Radiative Feedback Analysis of CO2 Doubling
and LGM Experiments
JSPS 5th University Allied Workshop (2 July 2008, Sunroute-Plaza Tokyo)
○M. Yoshimori, A. Abe-Ouchi
CCSR, University of Tokyo
and
T. Yokohata
National Institute for Environmental Studies
Climate 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., 2004
Stainforth 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)
• 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)
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 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）
PRP Feedback Analysis
(Wetharald and Manabe, 1988)
feedback
P：radiative damping, WV：water vapor, LR：lapse rate
A：surface albedo, C：cloud
：climate sensitivity [K/(W/m**2)]
SW flux
at TOA
sfc albedo
temperature
mixing ratio
clouds
Radiation
Model
LW flux
at TOA
PRP Feedback Analysis
(Wetharald and Manabe, 1988)
feedback
P：radiative damping, WV：water vapor, LR：lapse rate
A：surface albedo, C：cloud
：climate sensitivity [K/(W/m**2)]
DSW flux
at TOA
sfc albedo
temperature
mixing ratio
clouds
Radiation
Model
Water Vapor
Feedback
DLW flux
at TOA
Adjusted Stratosphere Radiative Forcing
LGM Ice Sheets
Sea level , vegetation, and dust distribution changes are not included in the forcing.
Climate Sensitivity w.r.t CTRL
F
(W/m2)
DT
(K)
l=DT/F
%
2xCO2
3.25
3.99
1.23
100
LGMGHG
-2.67
-3.09
1.16
94
LGM
LGM*
LGM**
-4.95
-5.37
-5.37
-5.20
-5.20
-4.88
1.05
0.97
0.91
86
79
74
*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
Less positive feedback in the LGM experiment
compared to the 2xCO2 experiment.
Global, annual mean feedback
strength w.r.t CTRL
Colman,
2003
+MIROC3.2
The total feedback strength in the LGM experiment is weaker than the 2xCO2
experiment, and it results from the weaker shortwave cloud feedback.
Water Vapor Feedback (DR/DT, W/m2/K)
DT: normalized by
global mean values
DT: 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 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 occurs in lower (higher) latitudes in cooling (warming)
experiments.
Cloud Feedback (W/m2/K)
The difference in the cloud feedback between LGM and 2xCO2
experiments is primarily due to shortwave component.
Cloud Amount
CTRL
2xCO2-CTRL
LGM-CTRL
LGM-2xCO2
Summary
• 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.
End