When Are We Committed to Crossing Critical (1.5 Or 2 °C) Temperature Thresholds?

When are we committed to crossing critical (1.5 or 2 °C) temperature thresholds?

Cristian Proistosescu1 Kyle Armour1 Gerard Roe1 Peter Huybers2

1University of Washington

2Harvard University

AGU Fall Meeting 2017 Courtesy of NASA’s Earth Observatory Two questions

1. When will we cross 1.5 or 2 °C global warming thresholds (e.g., following high or low emission scenarios) – subject to constraints from the observed global energy budget?

2. When will we be geophysically committed to crossing 1.5 or 2 °C global

warming thresholds? What do CMIP5 models say?

CMIP5 projections (IPCC AR5)

2 °C

Figure 12.5:Figure Time 12.5:Figure series Time of12.5: globalseries Time ofannual globalseries mean ofannual global surface mean annual air surface temper mean air aturesurface temper anomalies airature temper anomalies (relativeature anomalies to(relative 1986–2005) to(relative 1986–2005) from to CMIP51986–2005) from CMIP5 from CMIP5 concentration-drivenconcentration-drivenconcentration-driven experime experiments. Projections nts.experime Projections arents. shown Projections are for shown each are RCPfor shown each for theRCP for multi each for themodelRCP multi for mean themodel multi(solid mean model lines) (solid andmean lines) the (solid and lines) the and the 5–95% range5–95% (±1.64 range5–95% standard (±1.64 range deviation)standard (±1.64 deviation)standard across the deviation) across distributi the across ondistributi of individualthe ondistributi of individual modelson of individual(shading). models (shading). Discontinuitiesmodels (shading). Discontinuities at 2100Discontinuities at 2100 at 2100 are due toare different due toare differentnumbers due to numbersofdifferent models ofnumbers performing models of performing models the extension performing the extension runs thebeyond extension runs the beyond 21st runs century the beyond 21st and century the have 21st andno century physical have andno physical have no physical meaning.meaning. Only one meaning.Only ensemble one Onlyensemble member one ensemble ismember used from ismember used each from modelis used each and from model numbers each and model numbersin the and figu innumbersre the indicate figu inre the theindicate figunumberre the indicate of number the of number of different differentmodels contributing modelsdifferent contributing models to the contributingdifferent to the differenttime to periods.the timedifferent Noperiods. ranges time No periods. are ranges given No are for ranges giventhe RCP6.0 arefor giventhe projections RCP6.0 for the projections RCP6.0 beyond projections 2100 beyond 2100 beyond 2100 as only twoas onlymodels twoas are onlymodels available. two are models available. are available.

Do Not Cite,Do Not Quote Cite,Do or Not Quote Distribute Cite, or Quote Distribute or Distribute 12-129 12-129 12-129 Total pages:Total 175 pages: Total 175 pages: 175 What do CMIP5 models say?

Final DraftFinal (7 JuneDraftFinal 2013) (7 June Draft 2013) (7 June 2013) Chapter 12 Chapter 12 Chapter 12 IPCC WGI IPCC Fifth WGI Assessment IPCC Fifth WGI Assessment Report Fifth Assessment Report§ Models Report may not agree with observed CMIP5 projections (IPCC AR5) global warming and energy budget constraints

§ Models may not span full range of plausible future warming

§ Computationally expensive to run different emissions scenarios, so can’t ask questions like, when are we 2 °C geophysically committed to 2 °C?

§ Not clear which physical factors are contributing to uncertainty in projected warming

Do Not Cite,Do Not Quote Cite,Do or Not Quote Distribute Cite, or Quote Distribute or Distribute 12-129 12-129 12-129 Total pages:Total 175 pages: Total 175 pages: 175 ARMOUR ET AL.: SEA ICEARMOUR REVERSIBILITY ET AL.: SEA ICE REVERSIBILITYX-5 X-5

Global radiative forcingGlobal (F )changesapproximatelylinearlywithtimeovertheCO radiative forcing (F )changesapproximatelylinearlywithtimeovertheCO2 2 ARMOUR ET AL.: SEA ICE REVERSIBILITY ARMOURX-5 ET AL.: SEA ICE REVERSIBILITY X-5 2 2 rampings, by about 3.7rampings, Wm per by 70 about yr, which 3.7 Wm is the periodper 70 ofyr, CO which2 doubling is the period or halving of CO2 doubling or halving Global radiative forcing (F )changesapproximatelylinearlywithtimeovertheCOARMOUR ET AL.: SEA ICEGlobal REVERSIBILITY radiative forcing (F )changesapproximatelylinearlywithtimeovertheCO2 X-5 2 [Myhre et al., 1998]. The[Myhre o↵set et in al. Figure, 1998]. 1 Thebetween o↵set warming in Figure (red) 1 between and cooling warming (blue) (red) and cooling (blue) Global radiative forcing2 (F )changesapproximatelylinearlywithtimeovertheCO2 2 rampings, by about 3.7 Wm per 70 yr, which is therampings, period of by CO about2 doubling 3.7 Wm or halvingper 70 yr, which is the period of CO2 doubling or halving trajectories implies a laggedtrajectories response implies of hemispheric-mean a lagged response annual-mean of hemispheric-mean surface tempera- annual-mean surface tempera- 2 [Myhrerampings, et al., 1998]. by about The 3 o.7↵set Wm in Figureper 70 1 yr, between which[Myhre is warming the et period al. (red), 1998]. of andCO2 The coolingdoubling o↵set (blue) or in halving Figure 1 between warming (red) and cooling (blue)

ture anomalies (TNH andture anomaliesTSH), as expected (TNH and from deepTSH), ocean as expected heat storage from deep [e.g., oceanHeld heat et storage [e.g., Held et trajectories[Myhre implies et al., a 1998]. lagged The response o↵set of in hemispheric-mean Figure 1 betweentrajectories annual-mean warming implies (red) a surface lagged and tempera- response cooling (blue) of hemispheric-mean annual-mean surface tempera- al., 2010]. In order to approximatelyal., 2010]. In order account to forapproximately this lag, we account consider for the this evolution lag, we of consider ice the evolution of ice trajectories implies a lagged response of hemispheric-mean annual-mean surface temperature anomalies (TNH and TSH), as expected from deepture anomalies ocean heat ( storageTNH and [e.g.,TSHHeld), as et expected from deep ocean heat storage [e.g., Held et area as a function of hemisphericarea as a function temperature of hemispheric rather than temperature time. A justification rather than for time. this A justification for this al., 2010].ture In anomalies order to ( approximatelyTNH and TSH account), as expected for thisal. from lag,, 2010]. wedeep consider In ocean order heat the to evolution approximately storage [e.g., of iceHeld account et for this lag, we consider the evolution of ice treatment is that annual-meantreatment Arctic is that sea annual-mean ice area has been Arctic found sea ice to declinearea has linearly been found with to decline linearly with area asal. a, 2010].function In of order hemispheric to approximately temperature account rather forarea than this as lag, a time. function we A consider justification of hemispheric the evolution for this temperature of ice rather than time. A justification for this increasing global-meanincreasing temperature global-mean across a range temperature of GCMs, across emissions a range scenarios, of GCMs, and emissions scenarios, and treatmentarea is as that a function annual-mean of hemispheric Arctic sea temperature ice area hastreatment rather been found than is that totime. decline annual-mean A justification linearly Arctic with for sea this ice area has been found to decline linearly with climates [Gregory et al.,climates 2002; Ridley [Gregory et al. et, 2008; al., 2002;WintonRidley, 2006, et al. 2008,, 2008; 2011].Winton Specifically,, 2006, 2008, 2011]. Specifically, increasingtreatment global-mean is that annual-mean temperature Arcticacross sea a range ice areaincreasing of GCMs, has been global-mean emissions found to scenarios, decline temperature linearly and across with a range of GCMs, emissions scenarios, and we extend the argumentswe of extendWinton the[2011], arguments relating of Winton hemispheric[2011], ice relating cover to hemispheric global forcing ice cover to global forcing climatesincreasing [Gregory global-mean et al., 2002; temperatureRidley et al., across 2008; Winton aclimates range, 2006, of [Gregory GCMs, 2008, et emissions 2011]. al., 2002; Specifically, scenarios,Ridley et and al., 2008; Winton, 2006, 2008, 2011]. Specifically, through through climates [Gregory et al., 2002; Ridley et al., 2008; Winton, 2006, 2008, 2011]. Specifically, we extend the arguments of Winton [2011], relating hemisphericwe extendOur the ice arguments cover approach to global of Winton forcing [2011], relating hemispheric ice cover to global forcing throughwe extend the arguments of Winton [2011], relatingthrough hemispheric ice cover to global forcing § Use a 2-layer ocean model (Held et al. 2010; dTu radiative radiativedTu cu =forcingTu + F + "(cTresponseud Tu=)(1)Tu + F + "(Td Tu)(1) Armour 2017) that includes the essential physics dt dt through governing global-mean surface warming: Ocean heat uptake dT dTu efficacy u cu = Tu + F + "(Td Tu)(1)cu = Tu + F + "(Td Tu)(1) dt dT dt dT d dTd u cd =upper(T oceanu T dT)(2)cd = (Tu Td)(2) cu = Tu + F + "(Td Tu)(1)dt u dt dt

deep ocean Td dTd dTd cd = (Tu Td)(2)cd = (Tu Td)(2) dt ~⌧ zˆ dt ⌧ x x dT ~⌧ zˆ ⌧ d !Te = ⇥ = yˆ !Te = ⇥ = yˆ (3) (3) cd = (Tu Td)(2)⇢f ⇢f dt ⇢f ⇢f ~⌧ zˆ ⌧ x ~⌧ zˆ ⌧ x !Te = ⇥ = yˆ !(3)Te = ⇥ = yˆ (3) ⇢f ⇢f 1 ~⌧ ⇢f ⇢f ~⌧ zˆ ⌧ x 1 ~⌧ we = zˆ we = zˆ (4) (4) !Te = ⇥ = yˆ ⇢ · r⇥(3)f ⇢f ⇢f ⇢ · r⇥f 1 ~⌧ 1 ~⌧ we = zˆ (4)we = zˆ (4) ⇢ · r⇥f @w @v @u @@ww⇢ · @r⇥v @fv @@uu @w @v @u 1 ~⌧ ~u = xˆ + ~u = yˆ + xˆ + zˆ yˆ + (5) zˆ (5) we = zˆ r⇥ @y @z r⇥@z @@(4)xy @z @x @@yz @x @x @y ⇢ · r⇥f ✓ ◆ ✓ ✓ ◆ ◆✓ ✓ ◆ ◆ ✓ ◆ @w @v @u @w @v @u @w @v @u @w @v @u ~u = xˆ + yˆ + ~uzˆ = (5) xˆ + yˆ + zˆ (5) r⇥ @y @z DRAFT@z @x @x @r⇥y December14,2017,5:39pm@y @z @z @x @x @ DRAFTy ✓ @w◆ @v ✓ @u ◆ @w ✓ DRAFT@v◆ @u ✓ ◆ ✓ December14,2017,5:39pm◆ ✓ ◆ DRAFT ~u = xˆ + yˆ + zˆ (5) r⇥ @y @z @z @x @x @y ✓ ◆ ✓ ◆ ✓ ◆ DRAFT December14,2017,5:39pmDRAFT DRAFT December14,2017,5:39pm DRAFT DRAFT December14,2017,5:39pm DRAFT ARMOUR ET AL.: SEA ICE REVERSIBILITY X-5

