e choice asin the subm itted NorESM historicalextended runsin the
CM IP5 archive.Forthe Sensitivity runs,observations are used where available from 1980 until2012 as
detailed below.
3.1.1.Carbon Dioxide
Thealternativecarbondioxideconcentrationswereobtained from theNOAA dataset(NOAA.2014,http://www.
esrl.noaa.gov/gm d/ccgg/trends/global.htm l.)andcoveredtheperiodof1980to2012.Figure1showstheabsolute
concentrationsofCO2 and resultantclim ateforcing relativeto preindustrialforcing asused in theReference
andSensitivityensem bles.Them ainfeatureoftheCO2concentrationsisasteadyincreasebetween1980and
2012thattotalsapproxim ately16% com pared to the1980leveloran increasein forcing of0.81W m 2.The
concentrationsare alm ostidenticalbetween the Reference and Sensitivity ensem bles,with only a sm all
differenceseen after2005.Thelargestdifferenceisseen in 2012and equatesto adecreaseof 0.02W m 2.
3.1.2.M ethane
The m ethane concentrationsforthe alternative forcingswere from the W orld M eteorologicalOrganization
(W M O)data set[Tsutsum ietal.,2009].Thisonly covered the period of1984 until2012,so concentrations
between 1980and 1983werem aintained attheirCM IP5values(Figure1).Thereisam arked system aticbias
betweentheCM IP5concentrationsusedintheReferencerunsandthealternativeconcentrationsusedinthe
Sensitivityruns.Thisresultsin an increase ofaround 2.5% in the forcing relative to preindustrialcaused by
m ethane oraround 0.01W m 2.The offsetisrelativelyinvariantwith tim e afterthe initialshockin 1983/1984.
Thisisinthespin-upperiodfortheensem bles(section4)andpredatesthehiatus,whichisthefocusofthepresent
studies,byapproxim ately15years.
3.1.3.NitrousOxide
Fornitrousoxide,thealternativeconcentrationswerealso obtained from theW M O dataset[Tsutsum ietal.,
2009].Since N2O hasbeen system atically observed since the late 1970s,these coverthe fullperiod ofthe
ensem blerunsfrom 1980until2012.Thealternativeforcingsshow am eandifferenceoverthehiatusperiod
of 0.0003W m 2 from the originalconcentrationsused in CM IP5 (Figure 1).Nitrousoxide concentrations
haveincreased steadilyoverthisperiod resulting in an increasein forcing of0.07W m 2.
JournalofGeophysicalResearch:Atm ospheres 10.1002/2015JD023859
OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8578
3.1.4.CFC-11
The m ostcom plete record ofCFC-11 concentration wasfound in the AGAGE data set(NASA-AGAGE,2014,
http://agage.eas.gatech.edu/data.htm .),and this was used to replace the concentrations ofCFC-11 forthe
Sensitivityruns.ItshouldbenotedthatinNorESM ,theCFC-11 eldisadjustedtoincorporatetheeffectoftrace
gasesincluding long-lived GHGsand ozone-depleting substances.Figure 1 com paresthe adjusted CFC-11
concentrationsandresultantclim ateforcingintheReferenceandSensitivityforcings.Thelargestchangesoccur
from 2009onward and resultin an increasein forcing relativeto preindustrialofapproxim ately0.004W m 2.
To incorporate the radiativeeffectofthetracegases,theyareconverted to an equivalentconcentration of
CFC-11and added tothat eld,givingrisetotheadjusted CFC-11.Thisisdoneusingthefollowingequation,
which utilizestheratio ofradiativeef cienciesofthetracegasescom pared to thatofCFC-11.
CFC-11equivalentofX pptð Þ¼ X pptð Þradiative efficiency ofX
radiative efficiency ofCFC-11
NorESM includesforcingsfrom num eroustracesgasesin thisway,asshown in Table1.Figure1 showsthe
adjustedCFC-11alongwiththetruem easureofCFC-11andtheequivalentconcentrationofalloftherem ain-
ing contributorytrace gasescom bined.Untilthe late 1980s,the com bined contribution ofthe trace gases
Figure1.(topleft)Concentrationsofcarbondioxide,(topright)m ethane,(m iddleleft)nitrousoxide,(m iddleright)CFC-11adjusted,and (bottom right)CFC-12in NorESM fortheReference(blue)and Sensitivity(red)ensem bles.Theseparation of(bottom left)CFC-11adjusted intoitscom ponentsofCFC-11(blacksolid)and tracegases(blackdashed)isalsogiven.Theconcentrationsofthetracegaseshavebeen converted to CFC-11equivalentconcentrationsusing therelativeradiativeef cienciesoftheindividualtracegases.Therightaxisineachplotshowstheforcingrelativetopreindustriallevels.Despiteappearancesoflinearityoversuch asm allrange,therightaxesforCO2,CH4,and N2O arenonlinear.
JournalofGeophysicalResearch:Atm ospheres 10.1002/2015JD023859
OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8579
wascom parable to thatofCFC-11;however,the successofthe M ontrealprotocol[UNEP OzoneSecretariat,
1987]anditssubsequentam endm entshasm eantthatCFC-11concentrationshavebeensteadilydecreasing
sincearound 1990,whilethetracegaseshavesteadilyincreased.Theneteffectisthatsince2008,thecom -
bined forcing ofthetracegasesism orethan doublethatofCFC-11 alone.
Sincethetracegasesaregenerallynotaswellm onitored asGHGs,itwasnotpossibleto nd alternativecon-
centrationsform anyofthem ,especiallyextending asfarbackas1980.Theirconcentrationsweretherefore
replaced using whateverobservationscould befound.CF4 and C2F6 wereboth replaced for2004until2012
from the AGAGE data set(NASA-AGAGE,2014,http://agage.eas.gatech.edu/data.htm .).CFC-113 was also
taken from the AGAGE dataset(NASA-AGAGE,2014,http://agage.eas.gatech.edu/data.htm .)forthe period
of1985 until2012.NOAA’s Halocarbons and otherAtm ospheric Trace Species Group (HATS)provided
the alternative concentrationsofHFC-134a,HCFC-141,CH3Br,CH3Cl,SF6,HCFC-22,HCFC-142,Halon-1211,
and Halon-1301 from 1994,1992,1993,1999,1995,1991,1992,1992,and 2004 respectively (NOAA HATS,
2014,http://www.esrl.noaa.gov/gm d/hats/data.htm l.).Finally,CFC-114 and CFC-115 were taken from the
AGAGEdataset(NASA-AGAGE,2014,http://agage.eas.gatech.edu/data.htm .),startingfrom 2006.Thism eans
that27% oftheannualconcentrationsofthetracegaseswerereplacedduringtheperiod of1980until2012.
Thisincreasestoaround40% fortheperiodof1990until2012andtoaround70% for2000until2012.Thenet
changein forcing bythesetracegasesoverthehiatusperiod is0.003W m 2.
3.1.5.CFC-12
The AGAGE data setprovided the alternative CFC-12 concentrations (NASA-AGAGE,2014,http://agage.eas.
gatech.edu/data.htm .).Thesecoverthefullperiod of1980until2012and areshown in Figure1along with the
CFC-12 concentrationsfrom the Reference ensem ble.The alternative forcingsrelative to preindustriallevels
areconsistentlyhigherthanthosefoundinCM IP5runs,witham eandifferenceofaround12pptor0.004W m 2.
