During the “Green Sahara” period (11,000 to 5000 years before the present), the Sahara desert received high amounts of rainfall, supporting diverse vegetation, permanent lakes, and human populations. Our knowledge of rainfall rates and the spatiotemporal extent of wet conditions has suffered from a lack of continuous sedimentary records. We present a quantitative reconstruction of western Saharan precipitation derived from leaf wax isotopes in marine sediments. Our data indicate that the Green Sahara extended to 31°N and likely ended abruptly. We find evidence for a prolonged “pause” in Green Sahara conditions 8000 years ago, coincident with a temporary abandonment of occupational sites by Neolithic humans. The rainfall rates inferred from our data are best explained by strong vegetation and dust feedbacks; without these mechanisms, climate models systematically fail to reproduce the Green Sahara. This study suggests that accurate simulations of future climate change in the Sahara and Sahel will require improvements in our ability to simulate vegetation and dust feedbacks.

Keywords

Here, we used leaf wax biomarkers preserved in marine sediment cores to create a continuous, spatiotemporal reconstruction of precipitation rates in western Sahara. Our reconstruction spans the last 25,000 years, describing both the onset and termination of the Green Sahara as well as conditions during the Last Glacial Maximum and the deglaciation. We used a transect of gravity cores from the West African margin that span the full meridional breadth of the Sahara (19°N to 32°N; Fig. 1 ). To reconstruct precipitation, we used paired measurements of the carbon and hydrogen isotopic composition of leaf waxes (δ 13 C wax and δD wax ) and a Bayesian regression approach (see Materials and Methods and the Supplementary Materials). δD wax is an excellent tracer for the hydrogen isotopic composition of precipitation (δD P ) ( 21 ). However, large changes in vegetation, such as those that occurred during the Green Sahara, have secondary effects on the signal ( 21 ). Correcting δD wax for vegetation impacts using paired δ 13 C wax improves the inference of δD P (see fig. S1 and the Supplementary Materials) ( 22 , 23 ). In turn, δD P has a strong (r = −0.72) log-normal relationship with the amount of rainfall in western Sahara (see fig. S5, Materials and Methods, and the Supplementary Materials), allowing us to quantitatively infer precipitation rates.

The timing and magnitude of the rainfall changes that established the Green Sahara are still not well characterized. Our best indicators to date come from pollen records recovered from paleolake deposits in the desert. These data suggest that the Sahara hosted steppe, savannah, and wooded grassland environments, with tropical plants migrating as far as 24°N ( 12 – 14 ). These shifts correspond to an increase in precipitation across the Sahara and Sahel of 500 mm/year or more ( 14 – 16 ). Likewise, sedimentological and lake-level studies suggest that permanent paleolakes extended at least to 28°N ( 17 ), associated with increases in rainfall of ca. 300 to 900 mm/year ( 18 – 20 ). However, most of the lacustrine deposits from which these data are derived are poorly dated and discontinuous. Thus, these quantitative estimates generally apply only to the early or middle Holocene and do not give a clear picture of how the Green Sahara evolved through time and space.

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 permanent 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.

Notably, we observe very wet conditions as far as 31°N ( Figs. 2B and 3A ). Our northernmost site is located offshore from Cape Ghir, Morocco: a region that presently experiences a December to March rainy season typical of the Mediterranean. However, it is likely that most of the observed increase is monsoonal. Modeling experiments using either prescribed or interactive vegetation suggest that the changes in atmospheric circulation during the early Holocene were large enough to advect monsoonal moisture up to 30°N ( 25 – 27 ). In addition, analysis of an idealized mid-Holocene Green Sahara simulation ( 27 ) indicates that 90% of the annual increase at 31°N occurs during June to September (fig. S3). It is therefore feasible that, at the peak of the Green Sahara, monsoonal moisture inundated the entire western Saharan region. However, the mid-Holocene simulation also shows a strong weakening of the Azores high in winter, raising the possibility that a larger increase in winter precipitation occurred than those simulated (fig. S3). A dual-season increase may be partly responsible for the exceptionally high rainfall rates that we infer at 31°N (median, 1280 mm/year; 1σ range, 560 to 2550 mm/year).

Black lines indicate median values; colors indicate posterior probability distributions. Black squares with error bars denote modern mean annual observational values and SDs [from the Online Isotopes in Precipitation Calculator ( 61 ) for δD P and Global Precipitation Climatology Centre version 6 ( 60 ) for precipitation]. Note that, because our δD wax precipitation regression is logarithmic, the uncertainty of our inferred precipitation rates increases at higher precipitation amounts; thus, probability densities are more broadly distributed during wet intervals.

