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.
It’s important to note that the green Sahara always would’ve turned back into a desert even without humans doing anything—that’s just how Earth’s orbit works, says geologist Jessica Tierney, an associate professor of geoscience at the University of Arizona. Moreover, according to Tierney, we don’t necessarily need humans to explain the abruptness of the transition from green to desert.
Instead, the culprits might be regular old vegetation feedbacks and changes in the amount of dust. “At first you have this slow change in the Earth’s orbit,” Tierney explains. “As that’s happening, the West African monsoon is going to get a little bit weaker. Slowly you’ll degrade the landscape, switching from desert to vegetation. And then at some point you pass the tipping point where change accelerates.”
Tierney adds that it’s hard to know what triggered the cascade in the system, because everything is so closely intertwined. During the last humid period, the Sahara was filled with hunter-gatherers. As the orbit slowly changed and less rain fell, humans would have needed to domesticate animals, like cattle and goats, for sustenance. “It could be the climate was pushing people to herd cattle, or the overgrazing practices accelerated denudation [of foliage],” Tierney says.
Which came first? It’s hard to say with evidence we have now. “The question is: How do we test this hypothesis?” she says. “How do we isolate the climatically driven changes from the role of humans? It’s a bit of a chicken and an egg problem.” Wright, too, cautions that right now we have evidence only for correlation, not causation.
But Tierney is also intrigued by Wright’s research, and agrees with him that much more research needs to be done to answer these questions.
“We need to drill down into the dried-up lake beds that are scattered around the Sahara and look at the pollen and seed data and then match that to the archaeological datasets,” Wright said. “With enough correlations, we may be able to more definitively develop a theory of why the pace of climate change at the end of the AHP doesn’t match orbital timescales and is irregular across northern Africa.”
Tierney suggests researchers could use mathematical models that compare the impact hunter-gatherers would have on the environment versus that of pastoralists herding animals. For such models it would be necessary to have some idea of how many people lived in the Sahara at the time, but Tierney is sure there were more people in the region than there are today, excepting coastal urban areas.
While the shifts between a green Sahara and a desert do constitute a type of climate change, it’s important to understand that the mechanism differs from what we think of as anthropogenic (human-made) climate change today, which is largely driven by rising levels of CO2 and other greenhouse gases. Still, that doesn’t mean these studies can’t help us understand the impact humans are having on the environment now.
“It’s definitely important,” Tierney says. “Understanding the way those feedback (loops) work could improve our ability to predict changes for vulnerable arid and semi-arid regions.”
Wright sees an even broader message in this type of study. “Humans don’t exist in ecological vacuums,” he said. “We are a keystone species and, as such, we make massive impacts on the entire ecological complexion of the Earth. Some of these can be good for us, but some have really threatened the long-term sustainability of the Earth.”