What really turned the Sahara Desert from a green oasis to a desert? | Science
When most people imagine an archetypal desert landscape – with its relentless sun, rippling sand, and hidden oases – they often imagine the Sahara. But 11,000 years ago, what we know today as the world’s largest hot desert would have been unrecognizable. The now parched northern strip of Africa was once green and vibrant, dotted with lakes, rivers, meadows and even forests. Where has all this water gone?
Archaeologist David Wright has an idea: Maybe humans and their goats tipped the scales, kicking off this dramatic ecological transformation. In a new study from the journal Frontiers in Earth SciencesWright pledged to argue that humans could be the answer to a question that has plagued archaeologists and paleoecologists for years.
The Sahara has long been subjected to periodic episodes of humidity and aridity. These fluctuations are caused by slight oscillations in the tilt of the Earth’s orbital axis, which in turn change the angle at which solar radiation enters the atmosphere. At repeated intervals throughout Earth’s history, there has been more energy from the sun during the monsoon season in West Africa, and during these periods – known as periods African wetlands – much more rain falls over North Africa.
With more rain, the area gets more greenery, rivers and lakes. All of this has been known for decades. But between 8,000 and 4,500 years ago, something strange happened: the wet-to-dry transition occurred much faster in some areas than could be explained by orbital precession alone, culminating in the Sahara Desert as we know it today. âScientists generally call this ‘bad parameterization’ of the data,â Wright said in an email. “Which means we have no idea what we’re missing here, but something’s wrong.”
As Wright examined the archaeological and environmental data (mostly sediment cores and pollen records, all dated to the same period), he noticed what appeared to be a pattern. Wherever archaeological records showed the presence of “pastoralists” – humans with their pets – there was a corresponding change in the types and variety of plants. It was as if, every time humans, their goats, and their cattle hopped through the grasslands, they had turned everything into brush and desert in their wake.
Wright thinks that’s exactly what happened. âBy overgrazing the grasses, they reduced the amount of moisture in the air – the plants give off moisture, which produces clouds – and improved albedo,â Wright said. He suggests that this may have triggered the end of the wet period more abruptly than the orbital changes can explain. These nomadic humans may also have used fire as a tool for land management, which would have exacerbated the speed at which the desert took hold.
It is important to note that the Green Sahara would always have become a desert again even without humans doing anything. In addition, according to Tierney, we don’t necessarily need humans to explain the abruptness of the transition from green to desert.
Instead, the culprits could be regular feedbacks from old vegetation and changes in the amount of dust. âAt first you have this slow change in Earth’s orbit,â Tierney explains. âAs this happens, the West African monsoon will weaken a bit. Slowly you will degrade the landscape, moving from desert to vegetation. And then at some point you pass the tipping point where change is accelerating.
Tierney adds that it’s hard to know what triggered the waterfall in the system because everything is so closely linked. During the last wet period, the Sahara was populated by hunter-gatherers. As the orbit slowly changed and less rain fell, humans would have had to domesticate animals, like cattle and goats, for food. “It could be the climate pushing people to keep livestock, or overgrazing practices have accelerated denudation [of foliage]Tierney says.
Which one came first? It’s hard to say with the evidence we have now. âThe question is: how to test this hypothesis? she says. âHow do we isolate climate change from the role of humans? It’s a bit of a chicken and egg problem. Wright also cautions that at present we only have evidence of correlation, not causation.
But Tierney is also intrigued by Wright’s research and agrees with him that a lot more research is needed to answer these questions.
“We have to dig into the dry lake beds that are scattered across the Sahara and look at the pollen and seed data and then match them to the archaeological data sets,” Wright said. “With enough correlations we may be able to more definitively develop a theory as to why the rate of climate change at the end of AHP does not match orbital timescales and is irregular across North Africa. . “
Tierney suggests that researchers could use mathematical models that compare the impact hunter-gatherers would have on the environment versus that of pastoralists raising animals. For such models it would be necessary to have some idea of ââhow many people were living in the Sahara at the time, but Tierney is sure there were more people in the area than there are today. ‘hui, with the exception of coastal urban areas.
Although the changes between a green Sahara and a desert are a type of climate change, it is important to understand that the mechanism differs from what we now consider anthropogenic (man-made) climate change, which is largely due to increased levels of CO2 and other greenhouse gases. Still, that doesn’t mean these studies can’t help us understand the impact humans have on the environment today.
âIt’s really important,â Tierney says. âUnderstanding how these feedbacks (loops) work could improve our ability to predict changes for vulnerable arid and semi-arid regions. “
Wright sees an even larger message in this type of study. âHumans don’t exist in ecological voids,â he said. âWe are a key species and as such we have massive impacts on the entire ecological complexion of the Earth. Some of them may be good for us, but some have really threatened the long term sustainability of the Earth. ”