Radiative Feedback Analysis of CO2 Doubling and LGM Experiments

○ M. Yoshimori, A. Abe-OuchiCCSR, University of Tokyo

andT. Yokohata

National Institute for Environmental Studies

JSPS 5th University Allied Workshop (2 July 2008, Sunroute-Plaza Tokyo)

Climate SensitivityClimate Sensitivity• Climate sensitivity is a convenient metric that represents gross size of the

climate change response.• Often expressed in terms of temperature change due to doubling of CO2

“Chaney report” (1979): 1.5-4.5 K IPCC TAR (2001): 1.5-4.5 K; IPCC AR4 (2007): 2.0-4.5 K

• Perturbed physics ensemble

Murphy et al., 2004Stainforth et al., 2005

There is still a substantial range of uncertainty in the estimate of climate sensitivity, particularly in the higher end. Needs more constraint!

Constraining climate sensitivity (1)Constraining climate sensitivity (1)

• Last 150 years or so Large uncertainty in forcing (esp. indirect effect of aerosols) Effect of ocean heat uptake

• Last 1-2 millennia Relatively small forcing (solar forcing) with large uncertainties

(incl. volcanic forcing) Consensus on the global or hemispheric mean temperature

reconstruction remains to be made

Constraining climate sensitivity (2)Constraining climate sensitivity (2)Last Glacial Maximum (LGM) ~ 21,000 years ago• Relatively known forcing• Estimation from reconstructions/data

Hoffert and Covey, 1992; Hansen et al., 1993, Lea, 2004• Comparison between models with specific sensitivities and data

Manabe and Broccoli, 1985; Hewitt and Mitchell, 1997;Broccoli, 2000; Hewitt et al., 2001

• Comparison between perturbed physics ensembles and data Annan et al., 2005; Schneider von Deimling et al., 2006

But, fundamental questions remain to be explored:• Climate sensitivity may be determined differently between future

warming and ice ages （ Hewitt and Mitchell 1997, Crucifix 2006, Hargreaves et al., 2007) ? If so, why?

Model Experiments and Feedback AnalysisModel Experiments and Feedback Analysis• Purpose

To quantify the strength of individual feedback processes for 2xCO2 and LGM experiments and to reveal similarities and differences

To obtain ideas on how temperature changes are determined in those experiments in the model

• Model AGCM (T42/L20)-slab version of MIROC3.2: jointly developed by

CCSR/NIES/FRCGC• Experiments

Preindustrial simulation （ CTRL ） Doubling of atmospheric CO2 （ 2xCO2 ） Last Glacial Maximum （ LGM ） CTRL+LGM greenhouse gas （ LGMGHG ）

PRPPRP Feedback AnalysisFeedback Analysis(Wetharald and Manabe, 1988)(Wetharald and Manabe, 1988)

SW fluxat TOA

LW fluxat TOA

RadiationModel

sfc albedo

temperature

mixing ratio

clouds

P ： radiative damping, WV ： water vapor, LR ： lapse rateA ： surface albedo, C ： cloud

： climate sensitivity [K/(W/m**2)]

feedback

SW fluxat TOA

LW fluxat TOA

RadiationModel

sfc albedo

temperature

mixing ratio

clouds

Water VaporFeedback

PRPPRP Feedback AnalysisFeedback Analysis(Wetharald and Manabe, 1988)(Wetharald and Manabe, 1988)

P ： radiative damping, WV ： water vapor, LR ： lapse rateA ： surface albedo, C ： cloud

： climate sensitivity [K/(W/m**2)]

feedback

Adjusted Stratosphere Radiative ForcingAdjusted Stratosphere Radiative Forcing

Sea level , vegetation, and dust distribution changes are not included in the forcing.

LGM Ice Sheets

Climate Sensitivity w.r.t CTRLClimate Sensitivity w.r.t CTRL

Less positive feedback in the LGM experimentcompared to the 2xCO2 experiment.

F(W/m2)

T(K)

=T/F %

2xCO2 3.25 3.99 1.23 100

LGMGHG -2.67 -3.09 1.16 94

LGMLGM*

LGM**

-4.95-5.37-5.37

-5.20-5.20-4.88

1.050.970.91

867974

*include the effect of q-flux difference due to different land-sea mask**include the thermodynamic effect of elevated surface due to LGM ice sheets

Global, annual mean feedback Global, annual mean feedback strength w.r.t CTRLstrength w.r.t CTRL

The total feedback strength in the LGM experiment is weaker than the 2xCO2 experiment, and it results from the weaker shortwave cloud feedback.

Colman, 2003+MIROC3.2

Water Vapor Feedback (Water Vapor Feedback (R/R/T, W/m2/K)T, W/m2/K)

T: normalized by global mean values

T: normalized by the zonal mean values

•Water vapor feedback per 1K warming is larger in the tropics.•The water vapor feedback is weaker in the LGM experiment compared to the 2xCO2 experiment because fractionally less tropical forcing in the LGM.

Lapse Rate Feedback (W/m2/K)Lapse Rate Feedback (W/m2/K)

•Lapse feedback is negative (positive) in the low (high) latitudes.•The lapse rate feedback is stronger in the LGM experiment compared to the 2xCO2 experiment because fractionally more extratropical forcing in the LGM.

Albedo Feedback (W/m2/K)Albedo Feedback (W/m2/K)

Albedo feedback occurs in lower (higher) latitudes in cooling (warming) experiments.

Cloud Feedback (W/m2/K)Cloud Feedback (W/m2/K)

The difference in the cloud feedback between LGM and 2xCO2 experiments is primarily due to shortwave component.

Cloud AmountCloud Amount

LGM-CTRL LGM-2xCO2

CTRL 2xCO2-CTRL

SummarySummary• Radiative forcing, climate sensitivity, and climate feedback strength

is quantitatively evaluated in a consistent manner for 2xCO2, LGMGHG, LGM experiments using MIROC3.2.

• The comparison between 2xCO2 and LGMGHG experiments reveals that there is an asymmetry in the cloud feedback between warming and cooling experiments.

• The comparison between LGM and LGMGHG experiments further reveal that the LGM ice sheets causes additional weakening in the cloud feedback, and it occurs in the shortwave component.

• These results suggest that a model with high climate sensitivity in the warming experiment does not necessarily yields the high climate sensitivity in the LGM. Nevertheless, the difference in climate sensitivity between the LGM and 2xCO2 is relatively small compared to the intermodel spread in the 2xCO2 experiment, and paleoclimate studies may be able to provide some constraint.

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