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. 2018 Oct 1;9(1):4018.
doi: 10.1038/s41467-018-06321-y.

Pastoralism may have delayed the end of the green Sahara

Chris Brierley  1 , Katie Manning  2   3 , Mark Maslin  4
Affiliations

Pastoralism may have delayed the end of the green Sahara

Chris Brierley et al. Nat Commun. .

Abstract

The climate deterioration after the most recent African humid period (AHP) is a notable past example of desertification. Evidence points to a human population expansion in northern Africa prior to this, associated with the introduction of pastoralism. Here we consider the role, if any, of this population on the subsequent ecological collapse. Using a climate-vegetation model, we estimate the natural length of the most recent AHP. The model indicates that the system was most susceptible to collapse between 7 and 6 ka; at least 500 years before the observed collapse. This suggests that the inclusion of increasing elements of pastoralism was an effective adaptation to the regional environmental changes. Pastoralism also appears to have slowed the deterioration caused by orbitally-driven climate change. This supports the view that modern pastoralism is not only sustainable, but beneficial for the management of the world's dryland environments.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1
Pastoralist–environment interactions. a Schematic of a human population expansion beyond the carrying capacity of the region exacerbating aridification. b Schematic of how the technological and cultural advances associated with sustainable pastoralism could help buffer changes to a fragile ecosystem
Fig. 2
Fig. 2
Reconstructions of Holocene northern Africa. a Palaeoclimate reconstructions showing the existence of wet conditions (thick solid lines), semi-arid conditions (thin solid) or arid conditions (dashed). The individual reconstructions are colored by region: Central Sahara (Blue), Eastern Sahara (Red), the Atlas & Hoggar Mountains (Orange). The records discussed in the text are Lake Yoa (green) and marine cores, (black), whilst records outside of the population regions are coloured gray. The records shown in order of their wet/dry transition, and the total number of wet/dry transitions is shown for each 500 year period. b Probability density of dated archaeological finds for the three regions in northern Africa, which can be used to infer the relative populations
Fig. 3
Fig. 3
The relationship between sapropel formation and orbital precession using two sapropel chronologies. Precessional curves associated with sapropel formation are aligned to the start of each occurrence according to the respective chronology. The coloured segments of these curves indicate the actual duration of the sapropel. The red horizontal line indicates the precession at the termination of the most recent sapropel. a A speleothem-tuned chronology provides well-constrained estimates of the onset and termination of sapropels over the past 250,000 years. b The past ten interglacial sapropels seen in a Mediterranean Sea level record
Fig. 4
Fig. 4
The last two glacial cycles. a The input times series of radiative forcing of carbon dioxide changes (black) and climatic precession (red). b Barium to aluminium ratio at Ocean Drilling Program site 968 in the Eastern Mediterranean. c The distribution of the roughly 12,000 ensemble members that exhibit seven ‘green’ events. The median (black), inter-quartile range (dark gray) and 5–95% range (light gray) are shown, along with the sapropel start (green) and end (blue) dates calculated from observations
Fig. 5
Fig. 5
Potential interaction between humans and the ecosystem during the end of the African humid period. a Histogram of the number of climate proxies (Fig. 1a) indicating an end of the African Humid Period within a 500 year window. b The population inferred summed probability distribution over the whole of northern Africa region along with its 5–95% confidence level. c The simulated sensitivity of northern Africa diagnosed from the model. Black dots show the number of not-implausible model settings with a threshold time, t∗, in each century (see Methods for details); the black line is a 3 point running average. The gray line vertical indicates the time of the maximum simulated sensitivity
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References

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