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 y 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 orbi...

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Published in:	Proceedings of the National Academy of Sciences - PNAS Vol. 118; no. 23; pp. 1 - 7
Main Authors:	Cheddadi, Rachid, Carré, Matthieu, Nourelbait, Majda, François, Louis, Rhoujjati, Ali, Manay, Roger, Ochoa, Diana, Schefuß, Enno
Format:	Journal Article Web Resource
Language:	English
Published:	Washington National Academy of Sciences 08-06-2021
Subjects:	African humid period green Sahara Climate change Climate models Earth sciences & physical geography Environmental changes Geophysics Holocene Lakes Monsoons Paleoclimate Paleoclimate reconstructions Physical Sciences Physical, chemical, mathematical & earth Sciences Physics Physique, chimie, mathématiques & sciences de la terre Pollen Precipitation Rainfall Sciences de la terre & géographie physique Seasonal distribution Summer Vegetation Vegetation cover Vegetation model simulations Wind Winter paleoclimate reconstructions African humid period Holocene vegetation model simulations green Sahara
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Summary:	The greening of the Sahara, associated with the African Humid Period (AHP) between ca. 14,500 and 5,000 y 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 Middle Holocene climate model simulations cannot sustain vegetation in the Sahara or account for the increased humidity in the Mediterranean region. Here, we present an 18,500-y pollen and leaf-wax δD record from Lake Tislit (32° N) in Morocco, which provides quantitative reconstruction of winter and summer precipitation in northern Africa. The record from Lake Tislit shows that the northern Sahara and the Mediterranean region were wetter in the AHP because of increased winter precipitation and were not influenced by the monsoon. The increased seasonal contrast of insolation led to an intensification and southward shift of the Mediterranean winter precipitation system in addition to the intensified summer monsoon. Therefore, a winter rainfall zone must have met and possibly overlapped the monsoonal zone in the Sahara. Using a mechanistic vegetation model in Early Holocene conditions, we show that this seasonal distribution of rainfall is more efficient than the increased monsoon alone in generating a green Sahara vegetation cover, in agreement with observed vegetation. This conceptual framework should be taken into consideration in Earth system paleoclimate simulations used to explore the mechanisms of African climatic and environmental sensitivity.
Bibliography:	scopus-id:2-s2.0-85107404632 1R.C., M.C., L.F., and E.S. contributed equally to this work. Edited by Francesco S. R. Pausata, University of Quebec in Montreal, Montreal, Canada, and accepted by Editorial Board Member Donald R. Ort April 20, 2021 (received for review December 17, 2020) Author contributions: R.C. and M.C. designed research; R.C., M.C., L.F., and E.S. performed research; M.C., L.F., and E.S. contributed new reagents/analytic tools; R.C., M.C., M.N., A.R., R.M., D.O., and E.S. produced and analyzed paleoclimate data; L.F. performed simulations; A.R. performed fieldwork with minor contributions to the paper; and R.C., M.C., L.F., and E.S. wrote the paper.
ISSN:	0027-8424 1091-6490 1091-6490
DOI:	10.1073/pnas.2024898118