Global radiative forcing (F )changesapproximatelylinearlywithtimeovertheCOARMOUR ET AL.: SEA ICE REVERSIBILITY 2 X-5

Global radiative forcing2 (F )changesapproximatelylinearlywithtimeovertheCO2 rampings, by about 3.7 Wm per 70 yr, which is the period of CO2 doubling or halving

2 [Myhrerampings, et al., 1998]. by about The 3 o.7↵set Wm in Figureper 70 1 yr, between which is warming the period (red) of andCO2 coolingdoubling (blue) or halving trajectories[Myhre implies et al., a 1998]. lagged The response o↵set of in hemispheric-mean Figure 1 between annual-mean warming (red) surface and tempera- cooling (blue) trajectories implies a lagged response of hemispheric-mean annual-mean surface temperature anomalies (TNH and TSH), as expected from deep ocean heat storage [e.g., Held et al., 2010].ture In anomalies order to ( approximatelyTNH and TSH account), as expected for this from lag, wedeep consider ocean heat the evolution storage [e.g., of iceHeld et area asal. a, 2010].function In of order hemispheric to approximately temperature account rather for than this lag, time. we A consider justification the evolution for this of ice treatmentarea is as that a function annual-mean of hemispheric Arctic sea temperature ice area has rather been found than totime. decline A justification linearly with for this increasingtreatment global-mean is that annual-mean temperature Arcticacross sea a range ice area of GCMs, has been emissions found to scenarios, decline linearly and with climatesincreasing [Gregory global-mean et al., 2002; temperatureRidley et al., across 2008; Winton a range, 2006, of GCMs, 2008, emissions 2011]. Specifically, scenarios, and climates [Gregory et al., 2002; Ridley et al., 2008; Winton, 2006, 2008, 2011]. Specifically, we extend the arguments of Winton [2011], relating hemisphericOur ice cover approach to global forcing throughwe extend the arguments of Winton [2011], relating hemispheric ice cover to global forcing § Use a 2-layer ocean model (Held et al. 2010; 4 Global surface temperature response Armour 2017) that includes the essential physics to abrupt CO quadrupling through governing global-mean surface warming: 2 dT 63 c u = T + F + "(T T )(1) u dt u d u dTu c = T + F + "(T T )(1)4 u dt u d u 2 Slow warming on timescale of the dTd deep ocean cd = (Tu Td)(2)21 dt Fast warming on dTd timescale of the cd = (Tu Td)(2)surface dt (°C) T change Temperature 0 0 50 100 150 x 0 50 100 150 ~⌧ zˆ ⌧ Year after CO quadrupling !T = ⇥ = yˆ (3) 2 e ⇢f ⇢f ~⌧ zˆ ⌧ x !T = ⇥ = yˆ (3) e ⇢f ⇢f 1 ~⌧ w = zˆ (4) e ⇢ · r⇥f 1 ~⌧ w = zˆ (4) e ⇢ · r⇥f @w @v @u @w @v @u ~u = xˆ + yˆ + zˆ (5) r⇥ @y @z @z @x @x @y ✓ @w◆ @v ✓ @u ◆ @w ✓ @v◆ @u ~u = xˆ + yˆ + zˆ (5) r⇥ @y @z @z @x @x @y ✓ ◆ ✓ ◆ ✓ ◆ DRAFT December14,2017,5:39pm DRAFT DRAFT December14,2017,5:39pm DRAFT ARMOUR ET AL.: SEA ICE REVERSIBILITY X-5

Global radiative forcing (F )changesapproximatelylinearlywithtimeovertheCOARMOUR ET AL.: SEA ICE REVERSIBILITY 2 X-5

Global radiative forcing2 (F )changesapproximatelylinearlywithtimeovertheCO2 rampings, by about 3.7 Wm per 70 yr, which is the period of CO2 doubling or halving

dTd cd = (Tu Td)(2)21 dt dTd cd = (Tu Td)(2) dt (°C) T change Temperature 0 0 50 100 150 x 0 50 100 150 ~⌧ zˆ ⌧ Year after CO quadrupling !T = ⇥ = yˆ (3) 2 e ⇢f ⇢f ~⌧ zˆ ⌧ x !T = ⇥ = yˆ (3) e ⇢f ⇢f 1 ~⌧ w = zˆ (4) e ⇢ · r⇥f 1 ~⌧ w = zˆ (4) e ⇢ · r⇥f @w @v @u @w @v @u ~u = xˆ + yˆ + zˆ (5) r⇥ @y @z @z @x @x @y ✓ @w◆ @v ✓ @u ◆ @w ✓ @v◆ @u ~u = xˆ + yˆ + zˆ (5) r⇥ @y @z @z @x @x @y ✓ ◆ ✓ ◆ ✓ ◆ DRAFT December14,2017,5:39pm DRAFT DRAFT December14,2017,5:39pm DRAFT ARMOURARMOUR ET AL.: ET AL.: SEA SEAICE REVERSIBILITY ICE REVERSIBILITY X-5X-5

GlobalGlobal radiative radiative forcing forcing (F )changesapproximatelylinearlywithtimeovertheCO (F )changesapproximatelylinearlywithtimeovertheCO2 2

ARMOUR ET2 AL.:ARMOUR2 SEAARMOUR ICE ET REVERSIBILITY AL.: ET AL.: SEA ICE SEA REVERSIBILITY ICE REVERSIBILITY X-5 X-5X-5 rampings,rampings, by about by about 3.7 Wm 3.7 Wm per 70per yr, 70 which yr, which is the is periodthe period of CO of2 COdoubling2 doubling or halving or halving ARMOUR ET AL.: SEA ICE REVERSIBILITY X-5 Global radiativeGlobal forcingGlobal radiative (F radiative)changesapproximatelylinearlywithtimeovertheCO forcing forcing (F )changesapproximatelylinearlywithtimeovertheCO (F )changesapproximatelylinearlywithtimeovertheCO2 [Myhre[Myhre et al. et, al.1998]., 1998]. The The o↵set o↵ inset Figure in Figure 1 between 1 between warming warming (red) (red) and and cooling cooling2 (blue) (blue) 2 ARMOUR ET AL.: SEA ICE REVERSIBILITY X-5 Global radiative forcing (F )changesapproximatelylinearlywithtimeovertheCO2 2 2 2 rampings, by aboutrampings, 3.7 Wm by aboutper 3 70.7 yr, Wm which per is 70 the yr, period which of is CO the perioddoubling of orCO halvingdoubling or halving trajectoriestrajectoriesrampings, implies implies a lagged by a about lagged response 3 response.7 Wm of hemispheric-mean ofper hemispheric-mean 70 yr, which annual-mean is2 annual-mean the period surface of surface2 CO tempera-2 doubling tempera- or halving Global radiative forcing2 (F )changesapproximatelylinearlywithtimeovertheCO2 rampings, by about 3.7 Wm per 70 yr, which is the[Myhre period et of al. CO,[ 1998].2Myhredoubling[Myhre The et al. et o or,↵ 1998]. al.set halving, in 1998]. Figure The The o↵ 1set between o↵ inset Figure in warming Figure 1 between 1 (red) between warming and warming cooling (red) (blue) (red) and and cooling cooling (blue) (blue) tureture anomalies anomalies (T (NHTandNH andTSH),TSH as), expected as expected from from deep deep ocean ocean heat heat storage storage [e.g., [e.g.,HeldHeld et et 2 [Myhrerampings, et al., 1998]. by about The 3 o.7↵set Wm in Figureper 70 1 yr, between which is warming the period (red) of andCO2 coolingdoubling (blue) or halving trajectoriesal., implies 2010].al.trajectories, 2010].trajectories In a lagged order In order implies to response approximately implies to approximately a lagged of a hemispheric-mean lagged response account response account of for hemispheric-mean this for of annual-mean hemispheric-mean this lag, lag, we consider we consider surface annual-mean the annual-mean evolution thetempera- evolution surface of surface ice of tempera- ice tempera- [Myhre et al., 1998]. The o↵set in Figure 1 between warming (red) and cooling (blue) trajectories implies a lagged response of hemispheric-meanture anomalies annual-meanareaarea asture ( a asT functionture anomaliesNH asurface functionand anomalies ofT tempera- hemisphericSH ( of),T hemispheric asNH ( expectedandTNH and temperatureTSH temperature from),T asSH), expected deep as rather expected ocean rather from than heat than from deeptime. storage time. deep ocean A justiﬁcation ocean [e.g.,A heat justiﬁcationHeld heat storage etfor storage this [e.g.,for this [e.g.,HeldHeld et et trajectories implies a lagged response of hemispheric-meanal., 2010]. In annual-mean orderal., 2010]. to approximately In surface order to tempera- approximately account for this account lag, we for consider this lag, the we evolution consider of the ice evolution of ice ture anomalies (TNH and TSH), as expected from deep oceantreatment heattreatment storage isal. that, 2010].is [e.g., that annual-mean Inannual-meanHeld order et to Arctic approximately Arctic sea ice sea area ice accountarea has beenhas for been found this found to lag, decline to we decline consider linearly linearly the with evolution with of ice

dTu ~⌧ zˆ ⌧ x x dTd ~⌧ dTdzˆdT ⌧ cu = Tu + F + "(Td Tu)(1)!Te =!T =⇥ ⇥= =d yˆ yˆ (3) (3) dt cd = (Teu cdTd)(2)cd = (=Tu(TTud)(2)Td)(2) dT dt ⇢f ⇢dtf dt⇢f⇢f c u = T + F + "(T T )(1) u dt u d u