3.1.6.Sum m ary ofGHGs
M ostoftheGHGsshow littlechangefrom theCM IP5concentrationsusedintheReferenceruns,withthelargest
changearisingfrom thealternativem ethaneconcentrations.Thecom binedeffectofalloftheGHGsisgivenin
Figure2.Theaveragedifferencebetween theseforcingsin the two ensem blesrelativeto preindustriallevels
Table 1. Long-Lived Trace Gases and Ozone-Depleting Substances Included in NorESM as Partofthe AdjustedCFC-11 Concentration
TraceGas RadiativeEf ciencyM ean Concentration Between
1980 and 2012(ppt)EquivalentConcentration
ofCFC-11(ppt) M odied From
CFC-11 0.25 239 239 1980CF4 0.1 71.2 28.5 2004C2F6 0.26 2.6 2.7 2004C6F14 0.49 0.02 0.03 –HFC-23 0.19 12.6 9.6 –HFC-32 0.11 1.3 0.6 –HFC-43-10 0.4 0.1 0.2 –HFC-125 0.23 2.0 1.8 –HFC-134a 0.16 17.2 11.0 1994HFC-143a 0.13 3.3 1.7 –HFC-227ea 0.26 0.3 0.4 –HFC-245fa 0.28 2.0 2.3 –SF6 0.52 3.8 8.0 1995CFC-113 0.3 66.7 80.0 1985CFC-114 0.31 15.3 18.9 2006CFC-115 0.18 6.4 4.6 2006Carb-Tet 0.13 97.5 50.7 –M CF 0.06 73.2 17.6 –HCFC-22 0.2 124.9 99.9 1991HCFC-141 0.14 8.9 5.0 1992HCFC-142 0.2 8.7 7.0 1992Halon 1211 0.3 3.0 3.6 1992Halon 1301 0.32 2.0 2.6 2004Halon 2402 0.33 0.3 0.4 –CH3Br 0.01 8.7 0.3 1993CH3Cl 0.01 520.6 20.8 1999
JournalofGeophysicalResearch:Atm ospheres 10.1002/2015JD023859
OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8580
overtheperiodof1980to2012is0.014W m 2;approxim ately0.01W m 2ofwhichcom esfrom thealternative
m ethane,while around 0.004W m 2 ofwhich com esfrom the alternative CFC-12.However,the com bined
effectofthe alternative greenhouse gasforcings,including the long-lived GHGsand ozone-depleting sub-
stances,isan increase ofonlyaround 0.6% oftotalGHG forcing relative to preindustrialand correspondsto
arelativeerrorofaround1.2% onthechangeofforcingsbetween1980and2012.Them eannetchangeinfor-
cing from GHGsincluding long-lived trace gases and ozone-depleting substancesforthe hiatusperiod is
0.012W m 2.
3.2.AnthropogenicAerosols
NorESM includesthreetypesofaerosolsotherthan volcanic:blackcarbon (BC),sulfurdioxide(SO2),and pri-
m aryorganicm atter(POM )[Kirkevågetal.,2013;Iversen etal.,2013].Theseareread into them odelasem is-
sions(kgm 2s 1)from both fossilfueland biom assburning sources.Blackcarbon isalso read in based on
em issionsfrom airtraf c.Thesearede nedateverylocationandoneverylevelasm onthlyaverages;however,
theyareonlyreadinforasingleannualcycleonceperdecade.Forexam ple,inthehistoricalrunsinCM IP5,12
m onthlyaverageswereread in for1850,1860,etc.until1990and 2000,with a nalannualcycleincluded for
2005,the end ofthe sim ulation.However,in the Reference runsdiscussed in thiswork,the 2005 pointis
ignoredandinterpolationisperform edbetweenthe2000historicalem issionsandthe2010RCP8.5em issions.
NorESM perform salinearweightingtodeterm ineaerosolem issionsfortheyearsbetweentheinputdecades.
Alternative aerosolem issionswere obtained from the Norwegian M eteorologicalInstitute and were taken
from the EU-Eclipse V4adata set[Klim ontetal.,2013a,2013b].These included new annualcyclesforeach
decadebetween1970and2000,andforeachyearbetween2005and2010.Theyalsoincludeda nalannual
cycleestim ated for2020to ensurethatNorESM could interpolateem issionsfor2011and 2012.Thealterna-
tiveem issionsdonotincludenew valuesforblackcarbonem itted byairtraf casthisdatawereunavailable.
TheoriginalReferenceandnew Sensitivityem issionsareshowninFigure3.Inordertoascertaintheradiative
im pactoftheperturbationsto theaerosolsem issions,weperform ed asetofauxiliarysim ulations.Because
the aim isto estim ate the effectrelative to Preindustrial(PI)atm ospheric burdens,three setsofrunsare
required.Theserunsareundertaken utilizing,respectively,thefollowing:Preindustrial(m ainlynatural)em is-
sionswithnotim evariationbeyond aseasonalcycle(PI),tim e-varyingCM IP5em issionsasused inReference
(Ref),and tim e-varying Eclipse V4aem issionsasused in Sensitivity(Sens).In allcases,the radiation (direct)
andcloud nucleation( rstindirecteffect)im pactsarecom putedin“online”m odeusingtheprescribed aero-
solconcentrationpathwaysprovidedbyNationalCenterforAtm osphericResearchaspartoftheCom m unity
Atm osphereM odel.Thereafter,theyarerecom puted in “of ine”m odeusing theaerosolconcentrationsand
aerosol-cloud-radiation calculations thatresultfrom the distinctem ission scenarios in PI,Ref,and Sens.
[Kirkevåg etal.,2013].The nalaerosolclim ate forcing wasestim ated asthe difference between Ref-PIand
Sens-PIin thecalculated of inetop ofatm osphere(TOA)netradiation.
The alternative em issionsshow higherlevelsofblack carbon after2000,which provide positive clim ate
forcing bydecreasing thealbedo and allowing theatm osphereto absorb m oreheat.Theyalso show higher
levelsofSO2,which serveto stronglycooltheatm osphere.Finally,Prim aryOrganicM atter(POM )em issions
Figure2.Com bined forcingsrelativetopreindustriallevelsforCO2,CH4,N2O,CFC-12,and theadjusted CFC-11,includingtracegasesforthe(left)Referenceensem ble(blue)and theSensitivityensem ble(red).(right)ThedifferenceofSensitivitym inusReferencein forcingsisalso shown.
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OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8581
are lowerwith am arked relative m inim um in 2009.POM em issionsare “brown”and so have both positive
and negative directeffects.However,theirindirecteffects are undoubtedly a negative RF [Myhre etal.,
2013,Table 8.4].By design,the Sensitivity aerosolem issions exhibit substantively greater interannual
variabilitythan thoseused in Referenceoverthehiatus.Thedifferencesbetween theaerosolancillariesare
substantialoverthisperiod.Allofthe aerosolsundoubtedly contribute to the difference in RF in Figure 3
(bottom right).However,Figure 3 suggeststhatthe changesin POM are dom inantasthe peak in 2009
coincideswith arelativem inim um in BC and thestrong m inim um in POM .M axim um relativechanges(note
thatFigure3yaxisrangesdonotstartfrom zero)aregreaterinPOM (~25% )thaninSO2(~5% )orBC (~10% ).
IfBC werethedom inantfactor,thepeakinthedifferencebetweenReferenceand SensitivityRFwouldoccur
in 2010.Furtheranalysisbeyond thecurrentstudywould beneeded to diagnosetheim pactsoftheindivi-
dualaerosolsin them odel.
Regardless,them ean netdifferencein radiativeforcing between theReferenceand theSensitivityaerosols
overthehiatusperiod is0.11W m 2,with thelargestdifferenceoccurring in 2009and reaching 0.28W m 2.
Changesinallthreesetsofem issionsarelikelytobeim portant,butthechangeinPOM em issionsdom inates
thechangein overallRFbetween theReferenceand theSensitivityforcings.
3.3.Solar
Solarforcing isincluded inNorESM asanannualm ean valueoftotalsolarirradiance,with diurnaland seaso-
nalcyclesinsolarforcingappliedthroughthem odelcode.ThevaluesusedintheNorESM CM IP5sim ulations
werebasedonLean[2000].Thesewereadjustedbyafactorof0.9965basedontheworkofW angetal.[2005]
to bring them in line with observationsfrom the TotalIrradiance M onitor(TIM ),a space-based instrum ent
launched in 2003 as part ofNASA’s Earth Observing System SOlar Radiation and Clim ate Experim ent
(SORCE).From 2009 onward,the solarforcing in CM IP5 wasarepeatofthelastfourcyclesofthe historical
Figure 3.Totalaerosolem issionsfor(top left)blackcarbon,(top right)prim ary organicm atter,and (bottom left)sulfurdioxidefrom biom assburning and fossilfuelsfortheReference(blue)and Sensitivity(red)em issionsused in thiswork.Theem issionsfortheReferencerunareprescribedateachdecadefrom 1970until2020;whiletheRCPscenariosincludeavalueat2005.In theSensitivityruns,theem issionsareprescribed ateach decadefrom 1970to 2000and ateach yearfrom 2005to2010,with a nalvalueprescribed at2020.(bottom right)Thenettop ofatm osphereforcingassociated withthechangein allaerosolsisalso shown.
