Several lines of geomorphological and stratigraphical evidence suggest that the widespread early post-glacial wet phase in tropical Africa between 12,500 and 5,000 yr BP (refs 1, 2) was interrupted by severe dry episodes during the intervals 11,000-10,000, 8,500-6,500 and 6,200-5,800 yr BP (refs 3-9). The magnitude of the second event has recently been questioned10. We present new stratigraphical and dating evidence from the Ziway-Shala lake basin, Ethiopia, indicating the severity and timing of dry phases during the interval 12,000-5,000 yr BP. Our findings have important implications for predictions of the climatic impact of increasing atmospheric levels of CO2. �� 1983 Nature Publishing Group.