The δD P and inferred precipitation rates from our core sites place important new constraints on the spatiotemporal evolution of the Green Sahara as well as the magnitude of rainfall change ( Figs. 2 and 3A ). The Green Sahara period (ca. 11,000 to 5000 yr B.P.) emerges at every site as the time interval with the most depleted δD P and the highest inferred rainfall rates ( Fig. 2 ). Median rainfall rates during the Green Sahara, across all sites, were 640 mm/year, but there is substantial spatiotemporal variability (1σ range, 250 to 1670 mm/year; Fig. 2B ). This represents a remarkable difference from modern-day rainfall rates in western Sahara, which range from 35 to 100 mm/year. However, these numbers agree with the pollen and lake-level estimates (see discussion above), and they are consistent with the proposed landscape: a mix of grasslands, shrubs, and tropical elements. Grasslands dominate landscapes at precipitation rates from 300 to 800 mm/year in Africa ( 24 ) and Sudanian group taxa, which require rainfall rates between 500 and 1500 mm/year, expanded to 25°N ( 14 ). Thus, in general, our precipitation rate estimates confirm the interpretation that a seasonal tropical climate dominated most regions of North Africa during the Green Sahara time ( 14 ).

DISCUSSION

Spatiotemporal variability and abrupt change during the Green Sahara Our rainfall reconstructions clarify both the timing and variability of high precipitation rates during the Green Sahara. Peak rainfall typically occurred between 11,000 and 6000 yr B.P., but conditions evolved differently by latitude. In general, our data show that the Green Sahara was relatively restricted at higher latitudes (31°N) and lasted longer at lower latitudes (Fig. 3A). At our lowest latitude site (19°N), humid conditions were established early during the deglaciation, with median rainfall rates during the Bølling-Allerød period (B/A) interstadial (14,500 to 12,800 yr B.P.) of 1430 mm/year (1σ range, 623 to 2740 mm/year) (Figs. 2B and 3A). δD wax records from the Sahel and tropical West Africa similarly show relatively depleted values during the B/A (23, 28), indicating that, at low latitudes in western Africa, fully humid conditions were established before the deglaciation was complete. In contrast, our more northerly sites show that Green Sahara conditions were not established until the early Holocene (Fig. 3A). Bioturbation forward modeling suggests that humid conditions terminated early at 31°N [median value, 6500 yr B.P.; 6.5 thousand years ago (ka)] as compared with the other three sites to the south (m = 5.0, 5.3, and 5.3 ka, respectively; figs. S4 and S5). This generally supports the hypothesis that the termination of the Green Sahara was time-transgressive, with areas farther away from the epicenter of the West African monsoon experiencing earlier aridification as the monsoon retreated (23); however, we do not see a clear time transgression in the termination dates between 19°N and 27°N. A time-transgressive response does not preclude the existence of a regionally abrupt termination of high precipitation rates. Our bioturbation modeling suggests that the end of the Green Sahara was likely abrupt (occurring within a few hundred years) at all four of our core sites (see the Supplementary Materials), in agreement with analyses of dust data from the same sites (10) and other δD wax -based records in East Africa (11). In addition to a wet B/A event, our lowest latitude site (19°N) records pronounced drying during the Younger Dryas (YD) (12,800 to 11,500 yr B.P.). Although the YD is generally a dry interval between 23°N and 31°N, it is not readily distinguishable from the B/A or the rest of the deglacial sequence because of the prevalence of dry conditions before the event (Fig. 3A). Similarly, Heinrich event 1 (17,500 to 14,500 yr B.P.), which is associated with dry conditions throughout the East African and Indian monsoon domains (29), is not prominently featured in any of our precipitation records from western Sahara (Fig. 3A), most likely because rainfall rates were low and the West African monsoon did not extend to the latitudes of our sites (19°N to 31°N) during late glacial times.

