What Really Turned the Sahara Desert From a Green Oasis Into a Wasteland?
//Summary - Level-C2//
The Sahara Desert was once a lush, green landscape but transformed dramatically around 11,000 years ago. Recent studies, like those by archaeologist David Wright, suggest that human activity, particularly overgrazing by domesticated animals, may have expedited this transformation by affecting plant diversity and atmospheric moisture. However, geologist Jessica Tierney argues that natural cycles related to Earth's orbital changes primarily drive these transitions, with human impact still debatable. Both agree more research is needed to clarify these influences.
A)
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Most people picture the Sahara when thinking of an archetypal desert landscape—its relentless sun, rippling sands, and hidden oases. But 11,000 years ago, what we now call the world's largest hot desert would have been unrecognisable. The desertified northern strip of Africa was once green and vibrant, dotted with lakes, rivers, grasslands, and even forests. So, where has all the water gone?
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Archaeologist David Wright has an idea: Maybe humans and their goats tipped the balance and triggered this dramatic ecological change. In a new study in Frontiers in Earth Science, Wright argues that humans could answer a question that has puzzled archaeologists and palaeoecologists for years.
B)
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The Sahara has long been subject to periodic periods of wetness and dryness. These fluctuations are caused by slight wobbles in the tilt of the Earth's orbital axis, changing the angle at which the sun's rays enter the atmosphere. Throughout the Earth's history, more energy from the sun has been emitted during the West African monsoon season, and during these periods—known as the African Humid Periods—much more rain falls over North Africa.
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With more rain comes more greenery, rivers and lakes. All this has been known for decades. But something strange happened between 8,000 and 4,500 years ago: The transition from wet to dry happened much faster in some areas than could be explained by orbital precession alone, resulting in the Sahara as we know it today. "Scientists usually call this 'poor parameterisation' of the data," Wright said by email. "Which means we have no idea what we're missing here - but something is wrong."
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As Wright pored over the archaeological and environmental data (mostly sediment cores and pollen records, all dated to the same period), he noticed what seemed to be a pattern. Wherever the archaeological record showed the presence of 'pastoralists' - people with their domesticated animals - there was a corresponding change in the types and diversity of plants. It was as if, each time humans and their goats and cattle moved across the grasslands, they had transformed everything in their wake into scrubland and desert.
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Wright believes that is precisely what happened. "By overgrazing the grasses, they reduced the amount of atmospheric moisture - plants give off the moisture that creates clouds - and increased the albedo," Wright said. He suggests that this may have triggered the end of the wet period more abruptly than can be explained by orbital changes. These nomadic people may also have used fire as a land management tool, which would have exacerbated the rate at which the desert took hold.
C)
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It's important to note that the green Sahara would have always turned back into a desert even if humans hadn't done anything - that's how the Earth's orbit works, says geologist Jessica Tierney, an associate professor of geosciences at the University of Arizona. Besides, Tierney says, we don't necessarily need humans to explain the abruptness of the transition from green to desert.
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Instead, the culprits could be regular old vegetation feedback and changes in the amount of dust. "First, you have this slow change in the Earth's orbit," Tierney explains. "As that happens, the West African monsoon gets a little weaker. Slowly, you're going to degrade the landscape from desert to vegetation. And then, at some point, you pass the tipping point where the change accelerates.
9)
Tierney adds that it's hard to know what triggered the cascade in the system because everything is so tightly interwoven. During the last wet period, the Sahara was populated by hunter-gatherers. As the orbit slowly changed and less rain fell, people would have had to domesticate animals such as cattle and goats for food. "It could be that the climate drove people to herd livestock or that the overgrazing practices accelerated the denudation [of foliage]," says Tierney.
10)
Which came first? It's hard to say with the evidence we have now. "The question is, how do we test this hypothesis?" she says. "How do we isolate the climate-driven changes from the human role? It's a bit of a chicken and egg problem." Wright also warns that we only have evidence of correlation, not causation.
However, Tierney is also intrigued by Wright's research and agrees that much more research is needed to answer these questions.