1 x1 ~⌧ ~⌧ x dTd x ~⌧ wzˆe =we =zˆ⌧ z~⌧ˆ z~⌧ˆ zˆ ⌧ ⌧ (4) (4) cd = (Tu Td)(2)!Te = ⇥ = !T⇢e =·⇢!r⇥Tyˆe =⇥f ⇥=f = yˆ yˆ (3) (3) (3) dt ⇢f ⇢f ⇢· fr⇥⇢f ⇢f ⇢f dT c d = (T T )(2) d dt u d @w @w@v @v @u @u@w @w @v @v@u @u ~⌧ zˆ ⌧ x ~u =~u = 1 xˆ + xˆ +~⌧ 1 1yˆ + y~⌧ˆ + ~⌧ zˆ zˆ (5) (5) r⇥r⇥ @ywe@=y@zzˆ@z w@ze =@wze@zˆ=x @xzˆ @x @x@y @y (4) (4) (4) !Te = ⇥ = yˆ ✓ (3) ◆ ✓ ◆ ✓ ◆ ⇢f ⇢f ✓ ⇢ · r⇥◆ f ✓ ⇢ · r⇥⇢ ◆· r⇥f ✓f ◆ ~⌧ zˆ ⌧ x !Te = ⇥ = yˆ (3) ⇢f ⇢f DRAFTDRAFT December14,2017,5:39pm December14,2017,5:39pm DRAFT DRAFT @w @v @w@u@w@v@w@v @u@v@u@w@u@w @v @v@u @u 1 ~⌧ ~u = ~u =xˆ~u+= xˆ +yˆxˆ++ yˆ +zˆ yˆ + (5)zˆ zˆ (5) (5) w = zˆ r⇥ @y r⇥ @zr⇥(4) @y@z@y@z@x@z @z@x@z@x@y@x @x @x@y @y e ⇢ · r⇥f ✓ ◆ ✓ ✓✓ ◆ ◆◆✓ ✓✓ ◆ ◆◆✓ ✓ ◆ ◆ 1 ~⌧ w = zˆ (4) e ⇢ · r⇥f DRAFTDRAFTDRAFT December14,2017,5:39pm December14,2017,5:39pm December14,2017,5:39pm DRAFT DRAFT DRAFT @w @v @u @w @v @u ~u = xˆ + yˆ + zˆ (5) r⇥ @y @z @z @x @x @y ✓ @w◆ @v ✓ @u ◆ @w ✓ @v◆ @u ~u = xˆ + yˆ + zˆ (5) r⇥ @y @z @z @x @x @y ✓ ◆ ✓ ◆ ✓ ◆ DRAFT December14,2017,5:39pm DRAFT DRAFT December14,2017,5:39pm DRAFT ARMOURARMOUR ET AL.: ET AL.: SEA SEAICE REVERSIBILITY ICE REVERSIBILITY X-5X-5

GlobalGlobal radiative radiative forcing forcing (F )changesapproximatelylinearlywithtimeovertheCO (F )changesapproximatelylinearlywithtimeovertheCO2 2

dTu Step 2: Drive model with timeseries~⌧ ~⌧zˆ z ˆof ⌧historicalx ⌧ x cu = Tu + F + "(Td Tu)(1)radiative forcingdT (dMeinshausen!T = ⇥ dT et=d al.dT 2011)d yˆ , with (3) c =e(!TTe =cdT⇥)(2)c ==(=Tu(yˆTTd)(2)T )(2)(3) dt priors drawn fromd forcing rangeu⇢f ⇢ dinf dIPCC⇢f AR5⇢f u d dTu dt dt dt cu = Tu + F + "(Td Tu)(1) dt

GlobalGlobal radiative radiative forcing forcing (F )changesapproximatelylinearlywithtimeovertheCO (F )changesapproximatelylinearlywithtimeovertheCO2 2

area as aincreasing functionarea of global-mean as hemispheric a function temperature of temperature hemispheric across rather temperature a range than of time. rather GCMs, A justification than emissions time. scenarios, A for justification this and for this al., 2010].ture In anomalies order to ( approximatelyTNH and TSH account), as expected for this from lag, wedeep consider oceanincreasing heat thearea evolution storage global-mean as a function [e.g., of ice temperatureHeld of hemispheric et across temperature a range of GCMs,rather than emissions time. scenarios, A justification and for this treatmentclimates isclimates thattreatment [ annual-meanGregorytreatment [Gregory is et that al. is et, Arctic that 2002; annual-meanal., 2002; annual-meanRidley seaRidley ice et area Arctic al. et, Arctichas 2008; al. sea, been 2008;Winton ice sea found areaWinton ice, area has 2006, to, decline been2006, has 2008, been found 2008, linearly2011]. found 2011]. to Specifically, decline with to Specifically, decline linearly linearly with with area asal. a, 2010].function In of order hemispheric to approximately temperature account rather for than this lag, time. we A consider justification the evolution for this of ice increasingwe global-mean extendweincreasing extend theincreasing arguments the temperature global-mean arguments global-mean of Winton of across temperatureWinton temperature[2011], a range[2011], relating across of relating GCMs, across hemispheric a range hemispheric a emissions range of ice GCMs, of cover ice scenarios, GCMs, cover toemissions global to emissions globaland forcing scenarios, forcing scenarios, and and treatmentarea is as that a function annual-mean of hemispheric Arctic sea temperature ice area has rather been found than totime. decline A justification linearly with for this climatesthrough [Gregorythroughclimates etclimates al., [Gregory 2002; [GregoryRidley et al. et et, 2002; al. al.,, 2002; 2008;RidleyRidleyWinton et al. et,, 2008; 2006, al., 2008;Winton 2008,Winton 2011]., 2006,, Specifically, 2006, 2008, 2008, 2011]. 2011]. Specifically, Specifically, increasingtreatment global-mean is that annual-mean temperature Arcticacross sea a range ice area of GCMs, has been emissions found to scenarios, decline linearly and with we extend the argumentswe extendwe extend the of Winton arguments the arguments[2011], of Winton relating of Winton[2011], hemispheric[2011], relating relating ice hemispheric cover hemispheric to global ice cover forcing ice cover to global to global forcing forcing increasing global-mean temperature across a range of GCMs, emissions scenarios, and climates [Gregory et al., 2002; Ridley et al., 2008; Winton, 2006, 2008, 2011]. Specifically,dTu dTu cu c = T=u +TF++F"+(T"d (TTu)(1)T )(1) through throughthrough dtu dt u d u climates [Gregory et al., 2002; Ridley et al., 2008; Winton, 2006, 2008, 2011]. Specifically, we extend the arguments of Winton [2011], relating hemisphericOur ice cover approach to global forcing we extend the arguments of Winton [2011], relating hemispheric ice cover to global forcing through dTu dTdTd udTdTd u § Use a 2-layer ocean model (Held et al. 2010; Step 1:cu Draw =priorsTu of+c dc uF , c +d c ,=u " =, ( ( T=andTdTu=u( +T T T ufromuFdu)(1))(2)++T dfitsF")(2) +(T"d (TTdu)(1)Tu)(1) Armour 2017) that includes the essential physics of 2-layerdt model to CMIP5dtdt modeldtdt response to through governing global-mean surface warming: CO2 forcing (Geoffroy et al. 2013) X - 12 ARMOUR ET AL.: SEA ICE REVERSIBILITY X - 12 ARMOUR ET AL.: SEA ICE REVERSIBILITY dTu Step 2: Drive model with timeseries~⌧ ~⌧zˆ z ôf ⌧historicalx ⌧ x cu = Tu + F + "(Td Tu)(1)radiative forcingdT (dMeinshausen!T = ⇥ dT et=d al.dT 2011)d yˆ , with (3) X - 12 ARMOUR ET AL.: SEA ICE REVERSIBILITYc =e(!TTe =cdT⇥)(2)c ==(=Tu(yˆTTd)(2)T )(2)(3) dt priors drawn fromd forcing rangeu⇢f ⇢ dinf dIPCC⇢f AR5⇢f u d dTu dt dt dt Tobs cu = Tu + F + "ECS(Td = TRu2)(1)/obs = R2 Tobs (63) dt ECS = R2 /⇥obs = R2 Step⇥ R obs3: UseH Bayesianobs inference to estimate(63) ⇥ ⇥ Robs Hobs posterior parameters/Tobs forcings based on 1 x1 ~⌧ ~⌧ x dTd ECS = R2 /obsobserved= R2 warming~⌧ andzˆ energy⌧ budget~⌧ z~⌧ˆ (seezˆ (63)⌧ ⌧ x ⇥ ⇥ R H we =we =zˆ zˆ (4) (4) cd = (Tu Td)(2)F2 also: Forestobs et!Te al.= 2002,obs⇥ 2006;= !T⇢ Stotte =·⇢!r⇥Ty ˆ&e ·=Forest⇥r⇥f ⇥= f2007)= yˆ yˆ (3) (3) (3) dt ECS = F2 ⇥ ⇢f ⇢f ⇢f ⇢f ⇢f⇢f dT ECS = ⇥ d (64) cd = (Tu Td)(2) F2 TobsF2 = 0.75 ± 0.2 °C (64) dt =ECSF2 =T⇥obs ⇥ = F ⇥ Q @w @w@v @v (Otto@ etu al.@ 2013;u@w @w @v @v@u @u F obs Q obs -2 ~⌧ zˆ ⌧ x obs obs =~u 0.65=~u ±= 0.27 Wm1 xˆ +2000-2009xˆ +~⌧ relative1 1 toyˆ +(64)y~⌧ˆ + ~⌧ zˆ zˆ (5) (5) r⇥F2r⇥Tobs @ywe@=y@zzˆ@z w@ze =@wze@zˆ=x @xzˆ @x @x@y @y (4) (4) (4) !Te = ⇥ = yˆ = ⇥ ✓ (3) ◆ 1860-1879)✓ ◆ ✓ ◆ ✓ -2 ⇢ · r⇥◆ f ✓ ⇢ · r⇥⇢ ◆· r⇥f ✓f ◆ ⇢f ⇢f x Fobs2 = 2.3Qobs ± 1 Wm ~⌧ zˆ ⌧ R 4 Wm2 (65) !T = ⇥ = yˆ R ⇡4 Wm (3) (65) e ⇢f ⇢f ⇡ DRAFTDRAFT2 December14,2017,5:39pm December14,2017,5:39pm DRAFT DRAFT R 4 Wm@w @v @w@u@w@v@w@v @u@v@(65)u@w@u@w @v @v@u @u 1 ~⌧ ~u⇡= ~u =xˆ~u+= xˆ +yˆxˆ++ yˆ +zˆ yˆ + (5)zˆ zˆ (5) (5) r⇥ @y r⇥ @zr⇥ @y@z@y@z@x@z @z@x@z@x@y@x @x @x@y @y we = zˆ ✓ 2 (4)◆ ✓ ✓ ◆ ◆ ✓ ✓ ◆ ◆ ✓ ◆ ⇢ · r⇥f H 0.7 Wm2 ✓ ◆ (66)✓ ◆ ✓ ◆ 1 ~⌧ H ⇡0.7 Wm (66) w = zˆ ⇡ (4) e 2 ⇢ · r⇥f H 0.7 Wm (66) DRAFTDRAFTDRAFT⇡ December14,2017,5:39pm December14,2017,5:39pm December14,2017,5:39pm DRAFT DRAFT DRAFT @w @v @u @w @v @u ~u = xˆ + yˆ + T 0.8C=0zˆ .8K(5) (67) r⇥ @y @z @z @x @xT ⇡[email protected]C=0.8K (67) ✓ @w◆ @v ✓ @u ◆ @w ✓ ⇡ @v◆ @u ~u = xˆ + yˆ + T 0.8C=0zˆ .8K(5) (67) r⇥ @y @z @z @x @x @y ✓ ◆ ✓ ◆ ✓ ⇡ ◆

