Page - 357 - in Loss and Damage from Climate Change - Concepts, Methods and Policy Options

14 IntegratedAssessment for IdentifyingClimateFinanceNeeds… 357 14.5.1 ClimateSensitivityandDamageFunctions EquilibriumClimateSensitivity (ECS) is oneof the keyparameters in climate sci- ence.ECS isdefinedas theequilibriumchange inglobal temperaturedue toadou- blingofatmosphericCO2overitspreindustrialvalue.Thismeasureistypicallychar- acterised as a distribution due to underlying uncertainty in the behaviour of some aspectsoftheclimatesystem.Studiesbasedonobservations,energybalancemodels, temperaturereconstructionsandglobalclimatemodels(GCMs)haveconcludedthat theprobabilitydensitydistributionofECSpeaksat around3°C,witha long tail of small but finite probabilities of very large temperature increases.According to the IPCC’sFifthAssessmentReport (IPCC2013), estimatesof theECSindicate that it is likely to be in the range of 1.5–4.5 °C (with high confidence) and very unlikely to be greater than 6 °C (medium confidence). The extreme temperature outcomes of the distribution function are sometimes referred to as “fat tails.” Some authors (Weitzman2009,2012)haveproposed thatdecisionsonclimatepolicyshouldactu- allybebasedontryingtoavoidextremeoutcomesoflowprobability.Theuncertainty rangeofECShas not been reduced substantially in the past three decades and it is notexpected tobereducedin thenear future(RoeandBaker2007).TypicallyIAMs use themost likely value for ECS (3 °C as in Sect. 14.3.2), but it is important to performasensitivityanalysis fordifferentvalues forECS. The other major sources of uncertainty, in this case from climate change eco- nomics, is thewayinwhichthedamagefunctionfromglobalwarmingisrepresented (seeSect.14.3.1).Damagefunctionsarerecognisedasbeingoneoftheweakestlinks in the economics of climate change (Pindyck 2013), because it is very difficult to obtainempiricaldataandbecauseresultscanbeverysensitivetoitsfunctionalform, particularlywhen high temperatures are considered. One of themost well-known damagefunctions is theoneusedbyNordhaus(DICE4model,NordhausandSztorc 2013),whichhasbeenrecentlyadoptedbytheUSEnvironmentalProtectionAgency (EPA2010) toprovidevaluesfor thesocialcostofcarbon.However, inorder tocap- ture thepossibilityof“tippingpoints”andabruptclimatechange,Weitzman(2012) hasproposedadifferentdamagefunctionthatcaptureslargeimpactsbeyonda4–6°C threshold based on an expert panel study involving 52 experts according towhich at this temperaturechange threeoutoffive important tippingpoints areexpected to emerge (seeLentonet al. 2008).Theseauthorsmentiondifferent processes suchas irreversiblemeltdownoftheGreenlandicesheet,disintegrationoftheWestAntarctic ice sheet, reorganisation ofAtlantic thermohaline circulation, amongothers. Some of these processesmay have a significant probability of occurring this century for climate conditions involvingmediumwarming (between2and4 °C) andeven low 4It is important tomention thatDICEdamage functions include the impacts after adaptation has occurred,soadaptationisalreadyincluded.Someauthors(see, forexample,Bruinetal.2009)have included thepossibilityof reducingdamage throughadaptation in IAMsso that theycan therefore capture the trade-offbetweenadaptationandmitigation.