Part one begins with the geology of the Sahara's underlying rocks and the origins of some of its larger-scale geomorphological features, created by "prolonged periods of uplift, erosion, submergence beneath ice caps, flooding from the sea, and widespread volcanic activity". The evidence for early human activity — "the hippo hunters of the Sahara" — comes from archaeology, rock art, and pollen.
"There were two main phases of prehistoric human occupation at Gobero. The first was between about 9700 and 8200 years ago... skeletons show that the people involved were very tall, and the chemical composition of their teeth indicates that they were more or less sedentary hunter-fishers. ...
Occupation came to an end when the lake dried out during a prolonged dry phase between 8200 and 7200 years ago, when there is no evidence of human occupation. ...
... a new group of people moved in to occupy the area around the lake between about 7200 and 4500 years ago. ... In addition to hunting and fishing for their food, they brought domestic cattle with them."
Part two is about sand and dust, and about wind and water. It explains why the Sahara is dry, looking at climatic patterns, atmospheric circulation, the drift into arid latitudes, and other changes. And it looks at where sand comes from, how sand dunes move, and the role of water in fashioning the landscape.
"It is all too easy to underestimate the role of water in fashioning the Saharan landscapes. The constant soughing of the wind, the seemingly unstoppable advance of active dunes, the burial of former river channels and lakebeds beneath a deep mantle of wind-blown sand, all testify to the power and presence of wind in the desert. . . It is safe to conclude that although there is a constant tug-of-war between wind and water in the present-day Sahara, the major elements of the Saharan landscape were carved from the rocks by running water long before the first wind-blown sand made an appearance. It was the now mostly defunct Saharan rivers that supplied the parent alluvial sands that were then fashioned into dunes as the desert winds grew stronger and the overall climate became more arid."
This also touches on the broader influence of Saharan dust, on the Amazon, Sweden, the Nile Valley, in marine sediments, and so forth.
Part three looks at the present Sahara. Droughts such as the 1968-73 Sahel Drought are driven by sea surface temperatures, the Indian Ocean, volcanic eruptions, and the Southern Oscillation, among other aspects of the broader climate. And Williams counters simplistic attributions of desertification to human action:
"Our answer to the question of whether humans caused the Sahara to become dry is a fairly resounding 'No!' Humans may have helped to aggravate the environmental impacts of the latest phase of climatic desiccation which set in between about 5000 and 4000 years ago, but this impact was quite localised and probably caused by overgrazing from herds of domestic cattle."
And he looks at how humans have adapted to living in the Sahara — ways of finding water, irrigation strategies, and adaptations to aridity and uncertainty — and at what the future might hold.
When the Sahara Was Green has excellent maps, with the most important repeated on the endpapers, sixteen pages of colour photographs, a very select eight book "further reading" list, notes, and an index. I found it both an easy read and highly informative, and recommend it to anyone interested in environmental history generally and not just to those curious about the Sahara specifically.
May 2025
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