D)
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"We need to go down to the dried-up lake beds scattered around the Sahara, look at the pollen and seed data, and then correlate that with the archaeological records," Wright said. With enough correlations, we might be able to develop a more definitive theory as to why the pace of climate change at the end of the AHP doesn't match orbital timescales and is irregular across northern Africa.
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Tierney suggests that researchers could use mathematical models that compare the impact of hunter-gatherers on the environment with that of herders. Such models would require some idea of how many people lived in the Sahara then, but Tierney is sure there were more people in the region than there are today, except in coastal urban areas.
13)
While the shifts between a green Sahara and a desert are a form of climate change, it's essential to understand that the mechanism differs from what we now think of as anthropogenic (artificial) climate change, primarily driven by rising CO2 and other greenhouse gases. But that doesn't mean these studies can't help us understand humans' impact on the environment right now.
"It's important," says Tierney. "Understanding how these feedback (loops) work could improve our ability to predict changes for vulnerable arid and semi-arid regions."
F)
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Wright sees an even broader message in this kind of study. "Humans don't exist in an ecological vacuum," he said. "We are a keystone species, and as such, we have massive impacts on the whole ecological complexion of the Earth. Some of these can be good for us, but some have threatened the long-term sustainability of the Earth."
What Really Turned the Sahara Desert From a Green Oasis Into a Wasteland?
10,000 years ago, this iconic desert was unrecognisable. A new hypothesis suggests that humans may have tipped the balance
// New words//
Archetypal: Representing a perfect or typical example of something.
The archetypal hero in many stories is brave and selfless.
Rippling sand: Sand that has wavy patterns or small waves on its surface.
The rippling sand at the beach created beautiful patterns under the setting sun.
Unrecognisable: Not able to be recognised or identified.
After the renovation, the old house was utterly unrecognisable.
Now-dessicated: Currently dried up.
The Now-dessicated lakebed is a stark reminder of the area's changing climate.
Archaeologist: A person who studies human history by excavating and analysing artefacts.
The archaeologist carefully excavated ancient pottery from the site.
Plagued: Continuously troubled or burdened by something.
A series of unexplained events plagued the village.
Paleoecologists: Scientists who study the ecosystems of the past.
Paleoecologists can deduce climate conditions from fossilised pollen.
Periodic bouts: Occurring at intervals, typically describing symptoms or occurrences.
She suffered periodic bouts of insomnia that affected her daily activities.
Humidity: The amount of moisture in the air.
The humidity in the air made the heat feel even more oppressive.
Aridity: The quality of being extremely dry.
The aridity of the desert landscape is inhospitable to most forms of life.
Wobbles: Lurches from side to side.
The top wobbles as it spins, slowly losing momentum.
The Earth's oEarth'saxis: The imaginary line around which Earth rotEarth
The tilt of the Earth's axis affects the severity of seasons.
Radiation: Energy that is emitted in the form of rays, waves, or
particles.
Ultraviolet radiation from the sun can damage the skin.
Penetrates: Passes into or through something.
The sunlight penetrates the water, illuminating the depths.
Monsoon: A seasonal prevailing wind which brings heavy rains.
The annual monsoon rains are vital for farming in the region.
Orbital precession alone: Refers solely to the gradual shift in the orientation of an astronomical body's orbital path.
The orbital precession alone can influence long-term climate changes.
Sediment cores: Cylindrical samples of Earth taken from the ground or the seabed to study its properties.
Scientists analysed sediment cores to understand historical climate conditions.
Pollen records: Collections of pollen samples used to study past climates and environments.
Pollen records can tell us a lot about the types of vegetation that existed thousands of years ago.
Pastoralists: People who raise livestock and often move them from place to place.
Pastoralists move their herds to new grazing lands as the seasons change.
Corresponding: Associated with or related to.
The increase in sales had a corresponding rise in profits.
Hopscotched: Moved quickly and intermittently from one place to another.
She hopscotched across the country, stopping in small towns along the way.
Overgrazing: Grazing land too heavily or frequently so the vegetation does not recover.
Overgrazing by livestock can lead to soil erosion and desertification.
Enhancing albedo: Increasing the reflective power of a surface.
