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      Pastoralism may have delayed the end of the green Sahara

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      Nature Communications
      Nature Publishing Group UK

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          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.

          Abstract

          Climate deterioration towards desertification in North Africa following the African Humid Period has previously been associated with the emergence of pastoralism. Here, using a climate-vegetation model, the authors show that pastoralism in fact likely slowed the deterioration of orbitally-driven climate change.

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          High-resolution carbon dioxide concentration record 650,000-800,000 years before present.

          Changes in past atmospheric carbon dioxide concentrations can be determined by measuring the composition of air trapped in ice cores from Antarctica. So far, the Antarctic Vostok and EPICA Dome C ice cores have provided a composite record of atmospheric carbon dioxide levels over the past 650,000 years. Here we present results of the lowest 200 m of the Dome C ice core, extending the record of atmospheric carbon dioxide concentration by two complete glacial cycles to 800,000 yr before present. From previously published data and the present work, we find that atmospheric carbon dioxide is strongly correlated with Antarctic temperature throughout eight glacial cycles but with significantly lower concentrations between 650,000 and 750,000 yr before present. Carbon dioxide levels are below 180 parts per million by volume (p.p.m.v.) for a period of 3,000 yr during Marine Isotope Stage 16, possibly reflecting more pronounced oceanic carbon storage. We report the lowest carbon dioxide concentration measured in an ice core, which extends the pre-industrial range of carbon dioxide concentrations during the late Quaternary by about 10 p.p.m.v. to 172-300 p.p.m.v.
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            Tipping elements in the Earth's climate system.

            The term "tipping point" commonly refers to a critical threshold at which a tiny perturbation can qualitatively alter the state or development of a system. Here we introduce the term "tipping element" to describe large-scale components of the Earth system that may pass a tipping point. We critically evaluate potential policy-relevant tipping elements in the climate system under anthropogenic forcing, drawing on the pertinent literature and a recent international workshop to compile a short list, and we assess where their tipping points lie. An expert elicitation is used to help rank their sensitivity to global warming and the uncertainty about the underlying physical mechanisms. Then we explain how, in principle, early warning systems could be established to detect the proximity of some tipping points.
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              Insolation values for the climate of the last 10 million years

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                Author and article information

                Contributors
                c.brierley@ucl.ac.uk
                Journal
                Nat Commun
                Nat Commun
                Nature Communications
                Nature Publishing Group UK (London )
                2041-1723
                1 October 2018
                1 October 2018
                2018
                : 9
                : 4018
                Affiliations
                [1 ]ISNI 0000000121901201, GRID grid.83440.3b, Department of Geography, , University College London, ; London, WC1E 6BT UK
                [2 ]ISNI 0000000121901201, GRID grid.83440.3b, Institute of Archaeology, , University College London, ; London, WC1E 6BT UK
                [3 ]ISNI 0000 0001 2322 6764, GRID grid.13097.3c, Department of Geography, , Kings College London, ; London, WC2R 2LS UK
                Author information
                http://orcid.org/0000-0002-9195-6731
                Article
                6321
                10.1038/s41467-018-06321-y
                6167352
                30275473
                b2617ff4-e089-4a60-a67f-354590db321f
                © The Author(s) 2018

                Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

                History
                : 10 April 2018
                : 23 August 2018
                : 29 August 2018
                Funding
                Funded by: FundRef https://doi.org/10.13039/501100000275, Leverhulme Trust;
                Award ID: RPG-2016-115
                Award Recipient :
                Funded by: FundRef https://doi.org/10.13039/501100000781, EC | European Research Council (ERC);
                Award ID: 249390
                Award Recipient :
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                © The Author(s) 2018

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