Early Holocene greening of the Sahara requires Mediterranean winter rainfall

The greening of the Sahara, associated with the African Humid Period (AHP) between ca. 14,500 and 5,000 years ago, is arguably the largest climate-induced environmental change in the Holocene; it is usually explained by the strengthening and northward expansion of the African monsoon in response to orbital forcing. However, the strengthened monsoon in early to mid- Holocene climate model simulations cannot sustain vegetation in the Sahara or account for the increased humidity in the Mediterranean region. In this article, we present an 18,500 year-long paleoclimate record from Lake Tislit in Morocco (32°N) that provides the first quantitative reconstruction of rainfall seasonality in northern Africa. The Tislit record shows that increased humidity in the AHP extended up to the North Saharan and Mediterranean regions due to increased winter rainfall, rather than summer monsoon rainfall. Based on this observation of past climate, we propose that, as a response to the orbital forcing, the AHP included a strengthening and a southward shift of the Mediterranean winter rainfall system in addition to the intensified summer monsoon, with an overlap of these rainfall zones in the Sahara. Using a mechanistic vegetation model in early Holocene conditions, we show that this hypothetical seasonal distribution of rainfall yields a more realistic representation of the Green Sahara. This new conceptual framework should be taken into consideration in Earth System paleoclimate simulations used to explore the mechanisms of African climatic and environmental sensitivity