JournalofGeophysicalResearch:Atm ospheres 10.1002/2015JD023859
OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8582
record. Som e concerns have been
raised over this approach since the
cyclefrom 1996until2008wasinreality
only12.2yearslong,not13yearsasitis
in the m odel[Lean and Rind,2009].
Furtherm ore,the two cycles prior to
1996 were also atypical, being two
of the shortest on record.The solar
forcing in our30-m em berensem bleof
Reference runs was identicalto those
of the CMIP5 sim ulations, including
therepeated cyclefrom 2009onward.
The Sensitivity ensem ble uses the
solarforcings from the Physikalisch-
MeteorologischesObservatorium Davos
(PM OD)thatare based on a com bina-
tion ofdata from severalinstrum ents:
Active Cavity Radiom eterIrradiance Monitor-I(ACRIM -I),ACRIM-II,ACRIM -III,Variability ofsolarIRradiance and
GravityOscillations(VIRGO),TotalIrradianceM onitor(TIM),and Earth Radiation BudgetExperim ent(ERBE).The
PM OD data are generally considered the m ostaccurate setofsolarforcings,given how itwasconstructed
[FröhlichandLean,1998;Fröhlich,2006].Uncertaintyin solarforcingsisdiscussed furtherin section 6.1.1.
Itshould benoted thatwhilethedifferencebetween thePM OD solarforcing used in Sensitivityand thatof
the originalNorESM CM IP runsused in Reference reaches 0.5W m 2 in 2009,thisisthe totalsolarirradi-
ance.TheEarth receivesthisradiation effectivelyasatwo-dim ensionaldiskratherthan athree-dim ensional
sphere,andhencewithaquarterofthesurfaceareathatittrulyhas.Therefore,theeffectiveforcingisshown
on the leftaxisin Figure 4,and thisisone quarterofthe actualTOA forcing.The difference between the
PM OD forcingsandtheNorESM originalforcingisthereforeuptoaround 0.125W m 2,althoughthisvaries
with tim e,with them ean overthe hiatusperiod being 0.08W m 2.Thisisan orderofm agnitude greater
than thechangein GHGsasdiscussed in section 3.1(Figure2)and potentiallycom parable,butofopposite
sign,to thehighlyuncertain effectofaerosolchanges(section 3.2).
3.4.Volcanic
Theforcing effectofvolcanicaerosolsisincluded in NorESM through azonalm assm ixing ratio specied for
each latitudeand on each verticallevel.In practice,them odelusesthezonalm ean colum n m assofaerosols
distributed overtheverticallevelsbyasim pleshapefunction.Thecolum n m assofvolcanicaerosolsused in
NorESM wasbased on the work ofSato etal.[1993].The lastvolcaniceruption included in thisdata setis
Pinatubo in 1991.Tracesofthiseruption existuntil2001,afterwhich,there isno volcanicaerosolforcing
in NorESM .SincePinatubo,therehavebeen aseriesofsm alleruptionseach adding sm allquantitiesofaero-
solsto theatm osphere[Santeretal.,2014;Ridleyetal.,2014].
Alternativevolcanicaerosolconcentrationswereobtainedfrom theNASA GoddardInstituteofSpaceScience
(NASA-GISS,2014,http://data.giss.nasa.gov/m odelforce/strataer/.)and arebased on theupdateofthetabulation
ofstratosphericaerosolopticalthicknessgiveninSatoetal.[1993].Figure5com paresthecolum nm assofvolcanic
aerosolsforthe originalNorESM concentrationsand the alternative concentrations,along with the associated
latitudinallyintegrated forcings.The forcingswere calculated using the sam e form ulaaswasused in IPCC AR5
(Table 8.SM .9),where RF= 25*Atm ospheric OpticalDepth in W m 2 [Hansen etal.,2005].The weaker
volcanoesare clearlym issing from the Reference concentrations.Furtherm ore,the aerosolsfrom the Pinatubo
eruption have decreased in peak m agnitude in the alternative concentrationsbuthave a greaterpresence at
m iddle and high latitudes.Assum ing the presence ofa long-term (m ultidecadal)response to episodic large
volcanoes[Gregory,2010],sucham isspecicationm ayhaveim plicationsform odelbehaviorwithinthehiatus
period even though thiseruption occurred priorto this.Specically,m odelsim ulationsusing the Reference
forcingsm ay show lowerinitialocean heatcontent(OHC)valuesthan sim ilarm odelsim ulationsusing the
alternativeforcingsasaresultofthestrongerspecied volcanicaerosolsin theReferenceforcing volcanic
seriesin theearly1990s.Thiscom plicatesaclean com parison ofthelikelyeffectoftheforcing differences.
Figure 4.Solarforcing in W m 2 ofthe Reference forcing from NorESM(blue)and theSensitivityforcingsfrom Physikalisch M eteorologischesObservatorium Davos(red).
JournalofGeophysicalResearch:Atm ospheres 10.1002/2015JD023859
OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8583
Them ean changein forcing for1980
to 2012 between the Reference and
Sensitivity forcings is 0.17W m 2.
Overthe hiatus period,the alterna-
tive forcings exhibit system atically
greaterloadingsthatwould im parta
cooling in uence.The m ean change
in forcings overthe hiatus period is
0.08W m 2,which com pares well
to the estim ate from Box 9.2 ofthe
IPCC reportofaround 0.09W m 2.
3.5.ForcingsNotAltered
3.5.1.StratosphericW aterVapor
Between 2000 and 2005,the strato-
spheric water vapor shows a sharp
decline;however,the concentrations
have since risen steadily until2012
[Dessleretal.,2013].A recentstudy
byUrban etal.[2014]hasshown that
anothersharp drop in stratospheric watervaporconcentrations has occurred overthe last18m onths.
Decreasesin stratospheric watervaporare expected to warm the stratosphere butcoolthe troposphere
[Solom on etal.,2010].Although thisideawasdisputed byKaufm ann etal.[2011],who found no statistically
signicantrelationship between changing stratosphericwatervaporand surfacetem peratures,Dessleretal.
[2013]quantied thefeedbackofstratosphericwatervaporto beapproxim ately0.3W m 2K 1.
Unfortunately,thestratosphericwatervaporconcentrationsarenotread into NorESM and cannottherefore
be changed directly.This,to som e extent,re ectsthe realitythatstratosphericwatervaporalthough de -
nitelyradiativelyim portantisstrictlyspeakingam echanism ofinternalfeedback/variabilityratherthanwhat
m ayclassicallybeconsidered asaforcing which isan externaldrivertothesystem .In an attem ptto include
theeffectofchanging stratosphericwatervapor,theeffectofm oistureon variouslevelswasadjusted inthe
longwaveand shortwavecom ponentsofthecode.Thiswould havetheeffectofchanging thewatervapor
on those levelsforradiative purposes.However,the resulting tem perature response to thischange was
contraryto existing theoryand literature on thissubjectand no explanation could be determ ined forthis.
Sincewecurrentlyhaveno otherm ethod foradjusting thestratosphericwatervaporin theNorESM m odel,
and theonlym ethod wedohavepro-
duces results that are inconsistent
withthecurrentliterature,stratospheric
water vapor was not altered in the
Sensitivityrunsin thisstudy.
3.5.2.Ozone
NorESM includes ozone concentra-
tions as a forcing ancillary eld.
These concentrations are spatially
com plete,being read in foreverygrid
point and on every m odel level.
However,the ozone concentrations
are included in the m odelon a tem -
porally interm ittentbasis.Forexam -
ple,in the historicalm odelruns,the
m onthly ozone concentrations fora
single yearare read into the m odel
every 10yearsfrom 1850 until1990,
and every 5years from 1995 until
2005 (Figure6).
1840 1860 1880 1900 1920 1940 1960 1980 2000 20207.5
8
8.5
9
9.5
10
year
ozon
e co
ncen
trat
ion
[1e−
6 m
ol/m
ol]
Figure 6.M onthly ozone concentrations input to NorESM once every10yearsuntil1990 and every5yearsuntil2005.
