IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group I: The Physical Science Basis
Figure 8.15

Figure 8.15. Sensitivity (in W m–2 °C–1) of the tropical net cloud radiative forcing (CRF) to SST changes associated with global warming (simulations in which CO2 increases by 1% yr–1). The inset shows the tropically averaged sensitivity Σ predicted by 15 AOGCMs used in this report: 7 models predict Σ < 0 and 8 models predict Σ > 0. The main panel compares the CRF sensitivity to SST predicted by the two groups of models in different regimes of the large-scale tropical circulation (the 500 hPa vertical pressure velocity is used as a proxy for large-scale motions, with negative values corresponding to large-scale ascending motions, and positive values to sinking motions).Thick lines and vertical lines represent the mean and the standard deviation of model sensitivities within each group; dotted lines represent the minimum and maximum values of model sensitivities within each dynamical regime. The discrepancy between the two groups of models is greatest in regimes of large-scale subsidence. These regimes, which have a large statistical weight in the tropics, are primarily covered by boundary-layer clouds. As a result, the spread of tropical cloud feedbacks among the models (inset) primarily arises from inter-model differences in the radiative response of low-level clouds in regimes of large-scale subsidence. Adapted from Bony and Dufresne (2005).