During the early Holocene epoch [11,000 to 5000 years before the present (yr B.P.)], the hyperarid Sahara was transformed into a mesic landscape, with widespread grasslands, variable tree cover, large per- manent lakes, and extensive river drainage networks. Evidence for this “Green Sahara” interval comes from paleolake deposits, pollen, and archaeological remains, indicating that humans inhabited, hunted, and gathered deep within the present-day desert (1–3). The Green Sahara was the most recent of a succession of wet phases paced by orbital precession that extends back to the late Miocene (4). When the precessional cycle approaches perihelion during boreal summer, the increase in insolation drives a strong land-sea temperature gradient over North Africa that strengthens the African monsoon, bringing rainfall deep into the Sahara (5). Climate model experiments demonstrate that oceanic and land surface feedbacks can amplify this initial response, resulting in even wetter conditions (6, 7). These feedbacks may also result in abrupt shifts between wet and dry regimes (7, 8), and some sedimentary records suggest that the Green Sahara terminated within centuries around 5000 yr B.P. (9–11). Given the marked and expansive nature of the climate changes associated with the Green Sahara, it is a useful case study of how gradual external climate forcing in arid environments can result in rapid, nonlinear responses, a particularly instructive lesson in our currently warming world.
In summary, our δDwax -inferred precipitation reconstructions from the West African margin provide a continuous and quantitative view of rainfall rates for the last 25,000 years, including the Green Sahara interval. Our data reveal important spatiotemporal aspects of this remarkable change in hydroclimate, including an extreme northward incursion of the African monsoon (31°N) from 9.5 ka to 7 ka and the presence of a prominent pause in Green Sahara conditions near 8 ka. The millennium-long duration of the 8 ka pause matches exceptionally well with the archaeological record and provides a climatic explanation for the observed occupational patterns, demographic response, and lifestyle changes of Neolithic humans. We speculate that the 8.2 cooling in the Northern Hemisphere initiated the pause and that land surface feedbacks prolonged it. Likewise, we show that strong vegetation and dust feedbacks are necessary to explain the magnitude and intensity of the African monsoon during the Green Sahara. The prominent role of dust in forcing the Green Sahara agrees with 20th century analyses of Sahel rainfall, suggesting that dust feedbacks are as important as sea surface temperature and vegetation changes in driving observed historical trends (50). Furthermore, the features seen in our data, including the rapid termination of the Green Sahara and the pro- longed 8 ka pause, are consistent with the idea that the Sahara has multiple stable states, mediated by vegetation or dust feedbacks (8, 51).