Figure 5.Colum n m assofvolcanicaerosolsin kgm 2 bylatitude with them eridionallyintegrated forcingsunderneath,forthe(top)Referenceforcingsfrom theNorESM CMIP5runsand the(bottom )Sensitivityforcings.Theseareboth based on theworkofSato etal.
JournalofGeophysicalResearch:Atm ospheres 10.1002/2015JD023859
OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8584
The input frequency of ozone into
NorESM is insuf cient to resolve
its decadal scale im pacts which
would require forcing estim atesthat
varied continuously in space and
tim e.W hile com plete observationsof
vertically resolved ozone across the
globeforthewholeperiod ofinterest
are available, since no hypothesis
has been proposed in the literature
professing the role ofozone in caus-
ing the currently observed warm ing
hiatus,the ozone driving eld has
rem ained unchanged in ourcurrent
experim ent.A m ore com plete study
would be able to incorporate m ore
accurate ozone elds into NorESM .
This would not only allow for the
investigation ofthe im pactofchan-
ging ozoneconcentration on decadal
tim escalesbutwould alsohelp quantifytheim portanceofm oretem porallycom pleteozonerepresentation
in futureruns.
3.5.3.Land Surface Changes
NorESM includestheCom m unityLandSurfaceM odelversion4asitslandsurfacecom ponent.Thelandusedata
wereunchanged betweentheReferenceandSensitivityrunsand isthesam easthelandusedataspeciedfor
CM IP5 sim ulations.These consistofland use estim atesfrom theHistoryDatabaseofthe GlobalEnvironm ent
sm oothedtocom bineseam lesslywiththefuturepredictionsundertheRCP8.5scenario.Thelandusedatawere
notm odied in thisstudyprim arilybecause atthe tim e ofthe projectsexecution,land use changeshad not
beenpositedasam ajorfactorincausingorexplainingtheglobalwarm inghiatus.Furtherm ore,wedidnot nd
an observationaldatasetofland usethatwassignicantlydifferentfrom thestandard inputofNorESM.
3.6.Sum m ary ofCom bined Radiative EffectsofChanged Forcings
In thissection,we have docum ented changesto anum berofforcing ancillariesre ecting the observational
understanding attim eofprojectinception in early2014.Changeswerem adeto GHGsand long-lived green-
house gases (LLGHGs)(section 3.1),anthropogenic aerosols (section 3.2),solarradiation (section 3.3),and
volcanicaerosols(section3.4).A num berofadditionalforcingswerenotaltered(section3.5).Alloftheforcings
thatwererevisitedwerealtered withsom ebecom ingm orepositive/lessnegative(LLGHGsandanthropogenic
aerosols)and otherslesspositive/m orenegative(solarandvolcanoes).On average,theSensitivityTOA forcing
isslightlylesspositiveovertheperiodofthehiatus.However,changestoallfourforcingsaretim evariantsuch
thatthe totaldifference in netforcing between the two ensem blesvariesconsiderablyovertim e (Figure 7).
During theend ofthetwentieth century,theforcing differencesarestronglydom inated byvolcanicaerosols,
with theElChichióneruption in1982and M ountPinatuboin1992;however,itshould benoted thatthealter-
nativeforcingsincludenonew anthropogenicaerosoldataduringthisperiod.Overthehiatusperiodof1998to
2012,thedifferencein solarforcing becom esapparentand ofcom parablem agnitudeto thevolcanicforcing.
Thecom bined effectofthevolcanicand solarforcingisneverthelessm orethancom pensatedforbythesharp
risein anthropogenicaerosolforcing,which resultsin anetpositivedifferencein forcingsbetween 2008and
2010.Them eannetforcingdifferencebetweentheReferenceandSensitivityensem blesoverthehiatusperiod
is 0.03W m 2,although itvariesbetween 0.10W m 2 and 0.07W m 2.Thelineartrend in forcing ism ore
positiveforSensitivitythan itisforReferenceoverthehiatusby0.08W m 2dec 1.
4.M odelConfiguration and Ensem ble Creation4.1.M odelSystem
This study uses the m edium resolution con guration ofthe Norwegian Earth System M odelversion 1
(NorESM 1-M )which has provided output to the fth Coupled M odelIntercom parison Project (CM IP5)
1980 1985 1990 1995 2000 2005 2010−0.5
−0.4
−0.3
−0.2
−0.1
0
0.1
0.2
0.3
year
Figure 7.Netdifference ofSensitivity m inus Reference in radiative forcing(black).Thiscom binesthedifferencesin forcingsforGHGsand LLGHGs(green),anthropogenicaerosols(red),volcanoes(blue),and solarirradiance(m agenta).
JournalofGeophysicalResearch:Atm ospheres 10.1002/2015JD023859
OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8585
[Tayloretal.,2012].The m odelisbased on the Com m unityClim ate System M odelversion 4 (CCSM 4)[Gent
etal.,2011]withim portantchangesbeingthereplacem entofthezleveloceancom ponentwithanisopycnic
coordinateocean m odel,im proving therepresentation ofwaterm asses,and theem ploym entofadvanced
chem istry-aerosol-cloud-radiation interaction schem es,enabling the m odelto account for the indirect
aerosolfeedback.The sea ice and land com ponents— the LosAlam osSea Ice M odeland the Com m unity
Land M odel— have been adopted from CCSM 4 withoutchanges.The atm ospheric and land com ponents
arecon gured on aregularhorizontalgrid with a1.9°×2.5°resolution.In thevertical,theatm osphericcom -
ponentcom prises26hybridsigm a-pressurelevelsextendingupto3hPa.Theoceanandseaicecom ponents
arecon guredonacurvilinearhorizontalgridwith1°resolutionalongtheequatorandthenortherngridsin-
gularityshiftedoverGreenland.Inthevertical,theoceancom ponentcom prisesatwo-layerbulkm ixed-layer
representation with 51 isopycnic layers beneath.A detailed description ofthe m odeland evaluation of
standard sim ulationsaregiven in Bentsen etal.[2013]and Iversen etal.[2013].
4.2.Experim entalSetup
W ehaveperform ed twosetsofsim ulations,oneReferenceensem blewhichusesthedefaultCM IP5forcings,
and one Sensitivity ensem ble which uses the alternative forcings outlined in section 3.The sim ulations
covered theperiod of1980until2012.Thisperiod waschosen forthreereasons.
1. Itincludestheperiod ofinterestforstudying theobserved hiatusofglobalwarm ing.
2. Sincethehiatusstarted in the1990s,thischoiceallowsthem odeloveradecadein ordertoadjusttothe
m odied forcings,thusavoiding sharp changesin theforcingsduring theperiod ofinterest.
3. Starting overa decade priorto the period ofinterestallowsthe individualrunsto spin-up and diverge
such thattheyexhibitabroad rangeofbasicstatesofthe m odeled Earth system (including cryosphere
and ocean)consistentwith them odelphysicsbythestartoftheperiod ofstudy.Theend dateisdriven
bytheavailabilityofsom eoftheSensitivityrun-based forcingsbeing solelythrough 2012.
To allow fora robustdetection ofany differencesin forced response and an elucidation ofthe effectsof
m odelinternalvariability on decadaltim e scales,each ensem ble com prisesofa totalof30 sim ulations.
The initialconditions are generated from the three m em bers ofNorESM ’s CM IP5 historicalexperim ent
[Bentsen etal.,2013]asfollows:Both setsaresplitinto threesubsetsof10sim ulationseach.Thesim ulations
ofeach subsetare allinitialized with the1980-01-01 stateofthesam e CM IP5 NorESM historicalsim ulation,
i.e.,thesim ulationsofsubset1areinitializedwiththestateofthe rstCM IP5historicalrealization,thesim ula-
tionsofsubset2areinitializedwiththestateofthesecondCM IP5historicalrealization,andthesim ulationsof
subset3 are initialized with the state ofthe third CM IP5 historicalrealization.The three historicalrunsthat
werethem selvesspun offthem odelcontrolrun separated byseveraldecadesand by1980haveeach been
running for130years.W ithin each subset,the initialspread isgenerated by adding m icroscopic(O(1 6K))
noise to the ocean m ixed-layertem peratures.The internalspread then growswith tim e and (atleastfor
theatm osphere)reachessaturation afteradecadeorso,i.e.,beforeourperiod ofprim aryinterest.
