The Sahara Desert was once green and lush. Then the humans appeared.
Today, the Sahara Desert is defined by rolling sand dunes, merciless sunshine and oppressive heat. But barely 10,000 years ago, it was lush and verdant. So what spurred the shift from forest to wasteland?
A new study suggests humans have played a big role. Author David Wright, an environmental archaeologist at Seoul National University, says when humans spread west of the Nile 8,000 years ago, they brought sheep, cows and goats with them. which engulfed, mowed and trampled the native vegetation. This transformed the landscape and changed the local climate.
âGoats are the main suspects,â Wright said. âI literally saw a goat eat a brick – they’re not picky eaters at all, and they eat a lot for their size. It wouldn’t take many goats in a stressed landscape to have a big enough impact.
If hungry, herbivorous mammals already dot the Sahara, why have domestic animals played such a transformative role? The answer is that wild animals don’t like spending a lot of time in the open air where they are easy targets for predators. However, cattle are happy to strip a field of grass under the watchful eye of a human guardian.
Herds of goats and other animals exposed soil that was previously hidden under 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 one on a hot, sunny day. The surface of the Earth works the same way. The fawn-colored earth and sand reflect more sunlight than grass and fierce brush.
âAs the sunlight is reflected, the energy associated with the light returns to the atmosphere, which warms it. In the tropics, a heated atmosphere tends to have fewer clouds than a colder atmosphere, âWright said. Less clouds mean less rain. This is what happened in the Sahara.
Wright believes overgrazing led to drought. The drought stunted the growth of vegetation, which further transformed the landscape, exacerbating the drought, in a feedback loop that ultimately produced a hot, dry and dusty desert about the size of the United States. United.
Scientists generally attribute the transformation of the Sahara to changes in Earth’s orbit, which deprived the tropics of sunlight, resulting in reduced summer precipitation. Wright says human migration has pushed the region to a tipping point. The landscape did not change slowly and evenly, as one would expect if variations in Earth’s orbit were the only factor. On the contrary, it has changed in spurts, following the spread of cattle. Everywhere humans went, they left the scrubland in their wake.
Wright says his hypothesis still leaves many questions unanswered. âWe have to dig into these ancient lake beds to get the vegetation records, to look at the archeology and to see what people were doing there,â Wright said. âIt is very difficult to model the effect of vegetation on climate systems. It’s our job as archaeologists and environmentalists to get the data, to help create more sophisticated models. “
Several researchers interviewed for this story, however, questioned 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, caused by changes in Earth’s orbit, could explain the phenomena described in the study. Plants absorb moisture from the soil and transpire it through their leaves, adding water vapor to the atmosphere. When vegetation disappears, the atmosphere loses a vital water source, which exacerbates drought.
Foley said Wright’s research offers “a stimulating idea worthy of further debate and study, but the current body of evidence does not support the hypothesis.”
Wright noted that 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. It has global ramifications. The Amazon rainforest traps huge amounts of heat-trapping carbon pollution. Its disappearance accelerates global warming.
Wright hopes we can learn from the mistakes of our ancestors. âAssuming my scenario is true, it’s not as if people thought 8,000 years ago that their goats were compromising the annual cycles of precipitation. All of these effects were unintended.
âThe past is a window to the future,â Wright said. âOnce an ecological threshold is crossed, it is very difficult to restore it and rebalance the destructive potential of the event. And it matters. “
Jeremy Deaton writes for Nexus Media, an underwritten newswire covering climate, energy, politics, art and culture. You can follow it on @deaton_jeremy.