The Sahara Desert is often regarded as an unforgiving, desolate landscape that spans across North Africa, characterized by its harsh rocks and towering sand dunes. However, evidence from ancient rock paintings and fossils indicates that it was once a thriving oasis where human settlements and diverse flora and fauna flourished.
A recent study by MIT researchers supports this notion, revealing that the Sahara and North Africa have experienced alternating wet and dry climates every 20,000 years. This climatic "pendulum" is primarily driven by the Earth's axial tilt as it orbits the sun, which in turn influences the distribution of sunlight between seasons.
When the Earth is tilted to receive maximum summer sunlight during its orbit, monsoon activity in North Africa intensifies, transforming the Sahara into a wetter, greener environment. Conversely, when the axial tilt decreases incoming sunlight, monsoon activity weakens, resulting in the arid climate we observe today.
David McGee, an associate professor in MIT's Department of Earth, Atmospheric, and Planetary Sciences, explains, "Our results suggest that North African climate history is dominated by this 20,000-year rhythm, alternating between a green and dry Sahara." This knowledge can help determine optimal timescales for human colonization of the Sahara Desert.
The study's methodology involved analyzing sediment cores extracted from the ocean floor off West Africa's coast. These cores contain layers of ancient sediments deposited over millions of years, with each layer potentially containing Saharan dust and remnants of life forms, such as microscopic plankton shells.
Led by first author Charlotte Skonieczny, the researchers examined sediment layers from the past 240,000 years, measuring the concentration of a rare thorium isotope to establish the rate of dust accumulation on the seafloor. Thorium, produced at a constant rate in the ocean from trace amounts of radioactive uranium, can be used to determine the rate of sediment accumulation.
The findings demonstrated that the Sahara experienced wet and dry climatic shifts every 20,000 years, in sync with monsoon activity and the Earth's periodic axial tilt. This groundbreaking discovery sheds light on the Sahara's dynamic environmental history and provides valuable insights into potential opportunities for human habitation in the future.
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