5.Characterization ofthe M odel
Asdiscussed previously,variousalternative hypothesesotherthan forcingsinvolving internalvariabilityor
feedbacks within the system have been proposed to explain the observed hiatus in globalwarm ing.
Beforedetailed analysisofthetwo ensem blescan beundertaken,itisim portanttoplaceNorESM in context
ofthewiderrangeofCM IP5m odelsand to establish whetherornottheintroduction ofthealternativefor-
cingshasim pacted these alternative m echanism sin the m odel.Figure 8 showsthe Equilibrium Clim ate
Sensitivity(ECS)againsttheTransientClim ateResponse(TCR)forarangeofCM IP5m odels(Thisisam odied
version ofFigure9.42 from Chapter9 ofIPCC AR5)[Flato etal.,2013].NorESM hasan ECSofapproxim ately
2.8°C and aTCR ofapproxim ately1.4°C and thuslieswellwithin thespread ofCM IP5m odels.W hileitdoes
lie furtherfrom the CM IP5 ensem ble m ean than the CCSM 4 m odelused in the workofSanteretal.[2014],
NorESM isapparentlynotunusualam ong theCM IP5m odelsforitsresponseto changesin clim ateforcings.
The powerspectrum density ofglobalm ean tem perature variance in the historicalsim ulationsofCM IP5
m odelsisgiven in Figure9(Thisisam odied version ofFigure9.33from Chapter9ofIPCC AR5)[Flatoetal.,
2013].NorESM isgenerally consistentwith observationsand showsreasonable powerathiatus-like tim e
scalesthatisin broad with both otherm odelsand theobservations.
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OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8586
5.1.Changesin Ocean HeatContent
Variousstudieshavediscussed thepossi-
bilityofincreased heatsequestrationinto
the world oceans[e.g.,Meehletal.,2011,
2013, 2014; Chen and Tung, 2014;
Allan et al.,2014;England et al.,2014].
Figure 10 showsthe anom aliesofglobal
OHC forthetwoensem blesintheshallow
ocean and the deep ocean,de ned as
the surface to 325m and below 325m ,
respectively. Both ensem bles show a
sim ilarincrease occurring between 1980
and 2012 ofapproxim ately 100ZJin the
shallow ocean and 200ZJ occurring in
thedeeperocean.ThetotalOHC increase
of approxim ately 300ZJ is sim ilar to
the observational estim ate reported in
the IPCC report [Rhein et al., 2013].
Both ensem bles show with a sim ilar
m agnitudelocalm inim aaround 1992resulting from thePinatubo eruption thataregreatestin theupper-
m ost325m .
Intheshallow ocean,theSensitivityensem bleshowsincreasedvariabilityoverthehiatusperiod,especiallyafter
2006 when the RCP8.5 forcingswere replaced with observationalestim ates.The Sensitivityensem ble in the
deep ocean hasapproxim atelythe sam e variabilityasthe Reference ensem ble butwith aslightlydecreased
m ean.Thisistobeexpected sincethetotaloceanheatcontentshouldbeverysim ilartothetotalEarthsystemheatcontentwhich iscontrolled bytheforcings.Sincein thenettheSensitivityforcingsshow adecrease,wewould expectthe totalOHC anom alyto also decreasesom ewhat.Figure 10 suggeststhatthetrendsin OHC
during thehiatusperiod of1998to 2012areslightlyhigherin theSensitivityensem blethan in theReference
ensem bleforboth theshallow and deep oceans.Figure10furthersuggeststhatthereisgreaterheatcontent
increaseinthedeepoceanandlessheatcontentincreaseintheshallow oceanduringthehiatusperiodthanin
thewarm ing periodoftheearly1990s.Toelucidatethis,acom parison ofthetrendsinOHC fordeep and shal-
low oceansoverthehiatusperiodisgivenforall30m em bersofboththeReferenceandSensitivityensem bles
(Figure11).Thetrendswerecalculatedfrom thegloballyaveragedOHC annualseriesbyordinaryleastsquares
(OLS)regression overthe period 1998–2012.As expected [Meehletal.,2013],there is an anticorrelation
wherebywhen near-surfaceuptakeisrelativelyrapid,deep ocean uptakeisrelativelyslow and viceversa.
ThetrendsofthetotalOHC wereexam ined forthehiatusperiod and found to be13.4ZJyr 1and 13.9ZJyr 1
fortheReferenceand Sensitivityensem bles,respectively.A student’sttestwasappliedtoshow thatthetrends
inOHCbetweenthetwoensem blesweredifferentatthe5% signicancelevel.Overall,theinclusionofthealter-
nativeforcingsappearstohavehadm inim alim pactonthebehaviorofthem odeledOHCintheshallow ocean.
In the deep ocean,the alterna-
tive forcings reduce the m ean
OHCintheSensitivityensem ble
com pared to the Reference
ensem ble,butincreased trends
over the last 15years act to
decrease this difference by
the end ofthe run in 2012.A
recent paper by M ori et al.
suggested the need fora high
num berofensem ble m em bers
(atleast80)in orderto detect
asignicantresponseinsurface
airtem perature to changes in
Figure 9.Globalclim ate variability as represented by power spectraldensity for1901–2010globalm ean surfacetem peratureforboth historicalCM IP5sim ulationsandtheobservations.Thegreyshadingprovidesthe5to95% rangeofthesim ulations.Thisisam odied version ofFigure9.33b from Chapter9oftheIPCC AR% [Flatoetal.,2013].
Figure 8.Com parison ofthe Equilibrium Clim ate Sensitivity(ECS)andTransientClim ateResponse(TCR)ofNorESM to otherCM IP5m odels.Thisisam odied versionofFigure9.42from Chapter9oftheIPCC AR5[Flato etal.,2013].
JournalofGeophysicalResearch:Atm ospheres 10.1002/2015JD023859
OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8587
boundaryconditions.AswillbediscussedinPart2,m orerunsintheSensitivityensem blebeganthehiatusper-
iodwithaLaNiñasituationandendwithanElNiñosituationthanintheReferenceensem ble.Thiscouldinpart
explain thedifferencein OHC observed here.
5.2.ENSO Variability and Trends
Theocean tem peraturesoftheEastern Pacichavebeenofparticularinterestduetotheirrolein drivingthe
El-Niño–SouthernOscillation(ENSO).Com paringthetem peratureanom aliesovertheENSO 3.4regionshows
thatthefrequency,m agnitude,and seasonalityofthevariationsareallcom parableto thosein theobserva-
tionsforboth theReferenceand Sensitivityensem bles(Figure12).Theperiodogram highlightsthatabroad
peak in variability occursbetween 2 and 7years,in agreem entwith the observed variability ofENSO.The
introductionoftheSensitivityforcingsappearstohavehadnosignicantim pactonENSO asitisreproduced
byNorESM ,based on astudent’sttestofthepowerspectrum .
5.3.TOA Radiation
Thenetim balanceofradiation atthetop ofatm osphereprovidesinsightinto both thechangesin radiative
forcing and the changesin clim ate response.Thisnetim balance hasbeen shown to capture interannual
variability in the clim ate system from both volcanoesand ENSO [Allan etal.,2014]and hasbeen shown
to change in response to lower
frequency unforced m odes such as
the Interdecadal Pacic Oscillation
[Brown etal.,2014].Figure 13 shows
this netim balance ofTOA radiation
from the Reference and Sensitivity
ensem bles,com paredtotheobserva-
tionsfrom theCloudsand theEarth’s
Radiation EnergySystem (CERES)and
the Earth Radiation BudgetSatellite
(ERBS) as prepared by Allan et al.
[2014].The observations only begin
in 1985 atthe startofthe ERBS data.
The volcaniceruptionsofElChichón
and Pinatubo are clearly visible in
the 1980s and 1990s, respectively,
although the observations do not
cover the period ofthe ElChichón
eruption.Thereisreasonableconcur-
rence between the observational
record and the m odel ensem bles
overtheperiod thattheyoverlap.
20 25 30 35 40 45 50 55 60 65 7070
75
80
85
90
95
100
105
110
Figure 11.Crossdependency ofOHC changesbetween shallow and deepoceans(boundaryde ned at325m )over1998–2012in theReference(blue)and Sensitivity(Red)ensem bles,witheachpointdescribingthevaluesforasingle-ensem blem em ber.TrendsinOHChavebeencalculatedfrom thegloballyaveraged OHC annualseriesbyOLSregression overtheperiod1998–2012.Thenum bersin theupperrightcornerdenoteR2 values.
