Commensurate comparisons of models with energy budget observations reveal consistent climate sensitivities Kyle C Armour University of Washington School of Oceanography and Dept of Atmospheric Sciences Cristian Proistosescu (UW) Tim Andrews (UK Met Office) Malte Stuecker (UW) Kelly McCusker (UW) Yue Dong (UW) Levi Silvers (GFDL) David Paynter (GFDL) Thorsten Mauritsen (MPI) Jonathan Gregory (Reading) Courtesy of NASA’s AGU Fall Meeting 2017 Earth Observatory
Standard Model of global climate response to forcing § Linearization of global top-of-atmosphere Q = λ T + F (TOA) energy budget global TOA global TOA global TOA radiation flux radiative radiative anomaly response to forcing [Wm -2 ] warming [Wm -2 ] [Wm -2 K -1 ][K]
Standard Model of global climate response to forcing § Linearization of global top-of-atmosphere Q = λ T + F (TOA) energy budget global TOA global TOA global TOA radiation flux radiative radiative anomaly response to forcing [Wm -2 ] warming [Wm -2 ] [Wm -2 K -1 ][K] Equilibrium warming (Q=0) in response to a doubling of atmospheric CO 2 (forcing ≈ 3.7 § − F 2 ⇥ Wm -2 ): ECS = − F 2 ⇥ Equilibrium climate sensitivity (ECS) λ
Estimates of climate sensitivity Q = λ T + F correspondence Energy budget constraints on climate response = 0.75 ± 0.2 °C ⇥ T obs Alexander Otto 1 *, Friederike E. L. Otto 1 , Olivier Boucher 2 , John Church 3 , Gabi Hegerl 4 , e Piers M. Forster 5 , Nathan P. Gillett 6 , − Q obs = 0.65 ± 0.27 Wm -2 s Jonathan Gregory 7 , Gregory C. Johnson 8 , ⇥ Reto Knutti 9 , Nicholas Lewis 10 , Ulrike Lohmann 9 , = = 2.3 ± 1 Wm -2 F obs y Jochem Marotzke 11 , Gunnar Myhre 12 , Drew Shindell 13 , Bjorn Stevens 11 and Myles R. Allen 1,14 � (years 2000-2009 relative to 1860-1879) ECS = − F 2 ⇥ λ F 2 ⇥ T obs = F obs − Q obs
Estimates of climate sensitivity correspondence 0.8 Otto et al. ECS Energy budget constraints on Probability density [1/°C] climate response 0.6 Alexander Otto 1 *, Friederike E. L. Otto 1 , Olivier Boucher 2 , John Church 3 , Gabi Hegerl 4 , e Piers M. Forster 5 , Nathan P. Gillett 6 , 0.4 s Jonathan Gregory 7 , Gregory C. Johnson 8 , Reto Knutti 9 , Nicholas Lewis 10 , Ulrike Lohmann 9 , y Jochem Marotzke 11 , Gunnar Myhre 12 , Drew Shindell 13 , Bjorn Stevens 11 0.2 and Myles R. Allen 1,14 � 0 ECS = − F 2 ⇥ 0 1 2 3 4 5 6 λ Equilibrium climate sensitivity [°C] F 2 ⇥ T obs = F obs − Q obs
Estimates of climate sensitivity correspondence 0.8 Otto et al. ECS Energy budget constraints on Probability density [1/°C] climate response median ECS: 2.0 °C 0.6 5-95% range: 1.2-3.9 °C Alexander Otto 1 *, Friederike E. L. Otto 1 , Olivier Boucher 2 , John Church 3 , Gabi Hegerl 4 , e Piers M. Forster 5 , Nathan P. Gillett 6 , 0.4 s Jonathan Gregory 7 , Gregory C. Johnson 8 , Reto Knutti 9 , Nicholas Lewis 10 , Ulrike Lohmann 9 , y Jochem Marotzke 11 , Gunnar Myhre 12 , Drew Shindell 13 , Bjorn Stevens 11 0.2 and Myles R. Allen 1,14 � 0 ECS = − F 2 ⇥ 0 1 2 3 4 5 6 λ Equilibrium climate sensitivity [°C] F 2 ⇥ T obs = F obs − Q obs
