Climate feedbacks in CCSM 3 under changing CO 2 forcing . Part 1 II : Variation of climate feedbacks and sensitivity with forcing

5 Are equilibrium climate sensitivity and the associated radiative feedbacks a constant property 6 of the climate system, or do they change with forcing magnitude and base climate? Using 7 the radiative kernel technique, feedbacks and climate sensitivity are evaluated in a fully 8 coupled general circulation model (GCM) for three successive doublings of carbon dioxide 9 starting from present day concentrations. Climate sensitivity increases by 23% between the 10 first and third CO2 doublings. Increases in the positive water vapor and cloud feedbacks are 11 partially balanced by a decrease in the positive surface albedo feedback and an increase in 12 the negative lapse rate feedback. Feedbacks can be decomposed into a radiative flux change 13 and a climate variable response to temperature change. The changes in water vapor and 14 Planck feedbacks are due largely to changes in the radiative response with climate state. 15 Higher concentrations of greenhouse gases and higher temperatures lead to more absorption 16 and emission of longwave radiation. Changes in cloud feedbacks are dominated by the 17 climate response to temperature change, while the lapse rate and albedo feedbacks combine 18 elements of both. Simulations with a slab ocean model (SOM) version of the GCM are used to 19 verify whether a SOM-GCM accurately reproduces the behavior of the fully coupled model. 20 Although feedbacks differ in magnitude between model configurations (with differences as 21 large as those between CO2 doublings for some feedbacks), changes in feedbacks between 22 CO2 doublings are consistent in sign and magnitude in the SOM-GCM and the fully coupled 23 model. 24