New Insights into the Greening of the Sahara Desert

A groundbreaking study has provided new insights into the periodic wet phases in the Sahara Desert, explaining why the region was periodically green. Published in Nature Communications, the research demonstrates that changes in Earth’s orbit around the sun were responsible for the wet phases, which occurred over the past 800,000 years. These periods of greening were suppressed during ice ages. The study, conducted by climate scientists from the University of Helsinki and the University of Bristol, involved simulating the historic intervals of Sahara’s greening for the first time.

The findings reveal that the greening of the Sahara was influenced by the effects of large, distant, high-latitude ice sheets in the Northern Hemisphere. The research shows that North African Humid Periods, as these wet phases are known, played a crucial role in providing vegetated corridors out of Africa. These corridors allowed for the dispersal of various species, including early humans, across the world.

The study also highlights the importance of Earth’s orbital precession in driving these humid events. Earth’s precession, which refers to its wobbling on its axis, impacts the seasonality and controls the strength of the African Monsoon. This, in turn, determines the spread of vegetation across the Sahara Desert.

The research utilized a recently-developed climate model that successfully simulates the North African Humid Periods and provides a better understanding of their driving mechanisms. The results confirm that these periods occurred every 21,000 years and were influenced by changes in Earth’s orbital precession. Warmer summers in the Northern Hemisphere intensified the West African Monsoon system, resulting in increased Saharan precipitation and the spread of savannah-type vegetation.

However, the study reveals that the humid periods did not occur during ice ages when large glacial ice sheets covered high latitudes. These ice sheets cooled the atmosphere and suppressed the expansion of the African monsoon system, preventing the greening of the Sahara. The research emphasizes the teleconnection between distant regions, which may have restricted the dispersal of species, including humans, out of Africa during glacial periods over the past 800,000 years.

The study’s findings have significant implications for understanding past human distributions and the evolution of species in Africa. The ability to model North African Humid periods is a major achievement that enhances our understanding of the Sahara’s ecological significance as a gate controlling species dispersal in and out of the continent. Moving forward, this research will contribute to improved modeling of future climate change and its impact on the Sahara Desert.

Source: University of Bristol – Edward Armstrong et al., “North African humid periods over the past 800,000 years,” Nature Communications (2023). DOI: 10.1038/s41467-023-41219-4