When most people imagine an archetypal desert landscape—with its relentless sun, rippling sand and hidden oases—they often picture the Sahara. But 11,000 years ago, what we know today as the world’s largest hot desert would’ve been unrecognizable. The now-dessicated northern strip of Africa was once green and alive, pocked with lakes, rivers, grasslands and even forests. So where did all that water go?
Archaeologist David Wright has an idea: Maybe humans and their goats tipped the balance, kick-starting this dramatic ecological transformation. In a new study in the journal Frontiers in Earth Science, Wright set out to argue that humans could be the answer to a question that has plagued archaeologists and paleoecologists for years.
The Sahara has long been subject to periodic bouts of humidity and aridity. These fluctuations are caused by slight wobbles in the tilt of the Earth’s orbital axis, which in turn changes the angle at which solar radiation penetrates the atmosphere. At repeated intervals throughout Earth’s history, there’s been more energy pouring in from the sun during the West African monsoon season, and during those times—known as African Humid Periods—much more rain comes down over north Africa.
With more rain, the region gets more greenery and rivers and lakes. All this has been known for decades. But between 8,000 and 4,500 years ago, something strange happened: The transition from humid to dry happened far more rapidly in some areas than could be explained by the orbital precession alone, resulting in the Sahara Desert as we know it today. “Scientists usually call it ‘poor parameterization’ of the data,” Wright said by email. “Which is to say that we have no idea what we’re missing here—but something’s wrong.”
As Wright pored the archaeological and environmental data (mostly sediment cores and pollen records, all dated to the same time period), he noticed what seemed like a pattern. Wherever the archaeological record showed the presence of “pastoralists”—humans with their domesticated animals—there was a corresponding change in the types and variety of plants. It was as if, every time humans and their goats and cattle hopscotched across the grasslands, they had turned everything to scrub and desert in their wake.
Wright thinks this is exactly what happened. “By overgrazing the grasses, they were reducing the amount of atmospheric moisture—plants give off moisture, which produces clouds—and enhancing albedo,” Wright said. He suggests this may have triggered the end of the humid period more abruptly than can be explained by the orbital changes. These nomadic humans also may have used fire as a land management tool, which would have exacerbated the speed at which the desert took hold.
