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

Abstract

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.

Early Holocene greening of the Sahara requires Mediterranean winter rainfall

“…The length of the dry season in arid regions has a direct impact on ecosystems ( 41 , 42 ). This effect of a double rainy season on vegetation would be enhanced by positive feedbacks from the vegetation and soil ( 15 , 20 , 22 ).…”

“…The green Sahara is an example of extreme environmental change, which highlights the region’s extraordinary sensitivity and the need to better understand its hydroclimatic variability. Current explanations for the greening of the Sahara point to the Earth’s orbital changes during the Early Holocene, leading to increased boreal summer (JJA) insolation, which drove the intensification and northward expansion of the JJA monsoon over northern Africa ( 15 , 18 ), aided by strong positive feedbacks from the land surface ( 19 – 22 ). Reproducing the green Sahara has posed a lasting challenge for climate modelers.…”

“…When all surface feedbacks are prescribed, simulated precipitation in the northern Sahara is still too low compared to paleoclimatic evidence for substantially increased moisture at 31° N ( 11 , 13 ) or too high in the 15 to 20° N range ( 20 ), creating incompatibility with prescribed vegetation ( 22 ). Additional sources of moisture ( 25 , 26 ) may have contributed to an AHP that extended toward the Mediterranean borderlands through different mechanisms.…”

The Northern Migrations from a drying Sahara (6,000 years BP): cultural and genetic influence in Greeks, Iberians and other Mediterraneans

“…Arrows show postulated migrations from Ethiopia to both West Africa and Greece, the latter probably occurring in pharaonic times, to give rise to other Sub-Saharan populations (Fulani, Mossi, Rimaibe; Burkina Faso) and Greeks. Humid Sahara Desert harboured people density before around 6,000 -5,000 years BC (Chandan & Peltier 2020).…”

Using the Mid‐Holocene “Greening” of the Sahara to Narrow Acceptable Ranges on Climate Model Parameters

“…Future work could compare convection-permitting simulations and ensembles of GCM simulations like those studied here for both present day and paleoclimate conditions to identify further ways to improve GCM parametrization schemes. Some have argued that the Green Sahara modeling problem may be resolved by altering model boundary conditions such as vegetation coverage or dust loading (Chandan & Peltier, 2020;Pausata et al, 2016). Here, we have shown that it is also possible to improve the simulation of precipitation through changes to the model parametrizations, many aspects of which are subject to significant uncertainty.…”