Rainfall regimes of the Green Sahara

Tierney, Jessica E.; Pausata, Francesco S. R.; deMenocal, Peter B

doi:10.1126/sciadv.1601503

Cited by 290 publications

(384 citation statements)

References 101 publications

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“…Plant species have survived past climate changes by adjusting their ranges locally and/or adapted physiologically/genetically to the new conditions. Although the Sahara was wetter than today between 11,000 and 5,000 BP up to 31 • N (Tierney et al, 2017), it did not contribute to the long-term persistence of the Mediterranean species. Under this more restrictive set of conditions species either went extinct or adapted.…”

Section: Introductionmentioning

confidence: 82%

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Microrefugia, Climate Change, and Conservation of Cedrus atlantica in the Rif Mountains, Morocco

et al. 2017

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This study reconstructs and interprets the changing range of Atlas cedar in northern Morocco over the last 9,000 years. A synthesis of fossil pollen records indicated that Atlas cedars occupied a wider range at lower elevations during the mid-Holocene than today. The mid-Holocene geographical expansion reflected low winter temperatures and higher water availability over the whole range of the Rif Mountains relative to modern conditions. A trend of increasing aridity observed after 6,000 years BP progressively reduced the range of Atlas cedar and prompted its migration toward elevations above 1,400 masl. To assess the impact of climate change on cedar populations over the last decades, we performed a transient model simulation for the period between 1960 and 2010. Our simulation showed that the range of Atlas cedar decreased by about 75% over the last 50 years and that the eastern populations of the range in the Rif Mountains were even more threatened by the overall lack of water availability than the western ones. Today, Atlas cedar populations in the Rif Mountains are persisting in restricted and isolated areas (Jbel Kelti, Talassemtane, Jbel Tiziren, Oursane, Tidighine) that we consider to be modern microrefugia. Conservation of these isolated populations is essential for the future survival of the species, preserving polymorphisms and the potential for population recovery under different climatic conditions.

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Section: Introductionmentioning

confidence: 82%

“…The narrow strip (<5 • latitude) between the Mediterranean Sea and the Saharan Desert currently supports rather arid climates, though this has not always been the case (Tierney et al, 2017). In this setting, mountains provide habitat heterogeneity and a variety of microclimates.…”

Section: Introductionmentioning

confidence: 99%

Microrefugia, Climate Change, and Conservation of Cedrus atlantica in the Rif Mountains, Morocco

et al. 2017

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“…However, pollen and macro-fossil evidence shows that boreal forest extended farther north than today in the mid-Holocene Prentice et al, 2000;Binney et al, 2017) and, except in Alaska and central Canada, extended to the Arctic coast during the LIG LIGA, 1991;Lozhkin and Anderson, 1995). Pollen and other biogeographical/geomorphological and paleohydrological evidence also indicates northward extension of vegetation into modern-day desert areas, particularly in northern Africa, both in the mid-Holocene (Drake et al, 2011;Hély et al, 2014;Larrasoana et al, 2013;Lezine et al, 2011;Prentice et al, 2000;Tierney et al, 2017) and during the maximum phase of the LIG (Castaneda et al, 2009;Hooghiemstra et al, 1992). Given the impact of increased woody cover on albedo and evapotranspiration, these vegetation changes should have profound impacts on the surface energy and water budgets and may help to explain mismatches between simulated and reconstructed high-latitude (Muschitiello et al, 2015) and monsoon climates Claussen and Gayler, 1997;Pausata et al, 2016) in both time periods.…”

Section: Sensitivity To Prescribed Vegetationmentioning

confidence: 99%

“…The mid-Holocene interval has been the focus for model simulations, model-model comparisons, paleodata synthesis, and model-data comparison since the beginning of PMIP, and this work has contributed to model evaluation and understanding of climate change in the last three major assessments of the Intergovernmental Panel on Climate Change (Flato et al, 2013;Folland et al, 2001;Hegerl et al, 2007;Jansen et al, 2007;Masson-Delmotte et al, 2013). The changes in insolation are characterized by enhanced seasonal contrast in the Northern Hemisphere (NH) (and reduced seasonal contrast in the Southern Hemisphere, SH), giving rise to warmer NH summers and a significant enhancement of the NH monsoons (COHMAP Members, 1988;Hély et al, 2014;Lezine et al, 2011;Saraswat et al, 2013;Tierney et al, 2017). Systematic benchmarking against pollen-based reconstructions of climate variables and lake-level-based waterbalance reconstructions (Braconnot et al, 2007b(Braconnot et al, , 2012Coe and Harrison, 2002;Harrison et al, 1998Harrison et al, , 2014Harrison et al, , 2015 has highlighted the fact that climate models persistently underestimate changes in the monsoon precipitation and produce too much continental drying (Harrison et al, 2015).…”

