Robust increase in effective climate sensitivity with transient warming

Transcription

1 Robust increase in effective climate sensitivity with transient warming Kyle C Armour Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology Ringberg 5 Courtesy of NASA s Earth Observatory

2 Standard Model of global climate response to forcing Linearization of global top-ofatmosphere (TOA) energy budget H = T + R + O(T ) global TOA radiation flux anomaly global TOA radiative response global TOA radiative forcing Temperature anomaly T has units of K or C H and R have units of W/m Global radiative feedback λ is negative (stabilizing) and has units of Wm - K -

3 Standard Model of global climate response to forcing Linearization of global top-ofatmosphere (TOA) energy budget H = T + R + O(T ) global effective heat capacity global TOA radiation flux anomaly global TOA radiative response global TOA radiative forcing Transient warming: H = c eff dt dt = T + R Rate of global heat content change Equilibrium warming (H=), e.g., for a CO doubling (forcing R ): T = R /.Equilibrium K climate sensitivity

4 Standard Model of global climate response to forcing H = c eff dt dt = T + R CO radiative forcing (W/m ) 8 Radiative forcing from an abrupt CO doubling R Year response to CO doubling 5 5 Year after CO doubling

5 Standard Model of global climate response to forcing Global TOA radiation flux in response to CO doubling H = c eff dt dt = T + R response to CO doubling TOA radiation flux H (W/m ) CO radiative forcing R Year after CO doubling Year after CO doubling

6 Standard Model of global climate response to forcing Global TOA radiation flux vs global surface temperature H = c eff dt dt = T + R response to CO doubling TOA radiation flux H (W/m ) CO radiative forcing R slope λ Equilibrium climate sensitivity T = R / 5 5 Year after CO doubling

7 Standard Model of global climate response to forcing Global TOA radiation flux vs global surface temperature H = c eff dt dt = T + R response to CO doubling TOA radiation flux H (W/m ) higher climate sensitivity lower climate sensitivity 5 5 Year after CO doubling

8 Standard Model of global climate response to forcing Global TOA radiation flux vs global surface temperature H = c eff dt dt = T + R response to CO doubling TOA radiation flux H (W/m ) more efficient deep ocean heat uptake higher climate sensitivity lower climate sensitivity 5 5 Year after CO doubling

9 Standard Model of global climate response to forcing Global TOA radiation flux vs global surface temperature H = c eff dt dt = T + R response to CO doubling TOA radiation flux H (W/m ) Oceans (heat capacity c eff ) Atmosphere (feedback λ) 5 5 Year after CO doubling

10 Energy balance constraints on climate sensitivity obs = R obs H obs T obs TOA radiation flux (W/m ) Observed radiative forcing R obs Observed temperature and ocean heat uptake T obs, H obs.5.5 (IPCC AR5) Estimate of long-term warming in response to present-day radiative forcing

11 Energy balance constraints on climate sensitivity Radiative forcing probability density (m /W).5 obs = R obs H obs T obs TOA radiation flux (W/m ) Observed radiative forcing R obs Observed temperature and ocean heat uptake T obs, H obs.5.5 Estimate of long-term warming in response to present-day radiative forcing (IPCC AR5)

12 Energy balance constraints on climate sensitivity Radiative forcing probability density (m /W).5 obs = R obs H obs T obs TOA radiation flux (W/m ) Observed radiative forcing R obs Observed temperature and ocean heat uptake T obs, H obs.5.5 Less long-term warming (IPCC AR5)

13 Energy balance constraints on climate sensitivity Radiative forcing probability density (m /W).5 obs = R obs H obs T obs TOA radiation flux (W/m ) Observed radiative forcing R obs Observed temperature and ocean heat uptake T obs, H obs.5.5 More long-term warming (IPCC AR5)

14 Energy balance constraints on climate sensitivity TOA radiation flux (W/m ) Radiative forcing probability density (m /W).5 Observed radiative forcing R obs Observed temperature and ocean heat uptake T obs, H obs.5.5 obs = R obs H obs T obs Probability density (/ C) T = R obs R median:.7 C 5-95% range:.-. C 5 Equilibrium climate sensitivity ( C) see also: Hansen et al (5), Roe and Armour (), Padilla and Vallis (), Otto et al (), Masters (), Lewis and Curry (), and many others

15 Climate response to an abrupt CO change CMIP5 fully-coupled GCMs forced by abrupt CO quadrupling Radiative forcing from an abrupt CO doubling response to CO quadrupling CO radiative forcing (W/m ) Year Year after CO quadrupling fully-coupled General Circulation Models (GCMs) from the Coupled Model Intercomparison Project, phase 5 (CMIP5)

16 Climate response to an abrupt CO change CMIP5 fully-coupled GCMs forced by abrupt CO quadrupling Radiative forcing from an abrupt CO doubling response to CO quadrupling CO radiative forcing (W/m ) Year Year after CO quadrupling fully-coupled General Circulation Models (GCMs) from the Coupled Model Intercomparison Project, phase 5 (CMIP5)

17 Climate response to an abrupt CO change CMIP5 fully-coupled GCMs forced by abrupt CO quadrupling TOA radiation flux (W/m ) 5 Global TOA radiation flux vs global surface temperature response to CO quadrupling 5 5 Year after CO quadrupling fully-coupled General Circulation Models (GCMs) from the Coupled Model Intercomparison Project, phase 5 (CMIP5)

18 Climate response to an abrupt CO change CMIP5 fully-coupled GCMs forced by abrupt CO quadrupling TOA radiation flux (W/m ) 5 Global TOA radiation flux vs global surface temperature response to CO quadrupling 5 5 Year after CO quadrupling fully-coupled General Circulation Models (GCMs) from the Coupled Model Intercomparison Project, phase 5 (CMIP5)

