Contributions of Different Cloud Types to Feedbacks and Rapid Adjustments in CMIP5

Cloud feedbacks and rapid adjustments to an abrupt quadrupling of CO2 are diagnosed in five CMIP5 models using cloud radiative kernels in combination with cloud top pressureand optical depthpartitioned cloud fractions. Upon CO2 quadrupling, clouds exhibit a rapid reduction in fractional coverage, cloud top pressure, and optical depth, each contributing roughly equally to a positive global mean net cloud adjustment. Uniform rapid reductions in mid-level clouds, opposed by large increases in marine low-level cloudiness, are especially important in reducing planetary albedo in every model. As the planet warms, clouds become fewer, higher, and thicker, and global mean net cloud feedback is positive in all but one model. As in CMIP3, high cloud changes dominate the inter-model spread in longwave (LW) and shortwave (SW) cloud feedbacks, but low cloud changes dominate that of net cloud feedback. The importance of the negative optical depth feedback relative to the amount feedback at high latitudes is even more marked than in CMIP3. We show that the negative LW cloud adjustment inferred in previous studies is primarily caused by a 1.3 W m−2 cloud masking of the CO2 forcing rather than by genuine cloud changes, and that the cloud feedback is 0.3 W m−2 K−1 more positive when cloud masking effects are accounted for. We also show that failure to account for rapid adjustments leads to an overestimate of the positive cloud amount and altitude feedbacks and an underestimate of the negative cloud optical depth feedback, leading to a 100% overestimate of the ensemble mean net cloud feedback.