Estimates of climate sensitivity 0.8 correspondence 8 0.6 Otto et al. ECS Energy budget constraints on Probability density [1/°C] 8 CMIP5 ECS 6 climate response 4 2 0.4 6 0 0 1 2 3 4 5 6 Alexander Otto 1 *, Friederike E. L. Otto 1 , Olivier Boucher 2 , John Church 3 , Gabi Hegerl 4 , e 0.2 Piers M. Forster 5 , Nathan P. Gillett 6 , 4 s Jonathan Gregory 7 , Gregory C. Johnson 8 , Reto Knutti 9 , Nicholas Lewis 10 , Ulrike Lohmann 9 , 0 y Jochem Marotzke 11 , Gunnar Myhre 12 , 0 1 2 3 4 5 6 Drew Shindell 13 , Bjorn Stevens 11 2 and Myles R. Allen 1,14 � 0 ECS = − F 2 ⇥ 0 1 2 3 4 5 6 λ Equilibrium climate sensitivity [°C] F 2 ⇥ T obs = F obs − Q obs (Armour 2017; see also Proistosescu & Huybers 2017)
Estimates of climate sensitivity 0.8 § Global energy budget constraints produce estimates of ECS that are quite a bit lower 8 0.6 Otto et al. ECS than ECS simulated by CMIP5 models Probability density [1/°C] § Are the models overly sensitive? 8 CMIP5 ECS 6 4 2 0.4 6 0 0 1 2 3 4 5 6 § Or is something else going on…? 0.2 4 0 0 1 2 3 4 5 6 2 0 ECS = − F 2 ⇥ 0 1 2 3 4 5 6 λ Equilibrium climate sensitivity [°C] F 2 ⇥ T obs = F obs − Q obs (Armour 2017; see also Proistosescu & Huybers 2017)
Like-with-like comparisons of climate sensitivity 0.8 § Emerging consensus: model-observational comparisons must be made in a like-with-like 8 0.6 Otto et al. ECS way Probability density [1/°C] 8 CMIP5 ECS 6 4 2 0.4 6 0 0 1 2 3 4 5 6 0.2 4 0 0 1 2 3 4 5 6 2 0 0 1 2 3 4 5 6 Equilibrium climate sensitivity [°C] (Armour 2017; see also Proistosescu & Huybers 2017)
Like-with-like comparisons of climate sensitivity 0.8 § Emerging consensus: model-observational comparisons must be made in a like-with-like 8 0.6 Otto et al. ECS way, accounting for possibility that: Probability density [1/°C] 8 CMIP5 ECS ⇥ 6 4 2 1) Feedbacks ( ) vary over time as the 0.4 6 0 λ 0 1 2 3 4 5 6 spatial pattern of warming evolves (Armour 2017; Proistosescu & Huybers 2017) 0.2 4 0 0 1 2 3 4 5 6 2 0 0 1 2 3 4 5 6 Equilibrium climate sensitivity [°C] (Armour 2017; see also Proistosescu & Huybers 2017)
Like-with-like comparisons of climate sensitivity 0.8 § Emerging consensus: model-observational comparisons must be made in a like-with-like 8 0.6 Otto et al. ECS way, accounting for possibility that: Probability density [1/°C] 8 CMIP5 ECS ⇥ 6 4 2 1) Feedbacks ( ) vary over time as the 0.4 6 0 λ 0 1 2 3 4 5 6 spatial pattern of warming evolves (Armour 2017; Proistosescu & Huybers 2017) 0.2 4 2) Feedbacks affected by the “efficacy” 0 of non-CO 2 forcings (Shindell 2014; 0 1 2 3 4 5 6 Kummer & Dessler 2014; Marvel et al. 2015) 2 0 0 1 2 3 4 5 6 Equilibrium climate sensitivity [°C] (Armour 2017; see also Proistosescu & Huybers 2017)
