Today, the Sahara Desert is defined by undulating sand dunes, unforgiving sun, and oppressive heat. But just 10,000 years ago, it was lush and verdant. So, what spurred the shift from woodland to wasteland?
“Goats are the prime suspects,” said Wright. “I've literally seen a goat eat a brick — they aren't picky eaters at all, and they eat a lot for their size. It wouldn't take many goats on a stressed out landscape to make a pretty big impact.”
If hungry, plant-eating mammals already dotted the Sahara, why did domesticated animals play such a transformative role? The answer is that wild animals don't like to spend a lot of time out in the open where they are easy targets for predators. However, livestock are happy to denude a field of grass under the watchful eye of a human guardian.
Herds of goat and others beasts exposed ground that was previously hidden beneath vegetation, with consequences for the local climate. You may have learned in school that brighter colors reflect more light, which is why it's more comfortable to wear a white shirt than a black shirt on a hot, sunny day. The Earth's surface works the same way. Tawny-colored dirt and sand reflect more sunlight than viridescent grass and brush.
“As that sunlight is reflected, the energy associated with the light goes back into the atmosphere, which heats it. In the tropics, a heated atmosphere tends to have fewer clouds than a cooler atmosphere,” said Wright. Fewer clouds mean less rain. That's what happened in the Sahara.
Wright believes that overgrazing led to drought. Drought stunted the growth vegetation, which further transformed the landscape, which worsened the drought, in a feedback loop that eventually produced a hot, dry, dusty desert roughly the size of the United States.
Scientists generally attribute the transformation of the Sahara to changes in the Earth's orbit, which deprived the tropics of sunlight, leading to a drop-off in summer rainfall. Wright says that human migration pushed the region to a tipping point. The landscape did not transform slowly and uniformly, as would be expected if variations in the Earth's orbit were the only factor. Rather, it changed in fits and starts, following the spread of livestock. Wherever humans went, they left scrubland in their wake.
Wright says his hypothesis still leaves plenty of unanswered questions. “We need to drill down into these former lake beds to get the vegetation records, look at the archaeology, and see what people were doing there,” said Wright. “It is very difficult to model the effect of vegetation on climate systems. It is our job as archaeologists and ecologists to go out and get the data, to help to make more sophisticated models.”
Several researchers interviewed for this story, however, cast doubt on Wright's explanation, including Jon Foley, climatologist and executive director of the California Academy of Sciences. Foley said the loss of vegetation across the Sahara, provoked by changes in the Earth's orbit, could explain the phenomena described in the study. Plants soak up moisture from the ground and sweat it through their leaves, adding water vapor to the atmosphere. When vegetation disappears, the atmosphere loses a key source of H2O, worsening drought.
Foley said Wright's research offers “a thought-provoking idea, worthy of more debate and study, but the current body of evidence does not prove the hypothesis.”
Wright noted this idea has a historical parallel. Humans transformed the climate in Asia and North America through grazing. And the loss of vegetation continues to affect the climate today. Scientists believe deforestation in the Amazon has fueled drought in the region, threatening further deforestation. This has global ramifications. The Amazon Rainforest soaks up huge quantities of heat-trapping carbon pollution. Its disappearance is accelerating global warming.
Wright hopes that we can learn from our ancestors' mistakes. “Assuming my scenario is true, it isn't like people 8,000 years ago were thinking that their goats were compromising the annual rainfall cycles. All of these effects were unintentional.”
“The past is a window into the future,” said Wright. “Once an ecological threshold is crossed, it is very difficult to restore it and rebalance the destructive potential of the event. And it matters.”