DRAFT December14,2017,5:39pm2 /eff DRAFT (68) 2 /⇥eff (68) ⇥ DRAFT December14,2017,5:39pm DRAFT 2 /eff (68) ⇥

T (69) T (69)

T (69)

Q = T + R (70) Q = T + R (70)

Q = T + R (70)

R2 Tobs T2 = R2 ⇥ = Tobs R2 (71) T2 ⇥= ⇥ = Qobs RobsR2 ⇥ (71) ⇥ Qobs Robs ⇥ R2 Tobs T2 = ⇥ = R2 (71) ⇥ Q R ⇥ obs obs Tobs T2 = Tobs R2 (72) T2 ⇥= Qobs RobsR2 ⇥ (72) ⇥ Qobs Robs ⇥ Tobs T2 = R2 (72) ⇥ Q R ⇥ obs obs

DRAFT December8,2017,5:53pm DRAFT DRAFT December8,2017,5:53pm DRAFT

DRAFT December8,2017,5:53pm DRAFT ARMOURARMOUR ET AL.: ET AL.: SEA SEAICE REVERSIBILITY ICE REVERSIBILITY X-5X-5

GlobalGlobal radiative radiative forcing forcing (F )changesapproximatelylinearlywithtimeovertheCO (F )changesapproximatelylinearlywithtimeovertheCO2 2

Global radiativeGlobal forcingGlobal radiative (F radiative)changesapproximatelylinearlywithtimeovertheCO forcing forcing (F )changesapproximatelylinearlywithtimeovertheCO (F )changesapproximatelylinearlywithtimeovertheCO2 [Myhre[Myhre et al. et, al.1998]., 1998]. The The o↵set o↵ inset Figure in Figure 1 between 1 between warming warming (red) (red) and and cooling cooling2 (blue) (blue) 2 2 2 2 rampings, by aboutrampings, 3.7 Wm by aboutper 3 70.7 yr, Wm which per is 70 the yr, period which of is CO the perioddoubling of orCO halvingdoubling or halving trajectoriestrajectoriesrampings, implies implies a lagged by a about lagged response 3 response.7 Wm of hemispheric-mean ofper hemispheric-mean 70 yr, which annual-mean is2 annual-mean the period surface of surface2 CO tempera-2 doubling tempera- or halving [Myhre et al.,[ 1998].Myhre[Myhre The et al. et o,↵ 1998]. al.set, in 1998]. Figure The The o↵ 1set between o↵ inset Figure in warming Figure 1 between 1 (red) between warming and warming cooling (red) (blue) (red) and and cooling cooling (blue) (blue) tureture anomalies anomalies (T (NHTandNH andTSH),TSH as), expected as expected from from deep deep ocean ocean heat heat storage storage [e.g., [e.g.,HeldHeld et et

trajectoriesal., implies 2010].al.trajectories, 2010].trajectories In a lagged order In order implies to response approximately implies to approximately a lagged of a hemispheric-mean lagged response account response account of for hemispheric-mean this for of annual-mean hemispheric-mean this lag, lag, we consider we consider surface annual-mean the annual-mean evolution thetempera- evolution surface of surface ice of tempera- ice tempera-

ture anomaliesareaarea asture ( a asT functionture anomaliesNH a functionand anomalies ofT hemisphericSH ( of),T hemispheric asNH ( expectedandTNH and temperatureTSH temperature from),T asSH), expected deep as rather expected ocean rather from than heat than from deeptime. storage time. deep ocean A justiﬁcation ocean [e.g.,A heat justiﬁcationHeld heat storage etfor storage this [e.g.,for this [e.g.,HeldHeld et et

al., 2010].treatment Intreatment orderal., 2010]. isal. to that, approximately 2010].is that In annual-mean order Inannual-mean order to approximately account to Arctic approximately Arctic sea for icethis sea account area ice lag, accountarea has we for beenhas consider this for been found this lag, foundthe to lag,we evolution decline consider to we decline consider linearly of the linearly ice evolution the with evolution with of ice of ice

area as aincreasing functionincreasingareaarea of global-mean as hemispheric a global-mean as function a function temperature of temperature temperature hemispheric of hemispheric across rather across temperature a temperature range than a range of time. rather GCMs, of AGCMs,rather justification than emissions than emissions time. time. scenarios, A for justification scenarios, thisA justification and and for this for this

treatmentclimates isclimates thattreatment [ annual-meanGregorytreatment [Gregory is et that al. is et, Arctic that 2002; annual-meanal., 2002; annual-meanRidley seaRidley ice et area Arctic al. et, Arctichas 2008; al. sea, been 2008;Winton ice sea found areaWinton ice, area has 2006, to, decline been2006, has 2008, been found 2008, linearly2011]. found 2011]. to Speciﬁcally, decline with to Speciﬁcally, decline linearly linearly with with

increasingwe global-mean extendweincreasing extend theincreasing arguments the temperature global-mean arguments global-mean of Winton of across temperatureWinton temperature[2011], a range[2011], relating across of relating GCMs, across hemispheric a range hemispheric a emissions range of ice GCMs, of cover ice scenarios, GCMs, cover toemissions global to emissions globaland forcing scenarios, forcing scenarios, and and

climatesthrough [Gregorythroughclimates etclimates al., [Gregory 2002; [GregoryRidley et al. et et, 2002; al. al.,, 2002; 2008;RidleyRidleyWinton et al. et,, 2008; 2006, al., 2008;Winton 2008,Winton 2011]., 2006,, Specifically, 2006, 2008, 2008, 2011]. 2011]. Specifically, Specifically,

we extend the argumentswe extendwe extend the of Winton arguments the arguments[2011], of Winton relating of Winton[2011], hemispheric[2011], relating relating ice hemispheric cover hemispheric to global ice cover forcing ice cover to global to global forcing forcing

dTu dTu cu c = T=u +TF++F"+(T"d (TTu)(1)T )(1) through throughthrough dtu dt u d u Our approach

dTu dTdTd udTdTd u Step 1:c Draw =priorsT of+c dc uF , c +d c ,= " =, ( ( T=andTTu=u( +T T T ufromFd)(1))(2)++T dfitsF")(2) +(T"d (TTu)(1)T )(1) u u dt u d uu d u 2-layer model historical warming of 2-layerdt model to CMIP5dt modeldtdt response to CO2 forcing (Geoffroy et al. 2013) X - 12 ARMOUR ET AL.: SEA ICE REVERSIBILITY X - 12 ARMOUR ET AL.: SEA ICE REVERSIBILITY Step 2: Drive model with timeseries~⌧ ~⌧zˆ z ˆof ⌧historicalx ⌧ x radiative forcingdT (dMeinshausen!T = ⇥ dT et=d al.dT 2011)d yˆ , with (3) X - 12 ARMOUR ET AL.: SEA ICE REVERSIBILITYcd =e(!TTeu=cdTd⇥)(2)c ==(=Tu(yˆTTd)(2)T )(2)(3) priors drawn fromdt forcing range⇢f ⇢dt inf dIPCCdt⇢f AR5⇢f u d Tobs ECS = R2 /obs = R2 Tobs (63) ECS = R2 /⇥obs = R2 Step⇥ R obs3: UseH Bayesianobs inference to estimate(63) ⇥ ⇥ Robs Hobs posterior parameters/Tobs forcings based on 1 x1 ~⌧ ~⌧ x ECS = R2 /obsobserved= R2 warming~⌧ andzˆ energy⌧ budget~⌧ z~⌧ˆ (seezˆ (63)⌧ ⌧ x ⇥ ⇥ R H we =we =zˆ zˆ (4) (4) F2 also: Forestobs et!Te al.= 2002,obs⇥ 2006;= !T⇢ Stotte =·⇢!r⇥Ty ˆ&e ·=Forest⇥r⇥f ⇥= f2007)= yˆ yˆ (3) (3) (3) ECS = F2 ⇥ ⇢f ⇢f ⇢f ⇢f ⇢f⇢f ECS = ⇥ (64)

