Sahara Desert Was Once Lush and Populated
[h=1]Sahara Desert Was Once Lush and Populated[/h]By Bjorn Carey | July 20, 2006 10:07am ET
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At the end of the last Ice Age, the Sahara Desert was just as dry and uninviting as it is today. But sandwiched between two periods of extreme dryness were a few millennia of plentiful rainfall and lush vegetation.
During these few thousand years, prehistoric humans left the congested Nile Valley and established settlements around rain pools, green valleys, and rivers.
The ancient climate shift and its effects are detailed in the July 21 issue of the journal Science.
Some 12,000 years ago, the only place to live along the eastern Sahara Desert was the Nile Valley. Being so crowded, prime real estate in the Nile Valley was difficult to come by. Disputes over land were often settled with the fist, as evidenced by the cemetery of Jebel Sahaba where many of the buried individuals had died a violent death.
But around 10,500 years ago, a sudden burst of monsoon rains over the vast desert transformed the region into habitable land.
This opened the door for humans to move into the area, as evidenced by the researcher's 500 new radiocarbon dates of human and animal remains from more than 150 excavation sites.
"The climate change at [10,500 years ago] which turned most of the [3.8 million square mile] large Sahara into a savannah-type environment happened within a few hundred years only, certainly within less than 500 years," said study team member Stefan Kroepelin of the University of Cologne in Germany.
Frolicking in pools
In the Egyptian Sahara, semi-arid conditions allowed for grasses and shrubs to grow, with some trees sprouting in valleys and near groundwater sources. The vegetation and small, episodic rain pools enticed animals well adapted to dry conditions, such as giraffes, to enter the area as well.
Humans also frolicked in the rain pools, as depicted in rock art from Southwest Egypt.
In the more southern Sudanese Sahara, lush vegetation, hearty trees, and permanent freshwater lakes persisted over millennia. There were even large rivers, such as the Wadi Howar, once the largest tributary to the Nile from the Sahara.
"Wildlife included very demanding species such as elephants, rhinos, hippos, crocodiles, and more than 30 species of fish up to 2 meters (6 feet) big," Kroepelin told LiveScience.
A timeline of Sahara occupation [See Map]:
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[*]22,000 to 10,500 years ago: The Sahara was devoid of any human occupation outside the Nile Valley and extended 250 miles further south than it does today.
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[*]10,500 to 9,000 years ago: Monsoon rains begin sweeping into the Sahara, transforming the region into a habitable area swiftly settled by Nile Valley dwellers.
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[*]9,000 to 7,300 years ago: Continued rains, vegetation growth, and animal migrations lead to well established human settlements, including the introduction of domesticated livestock such as sheep and goats.
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[*]7,300 to 5,500 years ago: Retreating monsoonal rains initiate desiccation in the Egyptian Sahara, prompting humans to move to remaining habitable niches in Sudanese Sahara. The end of the rains and return of desert conditions throughout the Sahara after 5,500 coincides with population return to the Nile Valley and the beginning of pharaonic society.
Camel caravan crosses the Sahara desert in Morocco. Image credit: Bachmont / CC BY 2.0.
The Sahara Desert is often cited as the world’s largest desert. This is not entirely true, however, as it is only the third largest desert after Antarctica and the Arctic and the world’s largest hot desert.
At over 9,400,000 km2, it covers nearly 10 percent of the African continent, including Algeria, Chad, Egypt, Libya, Mali, Mauritania, Morocco, Niger, Sudan and Tunisia.
Geological evidence suggests that the desert is no more than 2 to 3 million years old.
However, a recent discovery of 7-million-year-old sand dune deposits in the northern Chad Basin has hinted at a much older age.
To unravel the Sahara’s mystery, Dr Contoux and his colleagues explored paleoclimate of North Africa over the past 30 million years using the Norwegian Earth System Model and the Community Atmosphere Model.
They identified the Tortonian stage (7-11 million years ago) of the Late Miocene epoch as the pivotal period for triggering North African aridity and creating the desert.
“The region undergoes aridification with the shrinkage of the Tethys – a giant ocean that was the origin of the modern Mediterranean, Black and Caspian Seas – during the Tortonian,” the scientists explained.
“The simulations are the first to show that the Tethys shrinkage has two main consequences for North African climate.”
