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P. Friedlingstein and IC Prentice Paris/Bristol/Exeter/Sidney + inputs from V. Masson-Delmotte

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The magnitude of the problem Brussels, May. 27th 2009 Uncertainty due to the carbon cycle uncertainty 2.6 – 4.1 °C2.4 – 5.6 °C 830 ppm 730 – 1000 ppm Higher [CO2], larger climate change IPCC, 2007

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Climate-Carbon Cycle Feedback CO 2 =EMI - F ao - F ab (1) T= CO 2 + T ind (2) with: F ao = ao CO 2 + ao T (3) F ab = ab CO 2 + ab T (4) (3) and (4) in (1), then (1) in (2) gives: T=1/(1-g) T unc with: g= ( ao + ab ) / (1+ ao + ab )

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Climate-Carbon Cycle Feedback T=1/(1-g) T unc = f T unc g= ( ao + ab ) / (1+ ao + ab ) g is the gain of the climate-carbon cycle feedback f=1/(1-g) f is the feedback factor and is the carbon cycle sensitivity to climate ( C/ T)

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Climate-Carbon Cycle Feedback Carbon cycle sensitivity to climate g Climate carbon cycle gain 0.04 – – 200 GtC/K

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What are the available observations ?

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Glacial interglacial CO 2 – Temperature

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Climate sensitivity is estimated here from 2xCO2 GCMs estimates, in the absence of physical feedbacks (black body response only). Two caveats Glacial interglacial CO 2 – Temperature

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1. Physical feedbacks Torn and Harte, 2006 Friedlingstein et al., 2006 is the climate sensitivity, accounting for all physical feedbacks g G-IG = 0.04*3.8/1.3 = 0.12

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Using the Full EPICA record

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Glacial interglacial CO 2 – Temperature ppm/K and taking from AR4 g G-IG = 0.08

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2. Does this help for future projections?

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Last Millennium and LIA

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dCO2/dT= 50.6 ppm/K dCO2/dT= 39.9 ppm/K

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Last Millennium and LIA

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dCO 2 /dT= 7.7 [ 1.7 – 21.4] ppm/K Confusion in terminology … dCO 2 /dT is neither g no

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Last Millennium and LIA dCO 2 /dT= 7.7 [ 1.7 – 21.4] ppm/K One could derive the gain g: (again, taking dT/dCO 2 from 2xCO2 sensitivity)

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biosphere Ocean i.e. time Last Millennium and LIA Or one could derive But one needs to know on millenium time scales …

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Interannual variability of CO 2 Gt. C per year SOI CO 2 Annual Growth Rate

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Interannual variability of CO 2 dCO2/dT= 2.9 ppm/K = -90 GtC/K g G-IG = 0.03

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Summary gainCarbon sensitivity to climate (GtC/K) G-IG * LIA * IAV0.03**-90 C4MIP models average *assuming 5.5, i.e. AF 0.15 **assuming equilibrium response Palaeo and historical CO2 variability could help to constraint Climate carbon cycle feedback Estimate of seems to be more robust than g across timescales

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Summary Palaeo and historical CO2 variability might help to constrain Climate carbon cycle feedback However, large uncertainties on data and on use of data Estimate of seems to be more robust than g across timescales. Is this accidental ? Do we get the right number for the right reason (right process) ? Best way is certainly not what I just presented... We should simulate the past rather than play with past data to infer future response

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