Figure10.Anom aliesofglobalm ean ocean heatcontentfortheReferenceensem ble(blue)and theSensitivityensem ble(red)in Zeta(1021)Joulesfor(left)shallow and (right)deep oceans.Shallow and deep oceansarede ned asfrom thesurfaceto 325m and below 325m respectively.
JournalofGeophysicalResearch:Atm ospheres 10.1002/2015JD023859
OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8588
5.4.ArcticSea Ice Extent
Finally,the Arctic sea ice extent,previously linked to cooling overm idlatitude Eurasia [Petoukhov and
Sem enov,2010;Outten and Esau,2012;Honda etal.,2009],isshown in Figure 14 forthe two ensem bles
and com pared to observationalestim ates from the NationalSnow and Ice Data Centre [Fettereretal.,
2002].TheNorESM m odelhasclearde cienciesinitsseasonalcycleofArcticregionseaice,havingaseasonal
cycle thatistoo sm all,even though the m ean extentoverthe calendaryearm ay be broadly correct.The
wintertim em axim um istoo sm all,and thesum m ertim em inim um too large.Thism isspecication m aylim it
thevalueofNorESM forconsidering atleastsom easpectsofseaice-atm ospherefeedbacks.
Differencesin thetrendsand variabilityofseaiceextentbetween thetwo ensem blesarenotsignicantat
the5% levelaccording toastudent’sttest;hence,theintroduction oftheSensitivityforcingshasnotsigni-
cantlyalteredthispotentialsourceofclim atefeedback.Inbothensem bles,them odelshowsaslightlyslower
decreaseinthewintertim eseaiceextentthanisseeninobservations,anditfailstoreproducethesteepdrop
in sum m ertim eseaiceobserved overthepast10–15years.
1981 1986 1991 1996 2001 2006 2011−4
−3
−2
−1
0
1
2
3
year
tem
pera
ture
ano
mol
y [K
]
101 10210−2
100
102
Period (months)
Pow
er
Figure12.Tem peratureanom aliesinthe(top)ENSO 3.4regionfrom HadCRUT4(black)[Moriceetal.,2012],theReferenceensem ble(blue),and Sensitivityensem ble(red),with theaccom panying (bottom )periodogram .Thesolid verticallinesontheperiodogram indicateperiodsof2yearsand 7years.
Figure 13.Netim balance in radiation atthe top ofthe atm osphere from the Reference ensem ble (blue)and Sensitivityensem ble(red),com pared to observationsfrom CERESand ERBS[Allan etal.,2014].Plotted arethe95% bootstrapcon dencebounds(shading),togetherwith ensem blem eans(solid line).
JournalofGeophysicalResearch:Atm ospheres 10.1002/2015JD023859
OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8589
6.Discussion
TherecentIPCCreport[Flatoetal.,2013]proposedthatthereducedtrendintheexternalforcings,largelyasa
resultofsolarand volcanicfactorswith apoorlyquantied contribution from anthropogenicaerosols,wasa
factorroughlyequalin im portto internalvariabilityin explaining thecurrentlyobserved warm ing hiatus.To
investigate the role offorcings,the forcingsin the Norwegian Earth System M odeldue to prim ary green-
housegases,solarirradiance,volcanicaerosols,anthropogenicand biom assburningaerosols,and anum ber
oflong-lived GHGsand ozone-depleting substanceshavebeen m odied based upon m ultiplestate-of-the-
artobservationalsources.Thesem odicationsnotonlyincluded new valuesfortheperiod of2006to 2012,
i.e.,beyond theCM IP5historicalrun,butalso m odicationsfortheperiod priorto 2006based on im proved
understandingand associated datasetinnovationsand updates.Toassessthepossibleroleofinternalvaria-
bility,eachsetofforcingswasrun30tim esstartingfrom distinctinitialconditionswith18yearsofspin-upto
ensure divergence.There are a num berofcaveatsrequired in both the forcingsand ensem ble creation
choiceswhich m ayim pactthesubsequentanalysisand thatweoutlinein thissection.
6.1.Forcings
The alternative forcings discussed in section 3 show differences between the Reference and Sensitivity
forcings;however,thenew forcingsshow onlyasm allchange(relativeto thecom m on signalofincreasing
netforcing)from those applied in the Reference runs,which were the CM IP5 forcings extended with
RCP8.5.Forthehiatusperiod of1998–2012,theSensitivityforcingsshowed differencesfrom theReference
forcingsofapproxim ately 0.02W m 2 due to the GHGs and long-lived trace gases, 0.08W m 2 due to
decreased solar forcing,0.11W m 2 due to anthropogenic aerosolchanges,and 0.08W m 2 due to
increased volcanic activity.Thus,the largestdifferenceswere found in the solarirradiance,volcanic,and
troposphericaerosols.
Thereisanim plicitassum ptionthatthedatasetsusedtocreatetheSensitivityforcingsinthisworkwerenot
them selvesm odied orextended unlesstheproducerofthatdatasetbelieved itwasanim provem enttodo
so.W hilethisisself-evidentfordatasetsthathavesolelybeen extended with new observationsthatdid not
previouslyexist,aquestion could beraised abouttheim provem entobtained bym odifying preexisting data
(e.g.,volcanic aerosolsfrom Pinatubo).Forthis reason,the authorsm ake no claim in thiswork thatthe
Sensitivityforcingsarebetterthan thosein theReferenceruns,onlythattheyhavebeen m odied to bring
them inlinewithcurrentunderstandingandaredifferentfrom thoseintheReferenceruns.Further,itshould
be noted thatthe black carbon em issionsfrom airtraf c,stratosphericwatervapor,land use albedo,and
ozone concentrationswere notm odied due to eithera lack ofsuitable observationaldata ora lack ofa
suitable m ethod form odifying the forcing in NorESM .The im plications ofthese om issionsby de nition
rem ainunknown.However,based uponMyhreetal.[2013],theforcingeffectsofozoneorblackcarbonfrom
airtraf careexpected to besm allcom pared with theim pactfrom otheraerosolsand thesolarforcing.
Asdetailed in section 2,there wasan inevitable trade-offin the experim entaldesign wherebywe allowed
solely 2 degreesoffreedom in the forcings.Yetitisbeyond dispute thatthere isuncertainty in m any of
theapplied forcings.Thoseforcingswhich changed m ostsubstantivelybetween Reference and Sensitivity,
Figure14.Arcticseaiceextentforwintertim e(Decem ber-January-February,topblacktraceandcolortraces)andsum m ertim e(June-July-August,lowerblacktraceand colortraces)from theReferenceensem ble(blue)and Sensitivityensem ble(red),com pared to theobservationalestim atesfrom theNationalSnow and IceDataCentre(black).Plotted arethe95% bootstrapcon dencebounds(shading),togetherwith ensem blem eans(solid line).
JournalofGeophysicalResearch:Atm ospheres 10.1002/2015JD023859
OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8590
solar, volcanic, and anthropogenic
aerosols are exactly those forwhich
there is greatest uncertainty [Myhre
etal.,2013].Hence,ifwe had chosen
differentrealizationsoftheseforcings
to apply,then the resulting RFwould
have differed.Here we lim it further
specic discussion to issues around
each ofthesethreeforcingsand how
theywereapplied.
6.1.1.Solar
W hile m odifying the solar forcing,
severalpossible alternative estim ates
of the observed evolution were
found, based on different sources
and different algorithm s.Figure 15
shows the three m ain alternatives
com pared to the original forcings
used in NorESM for CM IP5 including the nalchoice outlined in section 3.The Institut Pierre Sim on
LaplacesolarforcingisidenticaltothatofNorESM until2009,whenthepreviouscyclesbegintoberepeated
inNorESM .Thesolarforcingfrom SORCEisfrom theTM Iinstrum entfor2003onward.Priortothis,theSORCE
forcings are based on previous reconstructions [Krivova et al.,2010]but include a sm alloffset based
on the TM Im easurem ents forthe period from 2003 onward.The solarforcings from the Physikalisch-
M eteorologischesObservatorium Davos(PM OD)are based on a com bination ofdata from severalinstru-
m ents:ACRIM -I,ACRIM -II,ACRIM -III,VIRGO,TIM ,andERBE.ThePM OD dataaregenerallyconsideredthem ost
accuratesetofsolarforcings,given how itwasconstructed [Fröhlich andLean,1998;Fröhlich,2006].Forthis
reason,and in orderto stretch ourSensitivitytestssince PM OD hasthelargestdifferencefrom theoriginal
NorESM runs,thisestim ateofsolarforcing wasused.However,driving lesforNorESM werecreated forall
threeofthesolarforcingsshown in Figure15,and futureworkcould exploresuch sensitivity.