Temperature change [°C] change Temperature F2 TobsF2 = 0.75 ± 0.2 °C (64) =ECSF2 =T⇥obs ⇥ = F ⇥ Q @w @v (Otto@ etu al. 2013;@w @v @u obs obs @w -2 @v @u @w @v @u Fobs Qobs =~u 0.65=~u ±= 0.27 Wm1 xˆ +2000-2009xˆ +~⌧ relative1 1 toyˆ +(64)y~⌧ˆ + ~⌧ zˆ zˆ (5) (5) r⇥F2r⇥Tobs @yw@=y@zzˆ@z w@ze =@wz@zˆ=x @xzˆ @x @x@y @y (4) (4) (4) = ⇥ ✓ e ◆ 1860-1879)✓ e ◆ ✓ ◆ Year ✓ -2 ⇢ · r⇥◆ f ✓ ⇢ · r⇥⇢ ◆· r⇥f ✓f ◆ Fobs2 = 2.3Qobs ± 1 Wm R 4 Wm2 (65) R ⇡4 Wm (65) ⇡ DRAFTDRAFT2 December14,2017,5:39pm December14,2017,5:39pm DRAFT DRAFT R 4 Wm@w @v @w@u@w@v@w@v @u@v@(65)u@w@u@w @v @v@u @u ~u⇡= ~u =xˆ~u+= xˆ +yˆxˆ++ yˆ +zˆ yˆ + (5)zˆ zˆ (5) (5) @y r⇥@z @y@z @z@x @z@x @x@y @x @y r⇥ 2 r⇥ @y @z @z @x @x @y H 0.7 Wm✓2 ◆ ✓ ✓✓ ◆ ◆◆✓(66)✓✓ ◆ ◆◆✓ ✓ ◆ ◆ H ⇡0.7 Wm (66) ⇡ 2 H 0.7 Wm (66) DRAFTDRAFTDRAFT⇡ December14,2017,5:39pm December14,2017,5:39pm December14,2017,5:39pm DRAFT DRAFT DRAFT

T 0.8C=0.8K (67) T ⇡0.8C=0.8K (67) ⇡ T 0.8C=0.8K (67) ⇡

2 /eff (68) 2 /⇥eff (68) ⇥

2 /eff (68) ⇥

T (69) T (69)

T (69)

Q = T + R (70) Q = T + R (70)

Q = T + R (70)

DRAFT December8,2017,5:53pm DRAFT DRAFT December8,2017,5:53pm DRAFT

DRAFT December8,2017,5:53pm DRAFT ARMOURARMOUR ET AL.: ET AL.: SEA SEAICE REVERSIBILITY ICE REVERSIBILITY X-5X-5

GlobalGlobal radiative radiative forcing forcing (F )changesapproximatelylinearlywithtimeovertheCO (F )changesapproximatelylinearlywithtimeovertheCO2 2

dTu dTu cu c = T=u +TF++F"+(T"d (TTu)(1)T )(1) through throughthrough dtu dt u d u Our approach

Step 2: Drive model with timeseries~⌧ ~⌧zˆ z ˆof ⌧historicalx ⌧ x radiative forcingdT (dMeinshausen!T = ⇥ dT et=d al.dT 2011)d yˆ , with (3) cd =e(!TTeu=cdTd⇥)(2)c ==(=Tu(yˆTTd)(2)T )(2)(3) priors drawn fromdt forcing range⇢f ⇢dt inf dIPCCdt⇢f AR5⇢f u d

Step 3: Use Bayesian inference to estimate posterior parameters/forcings1 based on~⌧ observed warming and energyx budget1 (see~⌧ x x ~⌧ wzˆe =we =zˆ⌧ z~⌧ˆ z~⌧ˆ zˆ ⌧ ⌧ (4) (4) also: Forest et!Te al.= 2002,⇥ 2006;= !T⇢ Stotte =·⇢!r⇥Ty ˆ&e ·=Forest⇥r⇥f ⇥= f2007)= yˆ yˆ (3) (3) (3) ⇢f ⇢f ⇢f ⇢f ⇢f⇢f

DRAFTDRAFT December14,2017,5:39pm December14,2017,5:39pm DRAFT DRAFT @w @v @w@u@w@v@w@v @u@v@u@w@u@w @v @v@u @u ~u = ~u =xˆ~u+= xˆ +yˆxˆ++ yˆ +zˆ yˆ + (5)zˆ zˆ (5) (5) r⇥ @y r⇥ @zr⇥ @y@z@y@z@x@z @z@x@z@x@y@x @x @x@y @y ✓ ◆ ✓ ✓✓ ◆ ◆◆✓ ✓✓ ◆ ◆◆✓ ✓ ◆ ◆

DRAFTDRAFTDRAFT December14,2017,5:39pm December14,2017,5:39pm December14,2017,5:39pm DRAFT DRAFT DRAFT ARMOURARMOUR ET AL.: ET AL.: SEA SEAICE REVERSIBILITY ICE REVERSIBILITY X-5X-5

GlobalGlobal radiative radiative forcing forcing (F )changesapproximatelylinearlywithtimeovertheCO (F )changesapproximatelylinearlywithtimeovertheCO2 2

dTu dTu cu c = T=u +TF++F"+(T"d (TTu)(1)T )(1) through throughthrough dtu dt u d u Our approach Final Draft (7 June 2013) Chapter 12 IPCC WGI Fifth Assessment Report dTu dTdTd udTdTd u Final Draft (7 June 2013) Chapter 12 IPCC WGI Fifth AssessmentStep 1:c Report Draw =priorsT of+c dc uF , c +d c ,= " =, ( ( T=andTTu=u( +T T T ufromFd)(1))(2)++T dfitsF")(2) +(T"d (TTu)(1)T )(1) u dt u dtdtudt d uu d u 2-layer model projections of 2-layer model to CMIP5 modeldt response to CO2 forcing (Geoffroy et al. 2013)

Step 4: Use the observationally-constrained Temperature change [°C] change Temperature parameter/forcing estimates to project @w @w@v @v @u @u@w @w @v @v@u @u warming,~u =~u and= committed1 xˆ + warming,xˆ +~⌧ 1following1yˆ + y~⌧ˆ + ~⌧ zˆ zˆ (5) (5) RCP2.6 and RCP8.5 emissions scenarios r⇥r⇥ @ywe@=y@zzˆ@z w@ze =@wze@zˆ=x @xzˆ @x @x@y @y (4) (4) (4) Figure 12.5: Time series of global annual mean surfaceYear air temperature anomalies (relative to 1986–2005) from CMIP5✓ ✓ ⇢◆ · r⇥◆ ✓ f ✓ ⇢ ·◆r⇥⇢ ◆· r⇥✓f ✓f ◆ ◆ concentration-driven experiments. Projections are shown for each RCP for the multi model mean (solid lines) and the Figure5–95% range12.5: Time(±1.64 series standard of global deviation) annual across mean the surface distributi air temperon of individualature anomalies models (relative (shading). to 1986–2005) Discontinuities from at CMIP5 2100 concentration-drivenare due to different numbers experime ofnts. models Projections performing are shown the extension for each runs RCP beyond for the the multi 21st model century mean and (solid have lines)no physical and the meaning.5–95% range Only (±1.64 one ensemble standard memberdeviation) is acrossused from the distributieach modelon ofand individual numbersDRAFT modelsin theDRAFT figu (shading).re indicate Discontinuities the number of at 2100 December14,2017,5:39pm December14,2017,5:39pm DRAFT DRAFT differentare due to models different contributing numbers of to models the different performing time periods. the extension No ranges runs arebeyond given the for 21st the centuryRCP6.0 and projections have no beyondphysical 2100 asmeaning. only two Only models one ensembleare available. member is used from each model and numbers in the figure indicate the number@ ofw @v @w@u@w@v@w@v @u@v@u@w@u@w @v @v@u @u different models contributing to the different time periods. No ranges are given for the RCP6.0 projections~u = beyond 2100 ~u =xˆ~u+= xˆ +yˆxˆ++ yˆ +zˆ yˆ + (5)zˆ zˆ (5) (5) as only two models are available. r⇥ @y r⇥ @zr⇥ @y@z@y@z@x@z @z@x@z@x@y@x @x @x@y @y ✓ ◆ ✓ ✓✓ ◆ ◆◆✓ ✓✓ ◆ ◆◆✓ ✓ ◆ ◆

DRAFTDRAFTDRAFT December14,2017,5:39pm December14,2017,5:39pm December14,2017,5:39pm DRAFT DRAFT DRAFT

Do Not Cite, Quote or Distribute 12-129 Total pages: 175

Do Not Cite, Quote or Distribute 12-129 Total pages: 175 ARMOURARMOUR ET AL.: ET AL.: SEA SEAICE REVERSIBILITY ICE REVERSIBILITY X-5X-5

GlobalGlobal radiative radiative forcing forcing (F )changesapproximatelylinearlywithtimeovertheCO (F )changesapproximatelylinearlywithtimeovertheCO2 2

DRAFTDRAFTDRAFT December14,2017,5:39pm December14,2017,5:39pm December14,2017,5:39pm DRAFT DRAFT DRAFT

Do Not Cite, Quote or Distribute 12-129 Total pages: 175

Do Not Cite, Quote or Distribute 12-129 Total pages: 175 ARMOURARMOUR ET AL.: ET AL.: SEA SEAICE REVERSIBILITY ICE REVERSIBILITY X-5X-5

GlobalGlobal radiative radiative forcing forcing (F )changesapproximatelylinearlywithtimeovertheCO (F )changesapproximatelylinearlywithtimeovertheCO2 2

DRAFTDRAFTDRAFT December14,2017,5:39pm December14,2017,5:39pm December14,2017,5:39pm DRAFT DRAFT DRAFT

