But what the model helped to explain is how this wetter climate reached south into equatorial and southern equatorial Africa. The Earth's rotational wobbles actually cause changes in sunlight to be opposite between the hemispheres, such that extra heating in the north is cooling in the south. This means orbital forcing can't explain the wet, humid period that extends well into southern Africa. My collaborators were able to run experiments with the model using only orbital forcing, as well as experiments using other factors that might cause rainfall to vary, such as greenhouse gas concentrations. By comparing these tests to a simulation in which every forcing changed simultaneously,
we showed that an increase in greenhouse gases during the last deglaciation was largely responsible for the wetter conditions in equatorial and southeastern Africa.