<P> The Neolithic Subpluvial, or the Holocene Wet Phase, was an extended period (from about 7500--7000 BCE to about 3500--3000 BCE) of wet and rainy conditions in the climate history of northern Africa . It was both preceded and followed by much drier periods . </P> <P> The Neolithic Subpluvial was the most recent of a number of periods of "Wet Sahara" or "Green Sahara", during which the Sahara region was much moister and supported a richer biota and human population than the present - day desert . </P> <P> The Neolithic Subpluvial began during the 7th millennium BCE and was strong for about 2,000 years; it waned over time and ended after the 5.9 kiloyear event (3900 BCE). Then the drier conditions that prevailed prior to the Neolithic Subpluvial returned; desertification advanced, and the Sahara Desert formed (or re-formed). Arid conditions have continued through to the present day . </P> <P> During the Neolithic Subpluvial, large areas of North, Central, and East Africa had hydrographic profiles significantly different from later norms . Existing lakes had surfaces tens of meters higher than today, sometimes with alternative drainages: Lake Turkana, in present - day Kenya, drained into the Nile River basin . Lake Chad reached a maximum extent of some 400,000 square kilometers in surface area, larger than the modern Caspian Sea, with a surface level about 30 meters (100 feet) higher than its twentieth - century average . Some shallower lakes and river systems existed in the subpluvial era that later disappeared entirely, and are detectable today only via radar and satellite imagery . </P>

When was the last time the sahara was green