Do Not Cite, Quote or Distribute 12-129 Total pages: 175

Do Not Cite, Quote or Distribute 12-129 Total pages: 175 Our approach Final Draft (7 June 2013) Chapter 12 IPCC WGI Fifth Assessment Report

Final Draft (7 June 2013) Chapter 12 IPCC WGI Fifth Assessment Report Final DraftFinal (7 JuneDraftFinal 2013) (7 June Draft 2013) (7 June 2013) Chapter 12 Chapter 12 Chapter 12 IPCC WGI IPCC Fifth WGI Assessment IPCC Fifth WGI Assessment Report Fifth Assessment Report Report 2-layer model projections CMIP5 projections (IPCC AR5) Temperature change [°C] change Temperature

Figure 12.5: Time series of global annual mean surfaceYear air temperature anomalies (relative to 1986–2005) from CMIP5 concentration-driven experiments. Projections are shown for each RCP for the multi model mean (solid lines) and the Figure5–95% range12.5: Time(±1.64 series standard of global deviation) annual across mean the surface distributi air temperon of individualature anomalies models (relative (shading). to 1986–2005) Discontinuities from at CMIP5 2100 concentration-drivenare due to different numbers experime ofnts. models Projections performing are shown the extension for each runs RCP beyond forFigure the the multi 12.5:21stFigure model century Time 12.5: meanFigure series and Time (solid have of12.5: globalseries lines)no Time physical ofannual and globalseries the mean ofannual global surface mean annual air surface temper mean air aturesurface temper anomalies airature temper anomalies (relativeature anomalies to(relative 1986–2005) to(relative 1986–2005) from to CMIP51986–2005) from CMIP5 from CMIP5 meaning.5–95% range Only (±1.64 one ensemble standard memberdeviation) is acrossused from the distributieach modelon ofand individual numbersconcentration-driven modelsin the concentration-drivenfigu (shading).re indicateconcentration-driven experimeDiscontinuities the number experiments. Projections of at 2100nts.experime Projections arents. shown Projections are for shown each are RCPfor shown each for theRCP for multi each for themodelRCP multi for mean themodel multi(solid mean model lines) (solid andmean lines) the (solid and lines) the and the differentare due to models different contributing numbers of to models the different performing time periods. the extension No ranges runs arebeyond given5–95% the for range21st 5–95%the centuryRCP6.0 (±1.64 range5–95% and standardprojections (±1.64 have range deviation)standardno (±1.64 beyondphysical deviation)standard across2100 the deviation) across distributi the across ondistributi of individualthe ondistributi of individual modelson of individual(shading). models (shading). Discontinuitiesmodels (shading). Discontinuities at 2100Discontinuities at 2100 at 2100 asmeaning. only two Only models one ensembleare available. member is used from each model and numbersare in due the to arefigu different duere indicate toare differentnumbers due the to number numbersofdifferent models of ofnumbers performing models of performing models the extension performing the extension runs thebeyond extension runs the beyond 21st runs century the beyond 21st and century the have 21st andno century physical have andno physical have no physical different models contributing to the different time periods. No ranges are givenmeaning. for meaning. theOnly RCP6.0 one meaning.Only ensemble projections one Onlyensemble member beyond one ensemble ismember used2100 from ismember used each from modelis used each and from model numbers each and model numbersin the and figu innumbersre the indicate figu inre the theindicate figunumberre the indicate of number the of number of as only two models are available. different differentmodels contributing modelsdifferent contributing models to the contributingdifferent to the differenttime to periods.the timedifferent Noperiods. ranges time No periods. are ranges given No are for ranges giventhe RCP6.0 arefor giventhe projections RCP6.0 for the projections RCP6.0 beyond projections 2100 beyond 2100 beyond 2100 as only twoas onlymodels twoas are onlymodels available. two are models available. are available.

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Do Not Cite, Quote or Distribute 12-129 Total pages: 175 Do Not Cite,Do Not Quote Cite,Do or Not Quote Distribute Cite, or Quote Distribute or Distribute 12-129 12-129 12-129 Total pages:Total 175 pages: Total 175 pages: 175 When are we going to cross 1.5 or 2 °C thresholds? Final Draft (7 June 2013) Chapter 12 IPCC WGIFinal Fifth DraftAssessment (7 June Report 2013) Chapter 12 IPCC WGI Fifth Assessment Report

Final Draft (7 June 2013) Chapter 12 IPCC WGIFinal Fifth DraftAssessment (7 June Report 2013) Chapter 12 IPCC WGI Fifth Assessment Report

2-layer model projections Year at which 2 °C is crossed ] yr Probability density [1/ Temperature change [°C] change Temperature

Figure 12.5: Time series of global annual mean surfaceYear air temperature anomalies (relative to 1986–2005)Figure 12.5: from Time CMIP5 series of global annual mean surfaceYear air temperature anomalies (relative to 1986–2005) from CMIP5 concentration-driven experiments. Projections are shown for each RCP for the multi model mean concentration-driven (solid lines) and the experime nts. Projections are shown for each RCP for the multi model mean (solid lines) and the Figure5–95% range12.5: Time(±1.64 series standard of global deviation) annual across mean the surface distributi air temperon of individualature anomalies models (relative (shading). to 1986–2005)Figure5–95% Discontinuities range12.5: from Time(±1.64 at CMIP5 2100series standard of global deviation) annual across mean the surface distributi air temperon of individualature anomalies models (relative (shading). to 1986–2005) Discontinuities from at CMIP5 2100 concentration-drivenare due to different numbers experime ofnts. models Projections performing are shown the extension for each runs RCP beyond for the the multi 21st model century mean andconcentration-drivenare (solid duehave to lines)no different physical and thenumbers experime ofnts. models Projections performing are shown the extension for each runs RCP beyond for the the multi 21st model century mean and (solid have lines)no physical and the meaning.5–95% range Only (±1.64 one ensemble standard memberdeviation) is acrossused from the distributieach modelon ofand individual numbers modelsin the figu (shading).re indicatemeaning.5–95% Discontinuities the numberrange Only (±1.64 oneof at 2100 ensemble standard memberdeviation) is acrossused from the distributieach modelon ofand individual numbers modelsin the figu (shading).re indicate Discontinuities the number of at 2100 differentare due to models different contributing numbers of to models the different performing time periods. the extension No ranges runs arebeyond given the for 21st the centuryRCP6.0 anddifferentare projections duehave to modelsno different beyondphysical contributing numbers 2100 of to models the different performing time periods. the extension No ranges runs arebeyond given the for 21st the centuryRCP6.0 and projections have no beyondphysical 2100 asmeaning. only two Only models one ensembleare available. member is used from each model and numbers in the figure indicateasmeaning. only the numbertwo Only models oneof ensembleare available. member is used from each model and numbers in the figure indicate the number of different models contributing to the different time periods. No ranges are given for the RCP6.0 differentprojections models beyond contributing 2100 to the different time periods. No ranges are given for the RCP6.0 projections beyond 2100 as only two models are available. as only two models are available.

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Do Not Cite, Quote or Distribute 12-129 Do Not Cite,Total Quote pages: or 175 Distribute 12-129 Total pages: 175 When are we going to cross 1.5 or 2 °C thresholds? Final Draft (7 June 2013) Chapter 12 IPCC WGIFinal Fifth DraftAssessment (7 June Report 2013) Chapter 12 IPCC WGI Fifth Assessment Report

Final Draft (7 June 2013) Chapter 12 IPCC WGIFinal Fifth DraftAssessment (7 June Report 2013) Chapter 12 IPCC WGI Fifth Assessment Report

2-layer model projections Year at which 2 °C is crossed ] yr Probability density [1/ Temperature change [°C] change Temperature

Figure 12.5: Time series of global annual mean surfaceYear air temperature anomalies (relative to 1986–2005)Figure 12.5: from TimeProbability CMIP5 series of global of annual mean surfaceYear air temperature anomalies (relative to 1986–2005) from CMIP5 concentration-driven experiments. Projections are shown for each RCP for the multi model mean concentration-driven (solid crossinglines) and the experime2.0 °C nts.along Projections are shown for each RCP for the multi model mean (solid lines) and the Figure5–95% range12.5: Time(±1.64 series standard of global deviation) annual across mean the surface distributi air temperon of individualature anomalies models (relative (shading). to 1986–2005)Figure5–95% Discontinuities range12.5: from Time(±1.64 at CMIP5 2100series standard of global deviation) annual across mean the surface distributi air temperon of individualature anomalies models (relative (shading). to 1986–2005) Discontinuities from at CMIP5 2100 concentration-drivenare due to different numbers experime ofnts. models Projections performing are shown the extension for each runs RCP beyond for the the multi 21st model century mean andconcentration-drivenare (solid duehave to lines)no different physicalRCP and thenumbers 2.6experime = 0.13ofnts. models Projections performing are shown the extension for each runs RCP beyond for the the multi 21st model century mean and (solid have lines)no physical and the meaning.5–95% range Only (±1.64 one ensemble standard memberdeviation) is acrossused from the distributieach modelon ofand individual numbers modelsin the figu (shading).re indicatemeaning.5–95% Discontinuities the numberrange Only (±1.64 oneof at 2100 ensemble standard memberdeviation) is acrossused from the distributieach modelon ofand individual numbers modelsin the figu (shading).re indicate Discontinuities the number of at 2100 differentare due to models different contributing numbers of to models the different performing time periods. the extension No ranges runs arebeyond given the for 21st the centuryRCP6.0 anddifferentare projections duehave to modelsno different beyondphysical contributing numbers 2100 of to models the different performing time periods. the extension No ranges runs arebeyond given the for 21st the centuryRCP6.0 and projections have no beyondphysical 2100 asmeaning. only two Only models one ensembleare available. member is used from each model and numbers in the figure indicateasmeaning. only the numbertwo Only models oneof ensembleare available. member is used from each model and numbers in the figure indicate the number of different models contributing to the different time periods. No ranges are given for the RCP6.0 differentprojections models beyond contributing 2100 to the different time periods. No ranges are given for the RCP6.0 projections beyond 2100 as only two models are available. as only two models are available.