Section: Introductionmentioning

confidence: 99%

The PMIP4 contribution to CMIP6 – Part 2: Two interglacials, scientific objective and experimental design for Holocene and Last Interglacial simulations

et al. 2017

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Abstract. Two interglacial epochs are included in the suite of Paleoclimate Modeling Intercomparison Project (PMIP4) simulations in the Coupled Model Intercomparison Project (CMIP6). The experimental protocols for simulations of the mid-Holocene (midHolocene, 6000 years before present) and the Last Interglacial (lig127k, 127 000 years before present) are described here. These equilibrium simulations are designed to examine the impact of changes in orbital forcing at times when atmospheric greenhouse gas levels were similar to those of the preindustrial period and the continental configurations were almost identical to modern ones. These simulations test our understanding of the interplay between radiative forcing and atmospheric circulation, and the connections among large-scale and regional climate changes giving rise to phenomena such as land-sea contrast and highlatitude amplification in temperature changes, and responses of the monsoons, as compared to today. They also provide an opportunity, through carefully designed additional sensitivity experiments, to quantify the strength of atmosphere, ocean, cryosphere, and land-surface feedbacks. Sensitivity experiments are proposed to investigate the role of freshwater forcing in triggering abrupt climate changes within interglacial epochs. These feedback experiments naturally lead to a focus on climate evolution during interglacial periods, which will be examined through transient experiments. Analyses of the sensitivity simulations will also focus on interactions between extratropical and tropical circulation, and the relationship between changes in mean climate state and climate variability on annual to multi-decadal timescales. The comparative abundance of paleoenvironmental data and of quantitative climate reconstructions for the Holocene and Last Interglacial make these two epochs ideal candidates for systematic evaluation of model performance, and such comparisons will shed new light on the importance of external feedbacks (e.g., vegetation, dust) and the ability of state-of-the-art models to simulate climate changes realistically.

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“…Nevertheless, δD wax studies from offshore of NW Africa covering the African Humid Period (∼ 15-5 ka) yield a robust humid signal among different records (Niedermeyer et al, 2010;Collins et al, 2013;Kuechler et al, 2013;Tierney et al, 2017), although the vegetation of this African Humid Period had no modern analogue (Watrin et al, 2009). Watrin et al emphasize that instead of a homogenous latitudinal shift of vegetation zones as a whole, individual plant species likely have an advantage over others.…”

Section: Plant-wax Provenance and Vegetation Sourcesmentioning

confidence: 99%

Hybrid insolation forcing of Pliocene monsoon dynamics in West Africa

2018

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Abstract. The Pliocene is regarded as a potential analogue for future climate with conditions generally warmer-thantoday and higher-than-preindustrial atmospheric CO 2 levels. Here we present the first orbitally resolved records of continental hydrology and vegetation changes from West Africa for two Pliocene time intervals (5.0-4.6 Ma, 3.6-3.0 Ma), which we compare with records from the last glacial cycle (Kuechler et al., 2013). Our results indicate that changes in local insolation alone are insufficient to explain the full degree of hydrologic variations. Generally two modes of interacting insolation forcings are observed: during eccentricity maxima, when precession was strong, the West African monsoon was driven by summer insolation; during eccentricity minima, when precession-driven variations in local insolation were minimal, obliquity-driven changes in the summer latitudinal insolation gradient became dominant. This hybrid monsoonal forcing concept explains orbitally controlled tropical climate changes, incorporating the forcing mechanism of latitudinal gradients for the Pliocene, which probably increased in importance during subsequent Northern Hemisphere glaciations.

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Rainfall regimes of the Green Sahara

Abstract: We estimate rainfall during the “Green Sahara” period.

Cited by 290 publications

References 101 publications

Microrefugia, Climate Change, and Conservation of Cedrus atlantica in the Rif Mountains, Morocco

Microrefugia, Climate Change, and Conservation of Cedrus atlantica in the Rif Mountains, Morocco

The PMIP4 contribution to CMIP6 – Part 2: Two interglacials, scientific objective and experimental design for Holocene and Last Interglacial simulations

Hybrid insolation forcing of Pliocene monsoon dynamics in West Africa

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