19 Climate response to an abrupt CO change CMIP5 fully-coupled GCMs forced by abrupt CO quadrupling TOA radiation flux (W/m ) 5 Global TOA radiation flux vs global surface temperature response to CO quadrupling 5 5 Year after CO quadrupling H = eff (t)t + R T eff (t) = R eff(t) Effective climate sensitivity (setting transient warming) could be smaller than equilibrium climate sensitivity (setting long-term warming)

20 Climate response to an abrupt CO change CMIP5 fully-coupled GCMs forced by abrupt CO quadrupling TOA flux / radiative forcing.8... Global TOA radiation flux vs global surface temperature....8 Realized warming fraction (T/T eq ) Realized warming fraction (T/T eq ).8... response to CO quadrupling 5 5 Year after CO quadrupling H = eff (t)t + R T eff (t) = R eff(t) Effective climate sensitivity (setting transient warming) could be smaller than equilibrium climate sensitivity (setting long-term warming)

21 Climate response to an abrupt CO change CMIP5 fully-coupled GCMs forced by abrupt CO quadrupling TOA flux / radiative forcing.8... Global TOA radiation flux vs global surface temperature....8 Realized warming fraction (T/T eq ) Realized warming fraction (T/T eq ).8... response to CO quadrupling 5 5 Year after CO quadrupling H = eff (t)t + R T eff (t) = R eff(t) Effective climate sensitivity (setting transient warming) could be smaller than equilibrium climate sensitivity (setting long-term warming)

22 Climate response to an abrupt CO change CMIP5 fully-coupled GCMs forced by abrupt CO quadrupling TOA flux / radiative forcing.8... Global TOA radiation flux vs global surface temperature....8 Realized warming fraction (T/T eq ) Realized warming fraction (T/T eq ).8... response to CO quadrupling 5 5 Year after CO quadrupling H = eff (t)t + R T eff (t) = R eff(t) Effective climate sensitivity (setting transient warming) could be smaller than equilibrium climate sensitivity (setting long-term warming)

23 Are we underestimating long-term global warming? Consider %/yr CO ramping simulations response to CO increase 8 Year

24 Are we underestimating long-term global warming? Consider %/yr CO ramping simulations response to CO increase 8 Year Number of models 5 T eff (t) = Year R eff(t) 5 Effective climate sensitivity ( C) Apparent equilibrium climate sensitivity estimated after years

25 Are we underestimating long-term global warming? Consider %/yr CO ramping simulations response to CO increase 8 Year Number of models Number of models 5 5 Effective climate sensitivity ( C) 5 T eff (t) = T eff (t) = Year R eff(t) Year 7 R eff(t) 5 Effective climate sensitivity ( C) Apparent equilibrium climate sensitivity estimated after 7 years

26 Are we underestimating long-term global warming? Consider %/yr CO ramping simulations response to CO increase 8 Year Number of models Number of models Number of models 5 5 Effective climate sensitivity ( C) 5 5 Effective climate sensitivity ( C) 5 T eff (t) = T eff (t) = Year R eff(t) Year 7 R eff(t) Equilibrium warming T = R / 5 Equilibrium climate sensitivity ( C)

27 Are we underestimating long-term global warming? Equilibrium climate sensitivity ( C) Consider %/yr CO ramping simulations 5 Effective climate sensitivity estimated at year 7 vs equilibrium climate sensitivity 5 Effective climate sensitivity ( C) Number of models Number of models Number of models 5 5 Effective climate sensitivity ( C) 5 5 Effective climate sensitivity ( C) 5 T eff (t) = T eff (t) = Year R eff(t) Year 7 R eff(t) Equilibrium warming T = R / 5 Equilibrium climate sensitivity ( C)

28 Are we underestimating long-term global warming? Consider %/yr CO ramping simulations Equilibrium climate sensitivity ( C) 5 Effective climate sensitivity estimated at year 7 vs equilibrium climate sensitivity 5 Effective climate sensitivity ( C) Number of models T /T eff Equilibrium climate sensitivity/ Effective climate sensitivity

29 Are we underestimating long-term global warming? Probability density (/ C) Observations of transient warming give us an estimate of the effective climate sensitivity median:.7 C 5-95% range:.-. C 5 Effective climate sensitivity ( C) = Probability density (/ C) Probability density 5 Equilibrium climate sensitivity ( C) Convolve with an estimate of T /T eff median:. C 5-95% range:.-5. C T /T eff Equilibrium climate sensitivity/ Effective climate sensitivity

30 Are we underestimating long-term global warming? Probability density (/ C) Observations of transient warming give us an estimate of the effective climate sensitivity median:.7 C 5-95% range:.-. C 5 Effective climate sensitivity ( C) = Probability density (/ C) Probability density 5 Equilibrium climate sensitivity ( C) Convolve with an estimate of T /T eff median:. C 5-95% range:.-5. C T /T eff Equilibrium climate sensitivity/ Effective climate sensitivity

31 Summary and conclusions Common assumption in climate sensitivity estimates is that global feedbacks are constant in time CMIP5 models show a robust increase in effective climate sensitivity over time Estimates of equilibrium climate sensitivity based on global energy balance constraints (observed global warming, heat uptake and radiative forcing) should be viewed as a lower bound TOA radiation flux (W/m ) TOA flux / radiative forcing Number of models Fraction of equilibrium warming (T/T eq ) 5 T /T eff Equilibrium climate sensitivity/ Effective climate sensitivity