Fig. 2

Nonlinearity in the surface albedo feedback factor for three consecutive doublings of CO₂. The feedback factor, defined as the ratio of the magnitude of the albedo feedback on the Planck feedback, is calculated using the radiative kernel technique⁸⁵ and zonal averages are plotted for three consecutive doublings of CO₂ concentrations in CCSM3. The global average feedback factor decreases⁸⁷ from 0.097 for 2xCO₂–CNTL to 0.053 for 8xCO₂–4xCO₂