Like-with-like comparisons of climate sensitivity 0.8 § Emerging consensus: model-observational comparisons must be made in a like-with-like 8 0.6 Otto et al. ECS way, accounting for possibility that: Probability density [1/°C] 8 CMIP5 ECS ⇥ 6 4 2 1) Feedbacks ( ) vary over time as the 0.4 6 0 λ 0 1 2 3 4 5 6 spatial pattern of warming evolves (Armour 2017; Proistosescu & Huybers 2017) 0.2 4 2) Feedbacks affected by the “efficacy” 0 of non-CO 2 forcings (Shindell 2014; 0 1 2 3 4 5 6 Kummer & Dessler 2014; Marvel et al. 2015) 2 3) Feedbacks depend on natural variability in the pattern of warming 0 0 1 2 3 4 5 6 Equilibrium climate sensitivity [°C] (Armour 2017; see also Proistosescu & Huybers 2017)
Like-with-like comparisons of climate sensitivity 0.8 § Emerging consensus: model-observational comparisons must be made in a like-with-like 8 0.6 Otto et al. ECS way, accounting for possibility that: Probability density [1/°C] 8 CMIP5 ECS ⇥ 6 4 2 1) Feedbacks ( ) vary over time as the 0.4 6 0 λ 0 1 2 3 4 5 6 spatial pattern of warming evolves (Armour 2017; Proistosescu & Huybers 2017) 0.2 4 2) Feedbacks affected by the “efficacy” 0 of non-CO 2 forcings (Shindell 2014; 0 1 2 3 4 5 6 Kummer & Dessler 2014; Marvel et al. 2015) 2 3) Feedbacks depend on natural variability in the pattern of warming 0 0 1 2 3 4 5 6 4) Different definitions of global-mean Equilibrium climate sensitivity [°C] temperature used in models vs observations (Cowtan et al. 2015; Richardson et al. 2016) (Armour 2017; see also Proistosescu & Huybers 2017)
1) Feedbacks vary as the pattern of warming evolves CMIP5 response to 4 × CO 2 (Andrews et al. 2015)
1) Feedbacks vary as the pattern of warming evolves CMIP5 response to 4 × CO 2 (Andrews et al. 2015)
1) Feedbacks vary as the pattern of warming evolves Localized patches of warming Patches for Green’s Function simulations a b Warming patch Warming patch c 1.5 1 0.5 0 (°C) Radiative response Radiative response d e 10 5 0 -5 -10 (Wm -2 ) Global feedback response to localized patches of CMIP5 response to 4 × CO 2 (Andrews et al. 2015) warming in NCAR’s CAM4 (Dong et al., in preparation) see also Zhou et al. 2017
1) Feedbacks vary as the pattern of warming evolves Global radiative feedback [Wm -2 K -1 ] -15 -10 -5 0 5 10 15 Global feedback response to localized patches of CMIP5 response to 4 × CO 2 (Andrews et al. 2015) warming in NCAR’s CAM4 (Dong et al., in preparation) see also Zhou et al. 2017; Proistosescu & Huybers 2017; Andrews & Webb 2017; Ceppi & Gregory 2017; Silvers et al. 2017; Zhou et al. 2016; Gregory & Andrews 2016; Rugenstein et al. 2016; Rose et al. 2014; Armour et al. 2013; many others
1) Feedbacks vary as the pattern of warming evolves CMIP5 response to 4 × CO 2 (Andrews et al. 2015)
1) Feedbacks vary as the pattern of warming evolves ⇥ § Feedbacks under transient warming ( ) are more λ ⇥ CMIP5 models 2.5 − negative than those at equilibrium ( ) λ 2 ⇥ § Inferred (or instantaneous ) climate sensitivity (ICS) is generally smaller than equilibrium climate 2 λ 2 ⇥ ECS/ICS = sensitivity (ECS) λ − ICS = − F 2 ⇥ ECS = − F 2 ⇥ 1.5 λ 2 ⇥ λ 1 0 40 80 120 Year after CO 2 quadrupling CMIP5 response to CO 2 forcing (Armour 2017) see also Proistosescu & Huybers 2017
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