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Final Draft (7 June 2013) Chapter 12 IPCC WGIFinal Fifth DraftAssessment (7 June Report 2013) Chapter 12 IPCC WGI Fifth Assessment Report

2-layer model projections Year at which 1.5 °C is crossed ] yr Probability density [1/ Temperature change [°C] change Temperature

Figure 12.5: Time series of global annual mean surfaceYear air temperature anomalies (relative to 1986–2005)Figure 12.5: from TimeProbability CMIP5 series of global of annual mean surfaceYear air temperature anomalies (relative to 1986–2005) from CMIP5 concentration-driven experiments. Projections are shown for each RCP for the multi model mean concentration-driven (solid crossinglines) and the experime1.5 °C nts.along Projections are shown for each RCP for the multi model mean (solid lines) and the Figure5–95% range12.5: Time(±1.64 series standard of global deviation) annual across mean the surface distributi air temperon of individualature anomalies models (relative (shading). to 1986–2005)Figure5–95% Discontinuities range12.5: from Time(±1.64 at CMIP5 2100series standard of global deviation) annual across mean the surface distributi air temperon of individualature anomalies models (relative (shading). to 1986–2005) Discontinuities from at CMIP5 2100 concentration-drivenare due to different numbers experime ofnts. models Projections performing are shown the extension for each runs RCP beyond for the the multi 21st model century mean andconcentration-drivenare (solid duehave to lines)no different physicalRCP and thenumbers 2.6experime = 0.41ofnts. models Projections performing are shown the extension for each runs RCP beyond for the the multi 21st model century mean and (solid have lines)no physical and the meaning.5–95% range Only (±1.64 one ensemble standard memberdeviation) is acrossused from the distributieach modelon ofand individual numbers modelsin the figu (shading).re indicatemeaning.5–95% Discontinuities the numberrange Only (±1.64 oneof at 2100 ensemble standard memberdeviation) is acrossused from the distributieach modelon ofand individual numbers modelsin the figu (shading).re indicate Discontinuities the number of at 2100 differentare due to models different contributing numbers of to models the different performing time periods. the extension No ranges runs arebeyond given the for 21st the centuryRCP6.0 anddifferentare projections duehave to modelsno different beyondphysical contributing numbers 2100 of to models the different performing time periods. the extension No ranges runs arebeyond given the for 21st the centuryRCP6.0 and projections have no beyondphysical 2100 asmeaning. only two Only models one ensembleare available. member is used from each model and numbers in the figure indicateasmeaning. only the numbertwo Only models oneof ensembleare available. member is used from each model and numbers in the figure indicate the number of different models contributing to the different time periods. No ranges are given for the RCP6.0 differentprojections models beyond contributing 2100 to the different time periods. No ranges are given for the RCP6.0 projections beyond 2100 as only two models are available. as only two models are available.

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Do Not Cite, Quote or Distribute 12-129 Do Not Cite,Total Quote pages: or 175 Distribute 12-129 Total pages: 175 correspondence Committed climate warming

To the Editor — Te perception that Te outcome of the ffeenth Conference represents a central consideration in this future climate warming is inevitable stands of Parties to the United Nations Framework discussion. Unavoidable future warming at the centre of current climate-policy Convention on Climate Change was the requires a substantial allocation of funds discussions. We argue that the notion of Copenhagen Accord1. In addition to a to adaptation. However, if mitigation could unavoidable warming owing to inertia in broad acknowledgement of the need for be successful in avoiding climate change, the climate system is based on an incorrect deep emissions cuts, the accord included a the decrease of greenhouse-gas emissions interpretation of climate science. Stable commitment from developed countries to should be the highest near-term priority. atmospheric concentrations of greenhouse provide fnancial resources to developing Climate change commitment is defned gases would lead to continued warming, nations, noting the need for such resources as the future warming to which we have but if carbon dioxide emissions could to be balanced between funding for committed ourselves by virtue of past be eliminated entirely, temperatures mitigation and funding for adaptation. Tis human activities. Because of the slow would quickly stabilize or even decrease balance of mitigation and adaptation is also response time of the climate system, the over time. Future warming is therefore clear at the national level, where decisions equilibrium climate consistent with current driven by socio-economic inertia, and must be made whether to allocate available levels of greenhouse gases will not be is only as inevitable as future emissions. funding to projects aimed at decreasing reached for many centuries. Tis so-called As a consequence, mitigationZero-emissions eforts to emissions, or to those climate intended to decrease commitment constant-composition commitment results minimize future greenhouse-gas emissions vulnerability to the anticipated impacts of as temperatures gradually equilibrate can successfully restrict future warming climate change. with the current atmospheric radiation § to a level that may avoid dangerous Tere are many issues and interests that imbalance, and has been estimated at Howanthropogenic much interference more warmingwith the climate willmust occurbe considered given in weighing no the furtherbenefts between human 0.3 °C andinfluence 0.9 °C warming onover theclimate? system. Te challenge of climate mitigation, of mitigation or adaptation response- next century2 (Fig. 1). although§ Constant daunting, atmosphericis fully within the scope composition strategies. We argue requires here that thecontinued perception emissions;Constant-composition the climate commitment iscommitment ofis humanbetter control. defined with respectthat to future past climate emissions warming is inevitable only ofen misinterpreted as the unavoidable warming that is yet to manifest in response to past greenhouse-gas emissions3. However, 1.5 the climate warming commitment from past greenhouse-gas emissions is more Constant composition correctly defned as a ‘zero-emissions commitment’ — that is, the future climate IPCC AR4 models change that would occur, should greenhouse- This study gas emissions be eliminated entirely4. In 1.0

e ( °C) response to an abrupt elimination of carbon dioxide emissions, global temperatures either Zero emissions remain approximately constant, or cool e chang slightly as natural carbon sinks gradually atur draw anthropogenic carbon out of the BERN2.5CC model atmosphere at a rate similar to the mixing of 0.5 HadCM3LC model heat into the deep ocean5–8 (Fig. 1). From this we conclude that the elimination of carbon Global temper dioxide emissions leads to little or no further climate warming; that is, future warming is defned by the extent of future emissions, (Matthews and Weaver 2010) rather than by past emissions. Tese contrasting interpretations of 0.0 climate commitment carry very diferent 1800 1900 2000 2100 2200 2300 implications for global climate policy. Year We argue here that future warming is not predetermined by climate inertia, but Figure 1 | Two representations of climate commitment. Constant-composition commitment (red line) rather is a consequence of socio-economic, represents the future warming associated with current atmospheric carbon dioxide concentrations, and cultural and behavioural inertia; as such, zero-emissions commitment (blue line) represents the warming commitment from past carbon dioxide mitigation should be the priority of both emissions (simulated here by the UVic Earth System Climate Model; refs 6 and 9). Also shown are the national and international agreements so constant-composition commitments simulated by the Intergovernmental Panel on Climate Change fourth as to minimize future emissions. Some assessment report (IPCC AR4) models (orange line, multimodel mean; yellow shading, range spanned amount of adaptation will be required in by models)2, the zero-emissions commitment simulated by the HadCM3LC and BERN2.5CC models response to climate impacts that are already (grey lines; refs 7 and 8, respectively) and historical temperature data (black line). Constant atmospheric being felt, and adaptation will clearly be carbon dioxide concentrations lead to continued warming for many centuries, whereas the elimination of necessary in key vulnerable areas as these carbon dioxide emissions leads to approximately stable or decreasing global temperature. impacts continue to manifest owing to

142 NATURE GEOSCIENCE | VOL 3 | MARCH 2010 | www.nature.com/naturegeoscience

ngeo_Correspondence run on_MAR10.indd 142 15/2/10 15:42:11 correspondence Committed climate warming

To the Editor — Te perception that Te outcome of the ffeenth Conference represents a central consideration in this future climate warming is inevitable stands of Parties to the United Nations Framework discussion. Unavoidable future warming at the centre of current climate-policy Convention on Climate Change was the requires a substantial allocation of funds discussions. We argue that the notion of Copenhagen Accord1. In addition to a to adaptation. However, if mitigation could unavoidable warming owing to inertia in broad acknowledgement of the need for be successful in avoiding climate change, the climate system is based on an incorrect deep emissions cuts, the accord included a the decrease of greenhouse-gas emissions interpretation of climate science. Stable commitment from developed countries to should be the highest near-term priority. atmospheric concentrations of greenhouse provide fnancial resources to developing Climate change commitment is defned gases would lead to continued warming, nations, noting the need for such resources as the future warming to which we have but if carbon dioxide emissions could to be balanced between funding for committed ourselves by virtue of past be eliminated entirely, temperatures mitigation and funding for adaptation. Tis human activities. Because of the slow would quickly stabilize or even decrease balance of mitigation and adaptation is also response time of the climate system, the over time. Future warming is therefore clear at the national level, where decisions equilibrium climate consistent with current driven by socio-economic inertia, and must be made whether to allocate available levels of greenhouse gases will not be is only as inevitable as future emissions. funding to projects aimed at decreasing reached for many centuries. Tis so-called As a consequence, mitigationZero-emissions eforts to emissions, or to those climate intended to decrease commitment constant-composition commitment results minimize future greenhouse-gas emissions vulnerability to the anticipated impacts of as temperatures gradually equilibrate can successfully restrict future warming climate change. with the current atmospheric radiation § to a level that may avoid dangerous Tere are many issues and interests that imbalance, and has been estimated at Howanthropogenic much interference more warmingwith the climate willmust occurbe considered given in weighing no the furtherbenefts between human 0.3 °C andinfluence 0.9 °C warming onover theclimate? system. Te challenge of climate mitigation, of mitigation or adaptation response- next century2 (Fig. 1). although§ Constant daunting, atmosphericis fully within the scope composition strategies. We argue requires here that thecontinued perception emissions;Constant-composition the climate commitment iscommitment ofis humanbetter control. defined with respectthat to future past climate emissions warming is inevitable only ofen misinterpreted as the unavoidable warming that is yet to manifest in response to past greenhouse-gas emissions3. However, 1.5 theZero-emissions climate warming commitment climate from commitment pastassuming greenhouse-gas cessation emissions is more of CO emissions Constant composition correctly defned as a ‘zero-emissions 2 commitment’only, temperatures — that is, the future climate stay flat or even IPCC AR4 models changedecline that would (see occur, also:should greenhouse- Solomon et al. This study gas emissions be eliminated entirely4. In 1.0

