The Sahara Desert Used To Be a Green Savannah and New Research Explains Why
- December 28, 2023
- Posted by: OptimizeIAS Team
- Category: DPN Topics
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The Sahara Desert Used To Be a Green Savannah and New Research Explains Why
Subject : Environment
Section: Biodiversity
Context:
- Algeria’s TassiliN’Ajjer plateau is Africa’s largest national park.
- Over 15,000 etchings and paintings are exhibited there, some as much as 11,000 years old according to scientific dating techniques, representing a unique ethnological and climatological record of the region.
The Green Sahara or North African Humid Period:
- It was a period approximately 6,000-11,000 years ago.
- There is widespread climatological evidence that during this period the Sahara supported wooded savannah ecosystems and numerous rivers and lakes in what are now Libya, Niger, Chad and Mali.
- Using marine and lake sediments, scientists have identified over 230 of these greenings occurring about every 21,000 years over the past eight million years.
- These greening events provided vegetated corridors which influenced species’ distribution and evolution, including the out-of-Africa migrations of ancient humans.
- These dramatic greenings would have required a large-scale reorganisation of the atmospheric system to bring rain to this hyper-arid region.
Why has North Africa greened approximately every 21,000 years over the past eight million years?
- It was caused by changes in the Earth’s orbital precession – the slight wobbling of the planet while rotating. This moves the Northern Hemisphere closer to the sun during the summer months.
- This caused warmer summers in the Northern Hemisphere, and warmer air is able to hold more moisture. This intensified the strength of the West African Monsoon system and shifted the African rain belt northwards. This increased Saharan rainfall, resulting in the spread of savannah and wooded grassland across the desert from the tropics to the Mediterranean, providing a vast habitat for plants and animals.
Earth’s changing orbit:
- The Earth’s orbit around the sun isn’t constant due to gravitational effects from celestial bodies like the moon and planets, causing Milankovitch cycles.
- These cycles impact the solar energy received by the Earth, the eccentricity of the Earth’s orbit, the tilt of the Earth’s axis (obliquity), and ice ages on Earth.
- The third Milankovitch cycle, or precession, involves Earth’s wobble on its axis across 21,000 years. This cycle significantly correlates with periods of increased humidity.
- Precession affects seasonal differences, intensifying them in one hemisphere while reducing them in the other.
- This change triggers more rainfall in North Africa during warmer Northern Hemisphere summers, initiating humid phases and promoting vegetation growth across the region.
Eccentricity and the ice sheets:
- The eccentricity cycle determines how circular Earth’s orbit is around the sun.
- The eccentricity indirectly influences the magnitude of the humid periods via its influence on the ice sheets.
Significance of the Sahara region:
- The Sahara acts as a gate. It controls the dispersal of species between north and sub-Saharan Africa, and in and out of the continent.
- The gate was open when the Sahara was green and closed when deserts prevailed.
Significance of the study findings:
- It shows the sensitivity of this gate to Earth’s orbit around the sun.
- High-latitude ice sheets may have restricted the dispersal of species during the glacial periods of the last 800,000 years.
Source: The Wire