When the Sahara Was Green How Our Greatest Desert Came to Be

When seeing the world through a deep-time lens, no landscape feature is permanent. The Sahara, for example, "only" came into existence some 7 mya. In that time, it has not always been the parched desert it is now but has been green and verdant numerous times, crisscrossed by rivers and home to hippos, turtles, fish and other animals and plants typical of wetter climes. In this book, retired earth scientist Martin Williams draws on a long lifetime of research and desert expeditions to give a very accessible introduction to the surprisingly complex geography of the Sahara, answering some very basic questions.

I first came across the notion of the Sahara not being a permanent barrier to animal and plant migrations in Ancient Bones. That book mentions how both desert and savannah ecosystems were in flux over time. Williams here provides an introduction for the general reader to both the deep and recent history of the Sahara, explaining how it was able to support such vibrant life, when and how it dried out, and whether humans are to blame. Next to scientific literature spanning about a century, he draws heavily on his two more technical books with Cambridge University Press.

When the Sahara Was Green shines on the geography and geomorphology front, which are Williams's home turf. He explains the geological and climatological reasons why the Sahara has become a desert. The obvious reason is plate tectonics. North Africa moved into the latitudinal zone where large-scale atmospheric circulation patterns pretty much dictate a dry climate. Beyond that, however, many other particularities have enhanced its aridity over the last ~7 Ma. For example, the sheer size of the continent means inbound winds lose moisture before reaching its heart, the presence of mountains creates rain shadows, and more distant influences include overall global cooling in the last 33 Ma after Antarctica became an isolated continent.

And yet, as Williams has observed first-hand in the field, there were wetter intervals during this timespan. The reasons why these happened are not always necessarily clear, but the evidence is: fossil soils, lake sediments that are now literally weathering out of the landscape in inverted relief, infilled river channels showing up on satellite and radar images, vertebrate and invertebrate fossils, remnant populations of e.g. monkeys living in mountain sanctuaries, remains of human habitation such as cemeteries, rock art, and artefacts... these are some of the many independent lines of evidence discussed here. The resulting narrative jumps back and forth in time between deep time and the most recent twenty-thousand years when the Sahara went from wetter to drier.

Even if 80% of the Sahara consists of something else, there is no getting away from sand and thus sand, sand dunes, and sand storms all get a prominent look in. This shows the deep-time connections and surprising complexity of today's environment. Where does all this sand come from? Surprisingly, rivers that over millions of years dumped their sediment loads, especially as the Sahara started becoming drier and rivers did not reach the ocean anymore. But also from mountains with soft, easily eroded mantles. These are the legacy of being chemically weathered by rainforests that grew there tens of millions of years ago. And where does all this sand go? Sand, together with dust from other parts of the Sahara, blows all the way across the Atlantic. Because it is rich in organic compounds—leftovers from former lake beds and river sediments—it fertilises the Amazon rainforest. In fact, there are four main dust trajectories in operation over the Sahara today, blowing dust towards different continents.

When the Sahara Was Green is richly illustrated with a colour plate section, figures reproduced from books and papers, hand-drawn diagrams, and, my favourite, illustrated endpapers. Two maps show countries and names of the numerous landmarks discussed in the book. I referred to them often.

As much as the geography and geomorphology part of the book is interesting and well-informed, the book falls a bit short elsewhere. The section on human evolution is rather brief, focusing on discoveries in the Middle Awash Valley in Ethiopia (Australopithecus and Ardipithecus). Graecopithecus is only mentioned in the context of the Saharan desert dust deposit it was found in in Greece. This fossil discovery was described in Ancient Bones where the idea was floated that the drying up of the Mediterranean Sea allowed the migration of animals and plants between Africa and Europe. How climate interacted with human evolution is thus not really discussed. The chapter that mentions desert adaptations in plants, animals, and humans borders on the anecdotal, giving a select few examples only. And one of the questions that is prominently asked in the beginning—Will the Sahara become green again in the future?—is answered with a brief "yes, but not for a long time" (p. 180) without further explanation. We have a reasonable idea of the near-future course of the tectonic plates, so Williams's answer invites a new why-question that unfortunately goes unanswered.

Much of this book is factual and Williams only sparingly inserts personal anecdotes. Instead, he reserves his voice for a different question. Did humans cause the Sahara to dry up? His answer is a strident "no!". He calls it an "oddly myopic view" and "sadly deluded notion" (p. 143) and notably attacks the Ehrlichs, even though they allow for both human and climatic factors. Williams leans toward pinning it entirely on natural factors, "the Sahara is dry today for good and sufficient geographical reasons that have nothing to do with humans" (p. 144–145) but admits that there are recent examples where human actions have certainly not helped. My takeaway from this is that I think he is right where deep time is concerned (a similar argument was made in The Sloth Lemur's Song regarding humans and deforestation in Madagascar), though with our current population size it seems entirely within our power to make things worse. It is hard to disagree with his warnings that we need both long-term data to establish genuine trends (a high degree of climatic variation is entirely normal in the Sahara) and must reckon with local variability (the Sahara is simply too large to make sweeping generalisations).

Given Williams's deep well of knowledge, this book could have been bogged down by technicalities and jargon. Instead, When the Sahara Was Green is admirably accessible to a broad audience with only basic knowledge of geography and earth sciences. Furthermore, the book stands out for the numerous clear and well-designed illustrations that explain complex concepts.