The alternation of vegetated and arid Sahara has long attracted wide interest in paleoclimate field. This study presents a suite of highly interesting simulations of the past 190,000 years with a state-of-the-art Earth system model of intermediate complexity. This set of experiments are rigorously designed and analyzed, which allow the authors to make an important discovery of what controls the onset of African Humid Period: i.e. a threshold in orbital forcing, which decreases at elevated CO2 concentration. Moreover, maximum rates of change in simulated vegetation extent during the onset and termination of African Humid Period correlate strongly with the rates of change of the orbital forcing. A factor separation analysis further confirms the dominant role of the orbital forcing in driving the amplitude of precipitation and vegetation extent for past African Humid Period. These results have broad implications to future variability of Sahara climate in response to orbital forcing changes and non-linear earth system feedbacks.