An early Holocene pause in Green Sahara conditions We observe a prominent reduction in precipitation during the early Holocene—around 8000 yr B.P. (8 ka)—at the sites spanning 19°N to 23°N (Figs. 2B and 3A). This 8 ka dry period is also seen in a nearby δD wax record from the Sahel (15°N) (28), in leaf wax and lake-level records from East Africa (11, 30), and in numerous lake-level reconstructions from across the Sahara (fig. S6) (1). We do not observe a clear 8 ka pause between 27°N and 31°N in our data, and a survey of existing records indicates that, although it may be expressed inland at these latitudes, the duration of the event is short (fig. S6). One explanation for the weak expression of the 8 ka dry period at these latitudes is that winter rainfall contributions obscured the event (see discussion above). The 8 ka pause in Green Sahara conditions appears to have lasted for a millennium or more. Bioturbation forward modeling indicates that a dry period of at least 1000 years is needed to explain the duration of the event at our sites (fig. S4). In addition, other proxy data across Africa suggest extended and severe drying at this time (fig. S6). The archaeological record from the Sahara provides further compelling evidence of a prolonged 8 ka dry period. In particular, the duration of the 8 ka pause in our precipitation reconstructions aligns with evidence from the Gobero site in Niger (17°N), where extensive radiocarbon dating indicates that there was an interruption in occupation from 8150 to 7150 yr B.P., during which time a lake nearby dried up (Fig. 4) (31). The 8 ka pause also begins slightly before a prominent mid-Holocene dip in Saharan demographics, estimated from a density of over 1000 radiocarbon dates from archaeological sites (Fig. 4) (32). The inferred demographic delay relative to the climatic event is expected as populations adjust to new environmental conditions; a similar delay is seen in the population response to the onset of Green Sahara conditions in the early Holocene (32). Fig. 4 The 8 ka pause in Green Sahara conditions. Green areas denote the probability distributions of the start and end of the 8 ka dry period at sites GC49 (23°N) and GC68 (19°N), on the basis of the inferred locations in the core (from bioturbation modeling) for an abrupt beginning and end, and Monte Carlo iteration of age model uncertainties. Black and gray dots with error bars represent the mean and 1σ ages for climatic events associated with the 8.2 cooling event in the Northern Hemisphere, including (i) the timing of the drainage of Lake Agassiz and Lake Ojibway (37); (ii) the timing of an abrupt rise in sea level, detected in the Netherlands (62); (iii) the duration of the 8.2 event in the Greenland ice cores (38); and (iv) the duration of the response in Cariaco Basin grayscale data (63). Shown in red is the timing and duration of a prolonged interruption in the occupation of the Gobero site by humans (31). The yellow bar indicates a Sahara-wide demographic decline (32). The archaeological record further suggests that the 8 ka pause is associated with a distinctive change in lifestyle. At Gobero, the humans occupying the site before the 8 ka pause were hunter-fisher-gatherers, whereas the humans that occupied the site after the pause had a more diversified diet that included cattle husbandry (31). More generally, widespread adoption of pastoralism in the Sahara (the raising of cattle, the “cattle cult,” and the practice of dairying) occurs after the 8 ka pause (33, 34). The temporary deterioration of climate conditions at 8 ka in the Sahara may have been an impetus to abandon hunting and gathering in favor of cattle herding, a more resilient strategy in the face of a fluctuating climate (35). What might have caused this mid-Holocene pause in humid conditions? The beginning of the 8 ka pause is roughly coeval with the “8.2 event” in the North Atlantic, a widespread cooling event in the Northern Hemisphere (36) caused by the sudden drainage of Lake Agassiz and Lake Ojibway (37) and a subsequent slowdown of the Atlantic Meridional Overturning Circulation. However, although the 8.2 event only lasted for a couple of hundred years (38), and its expression in the Northern Hemisphere rarely exceeds 500 years (36), our data suggest a dry period lasting ca. 1000 years. This leaves us with two possibilities: (i) the 8 ka arid phase was coincident with but unrelated to the 8.2 cooling event, or (ii) the 8 ka pause was directly related to the 8.2 event and climatic feedbacks amplified its impact and prolonged its duration in the Sahara. Regarding the first possibility, one hypothesis is that, at peak Green Sahara conditions, the monsoonal system extended so far north that it left the West African tropics drier. Some mid-Holocene (6 ka) model simulations do show evidence of drier conditions below ca. 10°N in response to a northward shift in the monsoon (27, 39). This may explain the prolonged nature of the 8 ka event in both lake-level and δD wax data from tropical West Africa (5°N) (23) but cannot reasonably explain the presence of the 8 ka pause at ca. 23°N (Fig. 3A and fig. S6). Although speculative, a direct relationship between the 8.2 event and the 8 ka pause fits better with the available data. The onset of the 8 ka pause agrees reasonably well with known events associated with the 8.2 event (Fig. 4). Furthermore, the presence of the B/A and YD events at our lower latitude site suggests that North Atlantic forcing affects the West African monsoon system. As we discuss in further detail below, vegetation and dust feedbacks likely played a large role in maintaining high precipitation rates during the Green Sahara. Whereas there is no evidence for increased dust flux near 8 ka (10), there is a fluctuation in the richness and abundance of vegetation types around this time (14). A short-lived drying caused by the 8.2 event may have reduced vegetative cover, leading to changes in albedo that prolonged the drying and made it more difficult for the Green Sahara ecosystem to recover. Thorough testing of this hypothesis requires high-resolution pollen records from the Sahara, as well as model simulations to assess the sensitivity of the Green Sahara to millennial-scale perturbation.