“First, it weakens the African summer monsoon circulations and dries out North Africa. Second, it enhances the sensitivity of the African summer monsoon and its associated rainfall to orbital forcing.”
“The Tortonian stage thus marks the time when North Africa shifted from a permanently lush, vegetated landscape to a landscape experiencing arid and humid cycles on orbital time scales.”
The results were published on September 18, 2014 in the journal Nature.
Zhongshi Zhang et al. 2014. Aridification of the Sahara desert caused by Tethys Sea shrinkage during the Late Miocene. Nature 513, 401–404; doi: 10.1038/nature13705
This is one area that needs further investigation. And why I say some of us are too fixated on Ancient Kemet. We may find older records of our people under the Sahara that rival or exceed what we think about Ancient Kemet.
Check out the video. It goes into some of this towards the end.
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THE SAHARA and Arabia were transformed abruptly from fertile land covered with shrubs and grasses into a parched desert in a "brutal" period of climatic change lasting 400 years, scientists have found.For once, humans were not to blame: the cause was not farming or overgrazing, as is usually thought. Scientists reckon that the Sahara began to turn into a desert after the Earth underwent one of its periodic changes in orientation, starting 9,000 and finishing about 6,000 years ago. Its tilt lessened from 24.14 degrees off vertical to its present 23.45 degrees, while the time when the planet is closest to the Sun shifted gradually from July to January.
Nobody knows what triggered the changes, but geologists think they were probably caused by shifts of material deep inside the Earth's molten core. They altered the pattern of sunshine on the Earth, with profound effects on many weather systems. The discovery has implications today, as many climate researchers are worried that gradual changes in the Earth's temperature caused by global warming will have marked effects on ocean currents - particularly those that warm Britain.
Before the change, the northern hemisphere received more summer sunlight, which amplified summer monsoons. But once the change was over, the new conditions created a vicious feedback loop between vegetation and climate. As the African monsoon lessened, plants began dying. As they stopped retaining water and releasing it back into the atmosphere, the rains lessened further, until rivers and streams dried up. The Sahara Desert now covers 3.5 million square miles.
The discovery emerged from a new computerised climate model made by a team of scientists at the Potsdam Institute for Climate Change.
"Our simulations show how interactions between vegetation, atmosphere and ocean current can lead to relatively abrupt climate changes - a process that might influence climate in future, too," said Martin Claussen, the team's leader.
"It was very severe, ruining ancient civilisations and socio-economic systems," he added. In the Sahara, "we find an abrupt decrease in vegetation, from a green Sahara to a desert shrubland within a few hundred years".
The ancient civilisations that had lived there - and left rock paintings - may have been forced to migrate to the more fertile Nile valley and other river valleys, where great civilisations developed.
Professor Claussen noted that changes in the Earth's orbit and tilt will continue to occur. As to their effects, he said: "What will happen in the future, frankly, we can only speculate."
How Earth's Orbit Shaped the Sahara
By Anuradha K. Herath, Astrobiology Magazine | December 20, 2010 06:08pm ET
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The Sahara, the world's largest desert, was once fertile grassland. This fact has been common knowledge in the scientific community for some time, but scientists are still grappling with historic data to determine whether that transition took place abruptly or gradually.At the European Geosciences Union General Assembly held in Vienna, Austria earlier this year, researchers presented new evidence showing that the eastern region of the Sahara desert, particularly the area near Lake Yoa in Chad, dried up slowly and progressively since the mid-Holocene period.
"The findings of this study are that the sedimentological and geochemical properties of the lake sediments confirm that the Sahara has been drying slowly from six thousand years ago to reach the present day conditions around 1,100 years ago," said lead author Pierre Francus, professor at the National Institute of Scientific Research in Quebec, Canada.
In this latest study, researchers analyzed the sedimentation in Lake Yoa on a yearly basis and dated it to determine when and how the Sahara region dried-up. Other studies have used climate modeling to determine the time period that the Sahara went dry and the prevailing climate conditions at that time.
The widely-held belief is that the Sahara dried up due to a change in the Earth's orbit, which affects solar insolation, or the amount of electromagnetic energy the Earth receives from the sun. In simpler terms, insolation refers to the amount of sunlight shining down on a particular area at a certain time, and depends on factors such as the geographic location, time of day, season, landscape and local weather.
