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Consistency and robustness of selected emergent constraints for equilibrium climate sensitivityAxel Lauer and Veronika Eyring
Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), Oberpfaffenhofen, Germany
CFMIP Meeting on Cloud Processes, Circulation and Climate Sensitivity25-28 September 2017, Tokyo, Japan
Equilibrium climate sensitivity (ECS)
Charneyreport
IPCC FAR
IPCC SAR
IPCC TAR
IPCC AR4
IPCC AR5
DefinitionProjectedequilibriumchangeinglobalannualmeansurfacetemperaturefollowingadoublingoftheatmosphericCO2concentration.0.0
1.0
2.0
3.0
4.0
5.0
6.0
1979 1990 1996 2001 2007 2013
ECS(K)
Emergent constraints
Emergentconstraints arearelationshipacrossanensembleofmodels,betweensomeaspectofEarthsystemsensitivityand anobservabletrendorvariationinthecurrentclimate
• Anecessarypropertyofemergentconstraintsisaphysicalbasisfortherelation.
• Offerthepotentialtoreduceuncertaintyinclimatefeedbacksandprojections.
• Canhelpguidingmodeldevelopmentontoprocessescrucialtothemagnitudeandspreadoffutureclimatechange andtoguidefutureobservations.
Selected published emergent constraints for equilibrium climate sensitivity
SelectedpublishedemergentconstraintsforECS
1) Precipitation – SouthernITCZindex(Tian,2015)
2) Humidity – Tropicalmid-tropospherichumidityasymmetryindex(Tian,2015)
3) Mixing – Lowertroposphericmixingindex(LTMI)(Sherwoodetal.,2014)
4) Clouds – Covarianceofdeseasonalized tropicallowcloud(TLC)shortwavereflectionwithSST(Brient andSchneider,2016)
Testconsistencyamongdifferentproposedemergentconstraintsandtheirsensitivityto:
• Modelensemble(CMIP3,CMIP5)• Observationaldataset(s)
ESMValTool version 1.1.0
http://www.esmvaltool.org/Eyringetal.,Geosci.ModelDev.,2016
• Communitydiagnosticsandperformancemetricstool fortheevaluationofEarthSystemModels(ESMs)
• Standardizedmodelevaluation canbeperformedagainstobservations,againstothermodelsortocomparedifferentversionsofthesamemodel
• manydiagnosticsandperformancemetricscoveringdifferentaspectsoftheEarthSystem(dynamics,radiation,clouds,carboncycle,chemistry,aerosol,sea-ice,etc.)andtheirinteractions
• Well-establishedanalysisbasedonpeer-reviewedliterature
• Currently≈70developersfrom28institutionsand>100users
GitHub:https://github.com/ESMValGroup/ESMValTool
Southern ITCZ index (Tian, 2015)
SoutheasternPacific30°S-0°,150°W-100°W
• Climatologicalannualmeanprecipitationbias
• Model– observation(mmday-1)• AveragedoversoutheasternPacific
Observational data
1) GlobalPrecipitationClimatologyProject(GPCP,1980-2005)2) CPCMergedAnalysisofPrecipitation(CMAP,1980-2005)3) TropicalRainfallMeasuringMission(TRMM,1998-2013)
Southern ITCZ index (Tian, 2015)
4.0 K
TRMM, CMIP5
Southern ITCZ index (Tian, 2015)TRMM GPCP CMAP
CM
IP3
CM
IP5
4.0 K 3.8 K 3.7 K
4.1 K 3.8 K 3.7 K
Southern ITCZ index (Tian, 2015)
4.1 K
3.7 K
Tropical mid-tropospheric humidity asymmetry index (Tian, 2015)
Observationallybaseddata
1) AtmosphericInfraredSounder(AIRS,2003-2010)
2) ERA-InterimReanalysis(1980-2005)
3) NCEP/NCARReanalysis1(1980-2005)
• Climatologicalannualmeanmid-troposphericspecifichumidityrelativebias• (model– observation)/observation(in%)• SHtropicalPacific(120°E-80°W,30°S-0°)– NHtropicalPacific(120°E-80°W,0°-
20°N)
NH tropical PacificSH tropical Pacific
Tropical Pacific
3.8 K
3.3 K
Tropical mid-tropospheric humidity asymmetry index (Tian, 2015)
Lower tropospheric mixing index (LTMI) (Sherwood et al., 2014)
Observationallybaseddata
1) ERA-InterimReanalysis(1980-2005)2) NCEP/NCARReanalysis1(1980-2005)
• Averagedovertropicaloceanregionofmeanascent(upper25%)• Small-scalemixingS=(DR700-850/100%- DT700-850/9K)/2• Large-scalecomponentofmixing=ratioofshallowtodeepoverturning
D=áDH(D)H(-w1)ñ /á-w2H(-w2)ñ• LowertroposphericmixingindexLTMI=S+D
Tropical region of mean ascent (ERA-Interim)
Lower tropospheric mixing index (LTMI) (Sherwood et al., 2014)
3.6 K
2.9 K
NCEP
ERA-Interim
Covariance of tropical low cloud (TLC) shortwave reflection with SST (Brient and Schneider, 2016)
Observationallybaseddata
1) Relativehumidity:ERA-InterimReanalysis,NCEP/NCARReanalysis1
2) TOAradiativefluxes:CERES,SRB,ISCCP-FD
3) SST:HadISST
• MeanoverTLCregions:25%oftropicaloceanarea(30°S-30°N)withlowest500hPa relativehumidity
• TLCreflection(atTOA)=- áScñ /áIñ (in%)• Regressioncoefficients:deseasonalized variationsofTLCshortwave
reflectionandseasurfacetemperature(in%perK)
Annual mean TLC regions (ERA-Interim)
Covariance of tropical low cloud (TLC) shortwave reflection with SST (Brient and Schneider, 2016)
3.7 K
3.3 K
ERAISCCP
HadISST
NCEPSRBHadISST
CMIP5
CMIP3
Southern Hemisphere Hadley cell extent (Lipat et al., 2017)
~3.0 K
ERANCEP
CMIP5
CMIP3
Conclusions and outlook• Allfourofthetestedemergentconstraintsare
sensitivetothemodelensembleand/ortheobservationaldatasets used
• EstimatedECSrangeintheoriginalfourstudies:3.8– 4.0K(2.4– 4.3K)
• Robustnessestimatesobtainedhere(additionalobservationsandensembles):2.9K– 4.1K
• Applyingcompilationofcurrentemergentconstraintstudies(forECS)donotallowtonarrowrangeofECSsignificantly
Outlook• Furtherselectionbasedonphysicalmechanisms andfundamentalprocesses• Focusonindividualfeedbacks toconstrainECS/TCR/TCRE(e.g.carboncycle)• Accountingforobservationaluncertainties• Investigationofadditionalpublishedemergentconstraints• Developmentofnewemergentconstraints
Emergentconstraint Originalrange Thisstudy
ITCZ bias 4.1 K 2.7– 4.1K
Humidityasymmetryindex
3.8K 3.3– 3.8K
LTMI 4.0K(3.0– ) 2.9– 3.6 K
TLCreflection 4.0K(2.4– 4.3K) 3.3– 3.7K