6.1.2.VolcanicForcing
RecentworkbyRidleyetal.[2014]thatpostdatestheproduction oftheensem blesand initialsubm ission of
thispapersuggeststhatthevolcanicforcingsfrom Sato etal.used in theReferenceforcingsand Sensitivity
forcingsare biased low.Thisisbased upon acom parison to otherdatasetsbased upon observationsfrom
severallidarsand otherinstrum ents.Specically,an assum ption regarding the lowerm ostlevelin which
stratosphericvolcanicaerosolscan reside leadsto an underestim ate ofthe stratosphericburden in m iddle
tohighlatitudes,particularlyinwinter.Thisrequiresareassessm entofhow toapplythisforcingsothestrato-
sphericaerosolloading basepressurelevelvariesseasonallyand latitudinally.Thisisnotthecaseineitherof
thecurrentancillariesin NorESM ,and forthatm atterm ostothercontem poraryGCM sand ESM s,which load
above a xed pressure levelwith no globalorseasonalredistribution in the verticalextentoverwhich the
aerosolloading isbeing spread.
Forthesm allvolcanoessince2000,observationssuggestthattheeffectsofthisunderestim ation m aybeas
greatashalfoftheforcing im plied byestim atessuch asSato etal.atsom elatitudes.Therealm agnitudeof
post-2000 volcaniceffectsisreported byRidleyetal.to be ofthe order 0.19±0.09W m 2.Therefore,the
Sensitivityrunsm aybe high biased relative to realitybythe orderof0.1W m 2.Asdiscussed in section 3,
theReferencerunshaveeffectivelyno loading throughoutthehiatusperiod and arethereforehigh biased
throughoutto an even greaterdegree,lacking even an episodicinjection clearly presentin eitherSato or
Ridleyetal.series.A subsequentstudyfocusing furtheron volcanicforcingsiscurrentlybeing undertaken.
6.1.3.Aerosols
First,theestim ateofforcing dueto theapplied anthropogenicaerosolsin section 3isincom pleteasitdoes
notaccountforthe directeffectofaerosolson longwave radiation,which isassum ed to be sm all.Further,
whileitaccountsforchangesin thecloud liquid waterpath,itdoesnotaccountforeitherchangesin cloud
fractionortheindirecteffectofaerosolsincoldorm ixed-phaseclouds(thelatterhavingalow levelofunder-
standing)[e.g.,seeKirkevåg etal.,2013].
Figure 15.Solarforcing in W m 2 ofthe Reference forcing from NorESM(blue),theInstitutPierreSim on Laplace(black),theSOlarRadiation andClim ateExperim ent(green),and theSensitivityforcingsfrom PhysikalischM eteorologischesObservatorium Davos(red).
JournalofGeophysicalResearch:Atm ospheres 10.1002/2015JD023859
OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8591
The aerosolissuesastheypertain to the presentanalysiswere highlighted in m anyareasofthe recentFifth
Assessm entReportwith,in particular,a whole chapterconcerned with aerosolsand clouds[Boucheretal.,
2013].Theradiativeeffectsofaerosolsrem ain an areaofactiveresearch ashighlighted byrecenthigh pro le
workon theissueby,forexam ple,Shindel[2014]and Stevens[2015].Relevantprincipalaspectsto thepresent
studyare asfollows.Hartm ann etal.[2013]concluded thatwhile regionalchangeswere apparentin regions
withsuf cientinsiturecords,thesearegrosslyincom pleteandthatcon denceinsatellite-basedaerosoloptical
depth recordsislow.Myhreetal.[2013]highlighted large uncertaintyin historicalaerosolforcingsand their
radiativeeffects,and thatthesecontinued to bethepredom inantsourceofuncertaintyin historicalforcings.
Kirtman etal.[2013,section 11.3.6.1]cautioned thatthe aerosolem ission-RCP scenariosin the shortterm
m ay notbe realistic,rem oving the aerosolburden too quickly.They furthernote thatwhen and where the
aerosolsareem itted m aybeim portant,so aconsideration oftheeffectofaerosolsshould notnecessarilybe
consideredinterm ssolelyoftheglobalm eanRF.Postdatingthereport,Shindel[2014]suggeststhattheef cacy
ofaerosolforcingsisunlikelytobeinvariantinspaceandtim e.Collinsetal.[2013]highlightaroughlyequalsplit
betweenm odelsthatprescribeaerosolburdensdirectlyorcalculatethem interactivelybaseduponprescribed
em issions(seetheirTable2.1).TheNorESM m odelconsidered herein usesprescribed em issions.
Given thatNorESM iscon gured such asto derive aerosolconcentrationsinteractively from em ission-based
driverancillaries,the use ofdirectobservationally based aerosolburdens was precluded from the outset.
Historicalem ission-based scenariosdepend upon the veracityofnationalem issionsinventorieswhich cannot
betakenasagiven.Inparticular,nationalem issionsinventoriestendtoconcernindustrialscalepollutionrather
thandom esticsourcepollution.Thism ayleadtosignicantunderreportingofem issionsincountrieswithheavy
dom esticem issionssuch asChinaandIndia.Thererem ainsconsiderableuncertaintyonwhatthecorrectem is-
sionswereandthelackofadequateobservationalcapabilitiesofspeciatedaerosolsgloballycom binedwiththeir
shortlifetim eprecludeseitheratop-downorabottom -upclosureofthebudgetasispossibleform anyLLGHGs.
Duringtheexperim entalsetup,anadditionalpossiblesetofem issionsdriverswasconsidered.However,this
yielded burdensofthe speciated aerosolsthatwere substantially and system atically differentto the CM IP
runs in allcases.Use ofsuch ancillaries,while they m ay wellbe closerto the truth,would have led to
large-scaleshockterm sbeing applied to them odelin term softheRF.W ecarefullyconsidered thepossible
realism oftheaerosoldriversweused in Sensitivity.Such an assessm entislim ited bythequestionablevera-
cityoftheRCPscenarios’short-term aerosolbehavior[Kirtm anetal.,2013].Figure16com parestheReference
and Sensitivity burdensto those forallfourRCP scenarios.Underthe assum ption thatthe RCP scenarios
describeplausibledecadalscalevariation,theappliedSensitivityforcingsdonotappeartobegrosslyunrea-
listic,although thePOM concentrationsaresom ewhatoutsidetheRCP bounds.
W econcludethatofthevariousforcingsaltered in thiswork,both theem issionsand theconcentrationsof
tropospheric aerosols have the greatestuncertainties associated with them ,which com e from both the
uncertaintiesin observations/em issionsofaerosolsand from the RCP scenariosdue to uncertaintiesin the
prediction offuture em issions.One studyhasshown thatincreased sulfurem issionsin Asia are im portant
at least regionally,since the em itted sulfur counteracts the anthropogenically driven warm ing [Neely
etal.,2013];however,recentwork by Murphy [2013]suggeststhatthe shiftofpollution from Europe to
South EastAsiahashad little effecton the clear-skyradiative forcing.The uncertaintiesin aerosolsm aybe
com parableto thecom bined effectsofchangesin thesolarand volcanicforcingsoverrecentyears,asseen
in thisstudy.In theSensitivityensem ble,theeffectstend to cancelwhich m ay,orm aynot,re ectthereal-
world situation.Only im proved aerosolem issionsancillariesbased upon m ore rigorousunderstanding of
the historicalrecord could close this issue.One aspect of the aerosolconcentrations that is m issing
from the CM IP5 sim ulationsentirely isthe interannualvariability which can be quite large and im portant,
given thesubannualresidencetim escaleofaerosols.Thisaspectcould beresolved iftheaerosolconcentra-
tionsorem issionswere prescribed in the m odelsforeach yearinstead offoreach decade.Innovationsin
reanalysesofaerosolsunderthe M onitoring Atm osphericCom position and Clim ate (M ACC)projectand its
follow-onsm ayperm itsuch im proved ancillariesin thefuture,atleastsincesom epointin the1980s.
