Observed and modeled rainfall trends over Africa
The left panel is the observed linear trend in observed rainfall (as estimated by the Climate Research Unit of the University of East Anglia) in units of mm/month per 50 years. Blue areas correspond to a trend towards wetter conditions and brown areas toward a drier climate.
The center panel is the same linear trend in the ensemble mean over 8 realizations of GFDL's latest global climate model (CM2), which is forced by estimates of the changing greenhouse gas composition of the atmosphere, as well as changing volcanic and anthropogenic aerosols, solar forcing, ozone concentrations, and land use.
The right panel is the same trend but for an ensemble mean of 10 simulations with AM2, the atmosphere/land component of CM2 running over observed sea surface temperatures (SSTs) and with fixed greenhouse gases and other forcings. The implication of the right panel is that the drying trend in the Sahel is encoded in the distribution of SSTs and that the atmosphere/land model is capable of translating these SST changes into precipitation trends.
The implication of the center panel is that at least part of the signal in SSTs that generates this drought pattern is a consequence of changes in forcing.
See Held et. al., 2005: Simulation of Sahel drought in the 20th and 21st centuries, Proc. Nat. Acad. Sci., 102(50), 17891-17896. doi:10.1073/pnas.0509057102.