The Sahara is not simply a static desert. Scientists have shown that over long times this region cycled through phases of greening and moisture that repeat roughly every 21,000 years, driven by the Earth’s orbit. In this way the Sahara transformed cyclically into forest, savannah, and desert.
A recent Nature Communications study describes how these moist and green phases occur periodically. The cause lies in orbital changes around the Sun, with these phases being dampened during ice ages.
For the first time, climate scientists have simulated the Sahara’s historic greening intervals, measuring their duration and intensity and showing how they align with episodes of Northern Hemisphere glaciation.
Lead author Edward Armstrong, a climate scientist associated with the University of Helsinki and the University of Bristol, notes that the cyclical transformation of the Sahara into savannah and forest ecosystems stands as one of the planet’s most striking environmental shifts.
“This study represents one of the earliest climate-modeling efforts to reproduce African wet spells at magnitudes comparable to paleoclimatic observations and to explain when and why these events happen,” he adds.
A Sahara with its rivers and lakes
There is wide evidence that the Sahara was periodically lush in the past, sustained by an abundance of rivers, lakes, and water-dependent wildlife. Hippos and other animals thrived during these wet periods before the region became arid.
These wetter intervals in North Africa may have provided vegetated corridors that connected continents and allowed many species, including early humans, to disperse across the globe.
Along the Earth’s orbit
The greenings are believed to result from changes in Earth’s orbital geometry, especially orbital precession. Precession is the slow wobble of the planet around its axis, which shifts the timing and strength of seasons over cycles of about 21,000 years. These orbital shifts modulate how much energy the Northern Hemisphere receives in summer and winter, which in turn drives the strength of the African monsoon and the expansion of vegetation across this vast region.
A major challenge has been that many climate models struggle to replicate the extent of these wet spells, leaving the exact mechanisms uncertain.
A newly developed climate model was used in this study to simulate the North African rainy periods and to better understand the triggers behind them.
The results show that North Africa’s wet spells recur every 21,000 years as a consequence of orbital motion, with summers becoming hotter in the Northern Hemisphere. The intensified West African monsoon boosted Saharan rainfall and helped spread savannah-type vegetation across the desert.
Disruption during ice ages
The findings also indicate that these wet periods did not occur during ice ages when large ice sheets dominated high latitudes. The cooling atmosphere and the strengthened ice cover suppressed the widening tendency of the African monsoon system.
This underscores a key link between these remote regions and the distribution of species, including humans, during the last 800,000 years of glaciation.
Co-author Paul Valdés, a professor of Physical Geography, explains that traditional climate models have struggled to capture the full greening of the Sahara. The revised model successfully represents past changes and adds confidence in understanding future shifts.
Co-author Miikka Tallavaara, an associate professor of hominid environments, emphasizes that the Sahara has acted as a gateway shaping how species moved between northern and sub-Saharan Africa and beyond.
“The door was open when the Sahara was green and closed when deserts dominated. This alternation of wet and dry phases influenced the spread and evolution of species in Africa. Being able to model North African wet periods improves our understanding of human distributions and the evolution of our species on the continent.”
Reference work: Nature Communications article on the study of green Sahara cycles.
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Notes from the environmental science team emphasize that ongoing modeling efforts will continue to refine our picture of North Africa’s climate history and its links to global biological patterns.