Rainfall in a Green Sahara

During the Green Sahara-period (11,000 to 5000 years BP), the Sahara desert received high amounts of rainfall, supporting diverse vegetation, permanent lakes and human populations of hunter-gatherers.

New research indicates that the Green Sahara extended to 31°N and likely ended abruptly[1]. Researchers found evidence for a thousand-year dry period some 8000 years ago, coincident with a temporary abandonment of occupational sites by Neolithic humans.

The rainfall rates inferred from that data are best explained by strong vegetation and dust feedbacks; without these mechanisms, climate models systematically fail to reproduce the Green Sahara. “It was 10 times as wet as today,” said lead author Jessica Tierney. Annual rainfall in the Sahara is now less than 35 mm in some places. Although other research had already identified the existence of the Green Sahara period, Tierney and her colleagues were the first to compile a continuous record of the region’s rainfall going 25,000 years into the past.

“What’s interesting is the people who came back after the dry period were different – most raised cattle. That dry period separates two different cultures. Our record provides a climate context for this change in occupation and lifestyle in the western Sahara.”

Tierney and her colleagues used cores of marine sediments taken off the coast of West Africa at four different sites. Because the cores were taken over a north-south distance of about 1,300 km – from offshore Cape Ghir (Morocco) to the northwest corner of Mauritania – the cores revealed both the ancient rainfall patterns and the areal extent of the Green Sahara.

By analyzing the leaf wax from ancient marine sediments, they determined the region’s past rainfall patterns and also gathered clues about what types of plants were growing.

The amount of solar radiation the Earth receives during the Northern Hemisphere summer depends on where the Earth’s precession is in its 23,000-year cycle. At the beginning of the Green Sahara, the Northern Hemisphere was closer to the sun during summer. Warmer summers strengthened the West African monsoon and delivered more rain. Toward the end of the Green Sahara, the Northern Hemisphere was farther from the sun and the West African monsoon was weaker.

[1] Tierney et al: Rainfall regimes of the Green Sahara in Science Advances – 2017