Enhancing albedo in urban areas can help reduce heat absorption.
Abruptly: Suddenly and unexpectedly.
The meeting ended abruptly when the power went out.
Exacerbated: Made the problem worse.
The delay was exacerbated by bad weather.
Abruptness: The quality of being sudden or steep.
The abruptness of the cliff face made it a challenge for climbers.
Culprits: The causes of a problem or the persons responsible for it.
Investigators identified the broken rail as one of the culprits in the train accident.
Vegetation: Plant life or total plant cover.
The dense vegetation in the jungle can be nearly impenetrable.
Intertwined: Twisted together or closely connected.
Their lives were deeply intertwined after years of friendship.
Hypothesis: A proposed explanation based on limited evidence intended to be tested through further investigation.
She hypothesised that increased sunlight would speed up the growth of the plants.
Correlation: A mutual relationship or connection between two or more things.
There is a strong correlation between exercise and health benefits.
Causation: The action of causing something.
The study aimed to prove causation, not just correlation, between diet and disease.
Scattered: Randomly dispersed or spread out over a wide area.
The farmer scattered seeds across the field.
Constitute: To make up or form something.
Small businesses constitute a large portion of the local economy.
Semi-arid: Characterised by light rainfall, usually between 10 and 20 inches a year.
The semi-arid climate supports grasslands and shrublands.
Vulnerable: Exposed to the possibility of being attacked or harmed.
Coastal cities are particularly vulnerable to hurricanes.
Regions: Areas or divisions, especially parts of a country or the world, with definable characteristics but not permanently fixed boundaries.
The wine regions of France are renowned for their fine production.
Complexion: the natural colour, texture, and appearance of a person's skin, especially the face.
Her complexion brightened with health after her vacation.
Add info No1)
The world's largest desert, the Sahara Desert, was once full of greenery.
https://gigazine.net/news/20190903-sahara-desert-green-landscape/
The Sahara Desert, which spreads across the northeastern part of the African continent, is known as the world's largest desert, excluding Antarctica. Still, several thousand years ago, the Sahara Desert had a completely different landscape than the 21st century.
The era when lakes and rivers existed in the Sahara was also referred to as the "Green Sahara" (African Wet Period), and the wet period peaked between 9,000 and 6,000 years ago. It is said that the Sahara, a land of rolling grasslands, also had sparse forests where acacia trees and other trees grew.
However, civilisation in the "Green Sahara," which was full of rich nature, ended abruptly. The Earth's precession, which occurs every 25,800 years, causes the Earth's axis to rotate, and about 5,000 years ago, the area of rainfall moved south of the Sahara, causing the Sahara to begin to dry out.
Add info No2)
The Sahara desert was a lush place. Cows appear in rock art from 4,000 years ago
https://karapaia.com/archives/52331684.html
Add info No3)
Earth's precession - Wikipedia
The Earth's axis of rotation also moves like the grinding motion of the spinning top mentioned above. This is called the precession of the Earth.
A spinning top in a place with gravity moves this way due to gravity's influence. Of course, the Earth is not supported by an axis on a flat surface like a spinning top, so it is simply "for the same reason as a spinning top." It is wrong to explain that.
The Earth's shape is a spheroid (oblate sphere) with a slightly bulging part at the equator. The tidal force due to the gravity of the sun and moon causes the bulge at the equator to align with the ecliptic plane. The reason for this is the torque being applied.
Precession is a phenomenon in which the Earth's 23.5-degree tilt changes gradually until it tilts to the opposite side after 13,000 years, causing the seasons to change.
The precession of the Earth causes the vernal equinox to move 50.3 arc seconds westward on the ecliptic every year. It moves by 1 degree (1 day's worth) in about 72 years and 30 degrees in about 2150 years.
The tilt of the equator changes relative to the ecliptic, and the vernal equinox moves: the winter solstice, spring equinox, summer solstice, and autumn equinox move.
In 13,000 years, the autumnal equinox will be at the current spring location, the winter solstice will be at the summer location, the vernal equinox will be at the autumn location, and the summer solstice will be at the winter location.
