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? Final DraftFinal (7 JuneDraftFinal 2013) (7 June Draft 2013) (7 June 2013) Chapter 12Chapter 12Chapter 12 IPCC WGIIPCC Fifth WGI AssessmentIPCC Fifth WGI Assessment Report Fifth Assessment Report Report 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 12Chapter 12Chapter 12 IPCC WGIIPCC Fifth WGI AssessmentIPCC 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 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 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 tempera- ture 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 ( approximately∆TNH 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