Climate scientist Gavin Schmidt, of NASA's Goddard Institute for Space Studies, explained that around 8,000 years ago, the Earth's orbit was slightly different to how it is today. The tilt changed from around 24.1 degrees to the present-day 23.5 degrees.
"Additionally, the Earth had its closest approach to the sun in the northern hemisphere (with) summer in August," Schmidt said. "Today, that closest approach is in January. So, summertime in the north was warmer back then than it is now."
The changes in the Earth's orbital tilt and precession (or the wobbling motion) occur because of gravitational forces emanating from other bodies in the solar system. To understand exactly what happens, picture a spinning top when it is slightly disturbed. Just like a top, the Earth too wobbles slightly about its rotational axis. This tilt changes between roughly 22 and 25 degrees about every 41,000 years, while the precession varies on about a 26,000-year period. These cycles have been determined by astronomers and validated by geologists studying ocean sediment records.
"If you get a long enough time series that can be well dated, you should be able to see frequencies in the data that correspond to the periods predicted by theory," Schmidt explained.
For a long time, the belief was that the Earth's tilt would change only insignificantly in the next century. However, recent research is suggesting that the effects of global warming?particularly the oceans?could cause a change in the Earth's axial tilt. Scientists from NASA's Jet Propulsion Laboratory say that the current melting of ice in Greenland is already causing the tilt to change at a rate of approximately 2.6 centimeters each year. They predict that his change could increase in the years ahead.
The changes in insolation caused by shifts in axial tilt have an impact on atmospheric weather patterns such as monsoons. Thousands of years ago when the northern hemisphere received more sunlight, it also intensified the monsoons. After the Earth's tilt changed, the monsoons decreased and the vegetation began to disappear. When there were no plants to retain water and release it back into the atmosphere, the rain progressively decreased. The resulting feedback loop between plant life and climate eventually created the current desert conditions.
There is now considerable evidence to show that the Sahara used to have a grassland ecosystem and was a much wetter place than it is now. However, the debate about how that transition occurred continues. The disagreement among scientists is in part due to the lack of paleo-environmental records from the region. Therefore, scientists must often resort to climate modeling.
In 1999, German scientists used computer simulations to model the Earth's climate thousands of years ago. They concluded that the climatic transition of the Sahara took place abruptly, within a possible span of about 300 years.
Nearly ten years later, another group of scientists studied the environmental changes in the northern Chad area during the past 6,000 years and came to the conclusion that the Sahara underwent a more progressive drying-up process.
Schmidt belongs to the group of scientists who think there is evidence for sudden changes in the Sahara.
"Given the very strong dependence of vegetation on water availability, the end of the 'Green Sahara' came about quite suddenly around 5,500 years ago," Schmidt said. "Thus, a very slow change in the orbit (led) to an abrupt collapse in that ecosystem."
Since the Sahara spans a massive area?covering nearly a third of the African continent?it is quite possible that parts of it dried up abruptly while it took other regions a longer period of time to transform into a desert.
"It seems that drying was progressive in our area, but it does not automatically mean that it was the case in other areas such as Western Sahara," said Francus. "We cannot rule out completely the possibility of abrupt drying. Understanding the regional differences in climate change is the next challenge for climate scientists."
Francus explained that abrupt climate changes have been documented in many places on Earth at various times in the past. One example he cited is the Younger Dryas, one of the most famous examples of abrupt climate change that occurred between (approximately) 12,800 and 11,500 years ago. According to the National Oceanic and Atmospheric Administration, the end of this period was particularly abrupt when, for example, in Greenland, temperatures increased 18 degrees Fahrenheit in about a decade.
"Many scientists think that abrupt climate changes are possible in the future, but the nature, direction and intensity of these changes will most probably be region-dependent," Francus said.
Francus also noted that there are some models that cannot predict an abrupt climate shift at all. Some scientists feel that there is not enough knowledge to understand the processes driving these changes primarily because it is difficult to model the soil moisture and cover.
Regardless of whether the Saraha dried up gradually or suddenly, most scientists agree that it is important to understand how the climate changed in the past and what kinds of natural forces affected those changes. That will help climate researchers determine the precise role human behavior plays on current climate change.
"The models that are used to predict future climate need to be tested, and using information from the past is one way to achieve this goal," Francus said.
Related to our chapter 12 study group discussion