23 July 2021
Fossil pollen reveals Sahara’s climate history
Published online 7 June 2021
Study provides a different take on what led to the greening of the Sahara during the early Holocene, more than 10,000 years ago.
Fossilised pollen records from a permanent lake in the High Atlas Mountains of Morocco suggest that Mediterranean winter rainfall played an important role in sustaining plant life in the Sahara during the African Humid Period, an interval between 14,000 to 5,000 years ago when much of West and North Africa was wetter than today.
The Sahara was once covered by grasslands, trees and lakes. Piecing together scenarios that may have led to the ‘green Sahara’ period presents an ongoing challenge to climate modelling scientists. The prevailing view is that the greening was attributable to a change in the Earth’s tilt that led to an increase in solar radiation reaching the Northern Hemisphere during the summer, which in turn caused a strengthening and northward shift of the African monsoon. But simulations have shown that an amplified African monsoon alone would not have been enough to sustain a green Sahara.
A study by researchers from France, Morocco, Belgium and Germany calls into question the influence of the monsoon, and suggests that a southward shift of Mediterranean winter rainfall was crucial to the greening of the northern Sahara.
The team investigated a unique slice of history in the form of an 8.5-metre sediment core extracted from Lake Tislit in Morocco. The lake is situated at what earlier studies have shown as the upper latitudinal limit of the monsoon, making it “an ideal location for testing whether the monsoon hit the area or not,” explains corresponding author, Rachid Cheddadi, a palaeoecologist at the University of Montpellier, France.
By analysing pollen grains preserved in the sediment and using radiocarbon dating, the researchers were able to infer which species of trees, shrubs and herbs dominated the north Saharan landscape at different points in time over the past 18,500 years.
They identified 78 pollen types, of which 10 were aquatic. They found evidence of a notable rise of Mediterranean plant species such as Olea, Acer, Phillyrea and Quercus evergreen during the African Humid Period, which they say is indicative of higher rainfall in the winter rather than in the summer.
Drawing on an extensive database of plant distribution and their climatic ranges, they were able to reconstruct and quantify winter, spring and summer rainfall in Morocco over the 18,500-year period.
Their reconstruction showed that summer rainfall remained relatively unchanged over the whole period, implying that the monsoon did not influence the greening of the north Saharan region. In contrast, both winter and spring rainfall increased by around 30% between 14,000 and 9,500 years ago, and reached their peak between 10,500 and 8,500 years ago.
Cheddadi says that the increased winter rainfall may have descended on northern Africa as a response to lower amounts of solar radiation received by the Northern Hemisphere in the winter.
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,” the researchers say. They add that their findings will help inform future simulations of climate variability over northern Africa.
Cheddadi, R. et al. Early Holocene greening of the Sahara requires Mediterranean winter rainfall. PNAS https://doi.org/10.1073/pnas.2024898118 (2021).