The Greening of the Sahara: Past Changes and Future Implications

Pausata, Francesco S. R.; Gaetani, Marco; Messori, Gabriele; Berg, Alexis; Souza, Danielle Maia de; Sage, Rowan F.; deMenocal, Peter B

doi:10.1016/j.oneear.2020.03.002

Cited by 135 publications

(132 citation statements)

References 191 publications

Supporting

Mentioning

123

Contrasting

Unclassified

Order By: Relevance

“…Another confounding factor that may have contributed to changes in local precipitation within the tropics is the expansion of the tropical rainforests in the PlioMIP simulations. For instance, a northward expansion of the tropical vegetation in Africa is known to increase precipitation in the Northern Hemisphere tropics, decrease trade winds and, thus warm the tropical North Atlantic 48 . It is important to note that the PlioMIP initiative focuses on a single interglacial time slice (KM5c at ~ 3.2 Ma) within the mid-Pliocene Warm Period 4 (3–3.3 Ma).…”

Section: Discussionmentioning

confidence: 99%

Drier tropical and subtropical Southern Hemisphere in the mid-Pliocene Warm Period

Pontes

Wainer

Taschetto

et al. 2020

Sci Rep

View full text Add to dashboard Cite

Thermodynamic arguments imply that global mean rainfall increases in a warmer atmosphere; however, dynamical effects may result in more significant diversity of regional precipitation change. Here we investigate rainfall changes in the mid-Pliocene Warm Period (~ 3 Ma), a time when temperatures were 2-3ºC warmer than the pre-industrial era, using output from the Pliocene Model Intercomparison Projects phases 1 and 2 and sensitivity climate model experiments. In the Mid-Pliocene simulations, the higher rates of warming in the northern hemisphere create an interhemispheric temperature gradient that enhances the southward cross-equatorial energy flux by up to 48%. This intensified energy flux reorganizes the atmospheric circulation leading to a northward shift of the Inter-Tropical Convergence Zone and a weakened and poleward displaced Southern Hemisphere Subtropical Convergences Zones. These changes result in drier-than-normal Southern Hemisphere tropics and subtropics. The evaluation of the mid-Pliocene adds a constraint to possible future warmer scenarios associated with differing rates of warming between hemispheres. The Earth has experienced many periods in which climate was warmer than present. Understanding atmospheric circulation and precipitation during past warm climates is useful to produce constraints on possible future changes. Here we analyse the Southern Hemisphere large-scale rainfall response in the mid-Pliocene Warm Period (~ 3 Ma mPWP; hereafter referred as mid-Pliocene). During this period high-latitude Sea Surface Temperatures (SST) were as high as + 9 °C and + 4 °C in the Northern and Southern Hemisphere, respectively 1 , compared to pre-industrial times (~ 1850 Common Era [C.E.]; Fig. 1). In addition, the Greenland ice sheet had 50-70% 2-4 less mass and the western Antarctica was ice-free 5,6. Atmospheric CO 2 concentrations were similar to today 7 (~ 400 ppm). Although the extent of the ice sheets for the end of the twenty-first century is still a topic of debate, especially due to model's limitations in simulating land-ice processes 8,9 , the mid-Pliocene is considered a useful analogue for the end-of-century climate 10. Rainfall proxy archives do not exist for the tropics and Southern Hemisphere subtropics in the mid-Pliocene. However, paleo reconstructions of ice sheets, SSTs, vegetation distribution, soils and lakes performed by the Pliocene Research, Interpretation and Synoptic Mapping (PRISM) project 1 are available to force climate models 4,12

show abstract

Section: Discussionmentioning

confidence: 99%

Drier tropical and subtropical Southern Hemisphere in the mid-Pliocene Warm Period

Pontes

Wainer

Taschetto

et al. 2020

Sci Rep

View full text Add to dashboard Cite

show abstract

“…The mid- to late Holocene, roughly 6 to 4 thousand years ago (ka), was characterized by one of the largest climate shifts since the last glacial termination—the end of the Green Sahara (also referred to as the African Humid Period), when a once-vegetated northern Africa transitioned to a hyper-arid desert landscape 2 – 5 . 'Both the nature and timing of this climate shift have been topics of great interest because it overlaps with societal upheavals across western Asia and the Middle East.…”

