At Lake Yoa, which is groundwater-fed, vegetation decreased and became desert vegetation between 4,700-4,300 and 2,700 years ago, while the lake became hypersaline 4,000 years ago. However, the climate there may have been affected by the Tibesti Mountains and the end of the AHP thus delayed, and fossil groundwater left by the AHP nourishes the lake to this day. In the central Sahara, water resources in the mountains persisted longer.