6.2.Ensem ble Experim entalDesign
Inthisstudy,wehavecreatedtwo30-m em berensem blestoinvestigatetheim pactoftheSensitivityforcings
and the internalvariabilityasdiagnosed bythe NorESM m odel.W hile thisisalarge num berofensem bles
com pared to the num berofhistoricalruns subm itted to the CM IP5 archive forany single m odel,these
JournalofGeophysicalResearch:Atm ospheres 10.1002/2015JD023859
OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8592
ensem blesare stillnotsuf cientlylarge to sam ple the entire solution space ofpossible realizationsforthis
m odelorto fully explore the effectsofim perfectknowledge ofthe forcingsoverthe period (section 6.1).
Given thechanceto repeattheexperim ent,therearecertain thingsthatwewould do differently.
IftheOHC initialstateisim portantforcorrectlysam pling thesolution spaceoftheinternalvariability[Meehl
etal.,2014],then starting from justthreestatesofOHC m ightnotbesuf cientunlessoneofthesestatesby
chance wasreasonablyproxim alto the 1980sOHC state.Furtherm ore,itm aytake longerthan the 18year
spin-up used here to attain reasonable spread in OHC statesin the abyssaloceans.Itwould be preferable
to start from a broaderrange ofinitialOHC states to betterassess whether the ocean initialstate is
potentiallyim portant.
W ith regardsto ensem ble design,when the runswere started,we had notdiagnosed the RF ofthe two
ensem bles.Given the large changesin the ancillariesform anyofthe forcings,itwasexpected thatthe RF
would differsubstantiallygloballyand certainlyregionally;and hence,two distinctpathwayswould suf ce
to answeratleastto rstorderwhatroleforcing m ayplay.In hindsight,theresulting globalRFissom ewhat
sim ilar(section 3.6).Itm ay wellbe the case thatallofthe applied ancillariesin Sensitivity are reasonably
proxim alto the unknown truth and thatthe CM IP-5 runsthrough fortuitouscalculation oferrorsgotclose
to this.Butitwould be incredibly naïve to place any faith in such an inference,given the uncertaintiesin
m anyoftheforcings(section6.1).Ifweweretorepeattheanalysis,wewouldrunasom ewhatlargernum ber
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 20202000
2200
2400
2600
2800
3000
3200
3400
year
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 20201
1.1
1.2
1.3
1.4
1.5
year
1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 20202
2.5
3
3.5
4
4.5
5
year
Figure 16.Com parison ofthetwo applied setsofaerosolem ission forcingsto thosefortheentiresuiteofRCP scenariosthrough 2020.
JournalofGeophysicalResearch:Atm ospheres 10.1002/2015JD023859
OUTTEN ETAL. INVESTIGATING RECENTHIATUS,1 8593
ofensem bleswithsom ewhatsm allerpopulationssothatwecould betterexploreforcinguncertaintyeffects
such thatwehad three orfourdifferentSensitivityensem blesand hence 4 or5 degreesoffreedom in the
appliedforcings.Effortstoexploreuncertaintiesinsolarirradiance,addressinadequaciesinvolcanicforcings
thathaveonlyveryrecentlybecom eapparent,and m orefullyconsideraerosolem issionsuncertaintywould
beprioritized.
Perhaps,m ostim portantly,theuseofasinglem odelinthisstudyensuresthattheresultsdonotsam plethe
entireparam eterspaceofrealizationsofthehiatusobtainablebyclim atem odels,ingeneral,undertherange
offorcingswe considered.The Sensitivityforcingsused in thisprojecthave been m ade available through
supplem entary m aterialboth in the interestsoftransparency in ourwork and in the hopesthatthey m ay
beofusetootherm odelinggroupsinterestedinperform ingcom parablestudiesoftheirown.W ewouldalso
encouragegroupsto im proveupon ourexperim entaldesign.
7.Sum m ary
Variousforcingsin theNorwegian Earth System m odelwerem odied to bring them in linewith new obser-
vationalorem ission-based estim ates.Thesewereappliedtoa30-m em berSensitivityensem blecoveringthe
period of1980until2012.A sim ilar30-m em berReferenceensem blewascreated using theCM IP5historical
forcingsextended with RCP8.5.Exploratoryanalysishasrevealed thattheinclusion ofthese alternative for-
cings did not greatly alter the ENSO variability or Arctic sea ice extent between the Reference and
Sensitivityruns.Ocean HeatContentdid show an increased positivetrend in responseto theSensitivityfor-
cings.ENSO behaviorissim ilarto thatobserved,whileseaiceextentshowsm arked errorsin them ean state
and trendsin allseasons.
These ensem blesprovide aunique,to ourknowledge,toolsetthatenablesan investigation ofthe relative
rolesofpositedforcingm isspecications(totheextentdescribedbytwoofabroadrangeofpossibleforcing
histories)and internalm odelvariabilitym echanism sin explaining the hiatusin surface tem peraturesfora
single clim ate m odel.Asan initial“ rstlook”atthe im pactofthese Sensitivity forcings,the annualm ean
globaltem peratureforthetwo ensem blesisgiven in Figure17.Thechangesto theforcings,including solar
and aerosols,appearto havelittleim pacton theglobalm ean tem peraturetrends.A thorough investigation
ispresented in Part2 ofthisstudy[Thorneetal.,2015],where the ensem blesare com pared to various
observationaldata sets,and both ensem ble m ean behaviorand individualm em bersofthe ensem bles
are discussed.
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Acknowledgm entsTheforcingsdataused in thisworkcam efrom theAdvanced GlobalAtm osphericGasesExperim ent(http://agage.eas.gatech.edu/index.htm ),theEarth Research System LaboratoryoftheNationalOceanicand Atm osphericAdm inistration (http://www.esrl.noaa.gov/gm d/ccgg/),theW orldM eteorologicalOf ce’sW orld DataCentreforGreenhouse Gases(http://ds.data.jm a.go.jp/gm d/wdcgg/),theGoddard InstituteofSpaceStudiesattheNationalAeronauticsand SpaceAdm inistration (http://data.giss.nasa.gov/m odelforce/strataer/),and theW orld Radiation CentreatthePhysikalisch-M eteorologischesObservatorium Davos,Switzerland(http://www.pm odwrc.ch/pm od.php?topic=tsi/com posite/SolarConstant),including unpublished datafrom theVIRGO Experim enton thecooperativeESA/NASA M ission SoHO.Theaerosoland aerosol-precursorem issionswereprovided bytheEU projectsEclipseandPegasos(http://eclipse.nilu.no/).ThisworkwaspartoftheEXPLAIN projectwhich wasfunded bytheFastTrackInitiativeschem eattheCentreforClim ateDynam icsattheBjerknesCentreand hasbeen supported bytheResearch CouncilofNorwaythroughprojectEVA (grant229771).Com putationaland storageresourceshavebeen provided byNORTUR(nn2345k)and NorStore(ns2345k).Theauthorswould liketo thankHongm eiLiforproviding codeand adviceforFigures8and 9,putting NorESM intocontextwith otherCM IP5m odels,andIselin M edhaug fordiscussionsregard-ing solarforcings.PaulStackhouse,Richard Allan,and Norm an Loeb areacknowledged fortheirexpertiseonTOA observations.Gavin Schm idtprovided adviceon theSchm idtetal.analysis.Drew Shindellprovided usefulinputon how to calculateradiativeequivalentforcingsforvolcanicandaerosolforcings.AlfKirkevåg helped setup and provided adviceon theinter-pretation ofthem odelrunsto derivetheradiativeim pactsoftheaerosolchanges.Jochem M arotzkeprovidedadviceon NorESM in thecontextoftheCM IP-5 ensem ble.Sim on Good pro-vided adviceon observed OHC recordssuitability.Finally,wethankGeraldM eehland two anonym ousreviewersforthorough and constructive reviews,which served to im provethepaper.
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