A new study based on the analysis of African dust indicates that the Sahara Desert shifts between wet and dry climatic conditions every 20,000 years.
The pattern is synchronised with monsoon activity in the region and the changes in the Earth's tilt on its axis, the study claims.
Archaeologists have unearthed a large number of fossils and ancient rock paintings from the Sahara region in recent decades, suggesting that the region was once home to human settlements and a variety of plant and animal life.
However, conditions are completely different today, with the Sahara one of the most inhospitable places on Earth.
In recent years, several studies have tried to estimate the climatic conditions that would have prevailed in the Sahara region thousands of years ago. Many of these studies were based on an analysis of sediment cores recovered from the Atlantic Ocean, off the west coast of Africa.
Each year, millions of tonnes of dust from Sahara Desert is swept away by winds from the northeast. This dust, which is deposited in the form of sediments into the Atlantic Ocean, also carries with it remains of life forms, such as the shells of plankton.
Over a period of tens of thousands of years, multiple layers of sediments build up, which eventually serve as a geological clue to the climate history of the region.
Past studies of these sediment cores have indicated that the Sahara swings between dry and wet periods every 100,000 years - a pattern that is associated with the Earth's ice-age cycle, which is believed to repeat itself every 100,000 years.
Climate models, however, challenge this theory, suggesting that the climate of Sahara region must be driven by its monsoon season. According to scientists, the strength of monsoon season is determined by the periodic tilting of the Earth's axis, which also affects the distribution of the sunlight between seasons.
In the current study, researchers from the Massachusetts Institute of Technology examined sediment cores dug up off the West African coast and covering 240,000 years. For each layer of the sediment, the team measured the concentration of a specific isotope of thorium to determine how fast the dust accumulated on the seafloor.
The results revealed that the Sahara actually shifted between wet and dry climatic conditions every 20,000 years, which is in sync with the monsoon activity in the region and the periodic tilting of the Earth.
With periodic tilting, the Earth shifts from increased solar flux to less, and back again. An increase in sunlight during summers intensifies the monsoon activity in North Africa that, in turn, makes it a greener, wetter region. When the amount of solar flux decreases, it leads to a drier, parched climate in the Sahara region.
The findings of the study are published in journal Science Advances.
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