Section: Introductionmentioning

confidence: 99%

End of Green Sahara amplified mid- to late Holocene megadroughts in mainland Southeast Asia

et al. 2020

View full text Add to dashboard Cite

Between 5 and 4 thousand years ago, crippling megadroughts led to the disruption of ancient civilizations across parts of Africa and Asia, yet the extent of these climate extremes in mainland Southeast Asia (MSEA) has never been defined. This is despite archeological evidence showing a shift in human settlement patterns across the region during this period. We report evidence from stalagmite climate records indicating a major decrease of monsoon rainfall in MSEA during the mid- to late Holocene, coincident with African monsoon failure during the end of the Green Sahara. Through a set of modeling experiments, we show that reduced vegetation and increased dust loads during the Green Sahara termination shifted the Walker circulation eastward and cooled the Indian Ocean, causing a reduction in monsoon rainfall in MSEA. Our results indicate that vegetation-dust climate feedbacks from Sahara drying may have been the catalyst for societal shifts in MSEA via ocean-atmospheric teleconnections.

show abstract

“…The important role of feedbacks in maintaining a fundamentally different climate state provides another incentive for studying the mid-Holocene, because understanding them can also help us better appreciate the changes in the northern parts of Africa to be expected from ongoing climate change and regional human activity (Pausata et al, 2020). For instance, Charney (1975) and Charney et al (1975) have shown that the increase in albedo arising from decrease in vegetation can induce persistent regional atmospheric subsidence leading to reduced precipitation and further reduction in vegetation, creating a positive feedback that supports persistent desertification.…”

Section: Introductionmentioning

confidence: 99%

African Humid Period Precipitation Sustained by Robust Vegetation, Soil, and Lake Feedbacks

Chandan

Peltier

2020

Geophysical Research Letters

View full text Add to dashboard Cite

The African Humid Period (∼11,000-5,000 years before present) was the most recent of several precessionally paced wet intervals during which an increase in the Northern Hemisphere summer incoming solar radiation intensifies the West African Monsoon leading to dramatic changes over northern Africa. However, insolation anomaly alone is not sufficient and feedbacks are essential for further amplification of the monsoon. The most significant feedbacks derive from the land surface, arising from changes to vegetation, soil properties, and distribution of surface water. We show that in contrast to previous studies that have explored the individual impacts of these feedbacks, a modern climate model yields a much greater increase in precipitation in response to their collective effect. Agreement with proxies is improved while the desert-steppe transition is pushed further northward than in any previous study. In the West African Sahel, intensities of summer daily mean and extreme precipitation increase by 150% and 90%, respectively. Plain Language Summary From approximately 11,000 to 5,000 years before present, the summer monsoon over northern Africa was considerably stronger than what it is today and as such this period has come to be called the African Humid Period. As a result of this, lakes, wetlands, and rivers sprung up in the now arid regions of northern Africa and made it possible for vegetation to migrate northward which "greened the Sahara." It is widely understood that the greater amount of summer solar radiation impinging on this region, arising from a modest variation in the Earth's orbital configuration at that time, kick-started this intensification. However, while this may have triggered the strengthening of the monsoon, it was not by itself sufficient to intensify it to the degree suggested by proxies that register the strength of palaeomonsoons. The interaction between the atmosphere and the greener and wetter land surface could have further invigorated the monsoon, but based on the results from modeling experiments performed thus far, it has been thought that even this additional interaction is not sufficient. Here, we show that land-atmosphere interaction does however have the potential to strengthen the monsoon sufficiently to obtain agreement with proxy estimates.

show abstract

The Greening of the Sahara: Past Changes and Future Implications

Cited by 135 publications

References 191 publications

Drier tropical and subtropical Southern Hemisphere in the mid-Pliocene Warm Period

Drier tropical and subtropical Southern Hemisphere in the mid-Pliocene Warm Period

End of Green Sahara amplified mid- to late Holocene megadroughts in mainland Southeast Asia

African Humid Period Precipitation Sustained by Robust Vegetation, Soil, and Lake Feedbacks

Contact Info

Product

Resources

About