e ( °C) response2009) to an abrupt elimination of carbon dioxide emissions, global temperatures either Zero emissions remain approximately constant, or cool e chang § We are not committed to any slightly as natural carbon sinks gradually atur draw anthropogenicmore warming carbon out of the in the pipeline BERN2.5CC model atmosphere at a rate similar to the mixing of 0.5 HadCM3LC model 5–8 heat into §the … deep but ocean what(Fig. 1).about From this non-CO 2 we conclude that the elimination of carbon Global temper forcing agents? dioxide emissions leads to little or no further climate warming; that is, future warming is defned by the extent of future emissions, (Matthews and Weaver 2010) rather than by past emissions. Tese contrasting interpretations of 0.0 climate commitment carry very diferent 1800 1900 2000 2100 2200 2300 implications for global climate policy. Year We argue here that future warming is not predetermined by climate inertia, but Figure 1 | Two representations of climate commitment. Constant-composition commitment (red line) rather is a consequence of socio-economic, represents the future warming associated with current atmospheric carbon dioxide concentrations, and cultural and behavioural inertia; as such, zero-emissions commitment (blue line) represents the warming commitment from past carbon dioxide mitigation should be the priority of both emissions (simulated here by the UVic Earth System Climate Model; refs 6 and 9). Also shown are the national and international agreements so constant-composition commitments simulated by the Intergovernmental Panel on Climate Change fourth as to minimize future emissions. Some assessment report (IPCC AR4) models (orange line, multimodel mean; yellow shading, range spanned amount of adaptation will be required in by models)2, the zero-emissions commitment simulated by the HadCM3LC and BERN2.5CC models response to climate impacts that are already (grey lines; refs 7 and 8, respectively) and historical temperature data (black line). Constant atmospheric being felt, and adaptation will clearly be carbon dioxide concentrations lead to continued warming for many centuries, whereas the elimination of necessary in key vulnerable areas as these carbon dioxide emissions leads to approximately stable or decreasing global temperature. impacts continue to manifest owing to

142 NATURE GEOSCIENCE | VOL 3 | MARCH 2010 | www.nature.com/naturegeoscience

ngeo_Correspondence run on_MAR10.indd 142 15/2/10 15:42:11 Zero-emissions climate commitment

2-layer model forcing ] -2 Zero-emissions climate commitment assuming cessation of CO2 emissions only, temperatures stay flat or even decline (see also: Solomon et al. 2009) § We are not committed to any more warming in the pipeline

§ … but what about non-CO2 Radiative[Wm forcing forcing agents?

Year Zero-emissions climate commitment

2-layer model forcing

Cessation of emission at

] 8:22am, Dec 15, 2017 -2 Jump due to loss of Zero-emissions climate commitment tropospheric aerosols assuming cessation of CO2 emissions only, temperatures stay flat or even decline (see also: Solomon et al. 2009) § We are not committed to any Long lifetime of CO 2 more warming in the pipeline (range of carbon cycle parameters § … but what about non-CO2

Radiative[Wm forcing from Joos et al. 2013) forcing agents?

Year Zero-emissions climate commitment Final Draft (7 June 2013) Chapter 12 IPCC WGI Fifth Assessment Report

Final Draft (7 June 2013) Chapter 12 IPCC WGI Fifth Assessment Report 2-layer model temperature response

Zero-emissions climate commitment assuming cessation of CO2 emissions only, temperatures stay flat or even decline (see also: Solomon et al. 2009) § We are not committed to any more warming in the pipeline

§ … but what about non-CO2

Temperature change [°C] change Temperature forcing agents?

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Do Not Cite, Quote or Distribute 12-129 Total pages: 175 Zero-emissions climate commitment Final Draft (7 June 2013) Chapter 12 IPCC WGI Fifth Assessment Report

2-layer model temperature response Committed warming (above pre-industrial) inferred from observed warming and global energy budget: § Armour & Roe (2011): 0.6 °C (0.3-7.2 °C, 5-95%) based on AR4 values

§ Mauritsen & Pincus (2017): 1.1 °C (0.7-1.8 °C, 5-95%) based on AR5 values and updated energy budget; 13% that we’re already committed to 1.5 °C

§ This work: 1.3 °C (0.9-1.9 °C, 5-95%)

Temperature change [°C] change Temperature based on Otto et. al energy budget; 14% that we’re already committed to 1.5 °C, 3% chance that we’re already

Figure 12.5: Time series of global annual mean surfaceYear air temper ature anomalies (relative to 1986–2005) from CMIP5committed to 2 °C concentration-driven experiments. Projections are shown for each RCP for the multi model mean (solid lines) and the 5–95% range (±1.64 standard deviation) across the distribution of individual models (shading). Discontinuities at 2100 are due to different numbers of models performing the extension runs beyond the 21st century and have no physical meaning. Only one ensemble member is used from each model and numbers in the figure indicate the number of different models contributing to the different time periods. No ranges are given for the RCP6.0 projections beyond 2100 as only two models are available.

Do Not Cite, Quote or Distribute 12-129 Total pages: 175 When are we committed to crossing 1.5 or 2 °C thresholds? Final Draft (7 June 2013) Chapter 12 IPCC WGIFinal Fifth Draft Assessment (7 June 2013) Report Chapter 12 IPCC WGI Fifth Assessment Report

Final Draft (7 June 2013) Year at which Chapter 1.5 12 °C is IPCC WGIFinal Fifth Draft Assessment (7 June 2013) Report Year at which Chapter2 °C 12is crossed IPCC WGI Fifth Assessment Report crossed or committed or committed

] crossed ] crossed yr yr

committed committed

Probability density [1/ Probability density [1/

Figure 12.5: Time series of global annual mean surfaceYear air temperature anomalies (relative toFigure 1986–2005) 12.5: Time from series CMIP5 of global annual mean surfaceYear air temperature anomalies (relative to 1986–2005) from CMIP5 concentration-driven experiments. Projections are shown for each RCP for the multi model mean concentration-driven (solid lines) and experimethe nts. Projections are shown for each RCP for the multi model mean (solid lines) and the Figure5–95% range12.5: Time(±1.64 series standard of global deviation) annual across mean the surface distributi air temperon of individualature anomalies models (relative (shading). toFigure5–95% 1986–2005) Discontinuities range12.5: Time(±1.64 from at series CMIP5 2100standard of global deviation) annual across mean the surface distributi air temperon of individualature anomalies models (relative (shading). to 1986–2005) Discontinuities from at CMIP5 2100 concentration-drivenare due to different numbers experime ofnts. models Projections performing are shown the extension for each runs RCP beyond for the the multi 21st model century meanconcentration-drivenare and due (solid have to different lines)no physical and numbers experimethe ofnts. models Projections performing are shown the extension for each runs RCP beyond for the the multi 21st model century mean and (solid have lines)no physical and the meaning.5–95% range Only (±1.64 one ensemble standard memberdeviation) is acrossused from the distributieach modelon ofand individual numbers modelsin the figu (shading).re indicatemeaning.5–95% Discontinuities the range numberOnly (±1.64 one of at ensemble 2100standard memberdeviation) is acrossused from the distributieach modelon ofand individual numbers modelsin the figu (shading).re indicate Discontinuities the number of at 2100 differentare due to models different contributing numbers of to models the different performing time periods. the extension No ranges runs arebeyond given the for 21st the centuryRCP6.0differentare and projections due have to models different no beyondphysical contributing numbers 2100 of to models the different performing time periods. the extension No ranges runs arebeyond given the for 21st the centuryRCP6.0 and projections have no beyondphysical 2100 asmeaning. only two Only models one ensembleare available. member is used from each model and numbers in the figure indicateasmeaning. only the two numberOnly models one of ensemble are available. member is used from each model and numbers in the figure indicate the number of different models contributing to the different time periods. No ranges are given for the RCP6.0 different projections models beyond contributing 2100 to the different time periods. No ranges are given for the RCP6.0 projections beyond 2100 as only two models are available. as only two models are available.

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Do Not Cite, Quote or Distribute 12-129 Do Not Cite,Total Quote pages: or 175Distribute 12-129 Total pages: 175 When are we committed to crossing 1.5 or 2 °C thresholds? Final Draft (7 June 2013) Chapter 12 IPCC WGIFinal Fifth Draft Assessment (7 June 2013) Report Chapter 12 IPCC WGI Fifth Assessment Report

] crossed ] crossed yr yr 50% chance of 50% chance of being committed being committed to 1.5 °C by 2031 to 2°C by 2047 committed committed

Probability density [1/ Probability density [1/

Do Not Cite, Quote or Distribute 12-129 Do Not Cite,Total Quote pages: or 175Distribute 12-129 Total pages: 175

] crossed ] crossed yr yr committed committed

Probability density [1/ Probability density [1/

Do Not Cite, Quote or Distribute 12-129 Do Not Cite,Total Quote pages: or 175Distribute 12-129 Total pages: 175

Do Not Cite, Quote or Distribute 12-129 Do Not Cite,Total Quote pages: or 175Distribute 12-129 Total pages: 175 Parting thoughts

§ We are committed to crossing warming thresholds up to a couple decades before those temperatures would be reached under ongiong emissions, due to the loss of tropospheric aerosols

§ Current climate commitment is about 1.3 °C (0.9-1.9 °C, 5-95%), with a14% that we’re already committed to reaching 1.5 °C above pre- industrial

Temperature change [°C] change Temperature Year § 50% chance of being committed to 1.5 °C by 2031, 2 °C by 2047 )

yr following RCP8.5

§ Model, constrained by observations of global warming and energy budget, is a great tool for probing sources of uncertainty in future warming and climate commitment (hint: primarily aerosol forcing uncertainty; carbon cycle uncertainty becomes important later in the century under high emissions)

Probability distribution (1/ Year Manuscript in preparation: Proistosescu, Armour, Roe & Huybers