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Credit: CC0 Public Domain
Algerian Tassili Najjer Plateau It is Africa’s largest national park. Inside the vast sandstone formations lies what is probably the world’s largest art museum. There he exhibited more than 15,000 etchings and paintings, some of which are 11,000 years old according to scientific dating, providing a unique ethnographic and climatological record of the region. It represents.
Curiously, however, these photographs do not depict the arid, barren terrain that exists in Tassili Najjer today. Instead, it depicts a vibrant savannah inhabited by elephants, giraffes, rhinos, and hippos. This rock art is an important record of past environmental conditions that prevailed in the Sahara Desert, the world’s largest desert. hot desert.
These images depict an era known as the Epoch Epoch, approximately 6,000 to 11,000 years ago. Green Sahara or the Wet Era of North Africa.climatologically wide range evidence During this time, the Sahara desert supported forest-savannah ecosystems and numerous rivers and lakes in present-day Libya, Niger, Chad, and Mali.
This greening of the Sahara Desert did not happen all at once. Scientists used ocean and lake sediments to identified Over the past 8 million years, more than 230 greening events have occurred approximately every 21,000 years. These greening events provided vegetation corridors that influenced the distribution and evolution of species, including the migration of ancient humans out of Africa.
These dramatic greenings would have required major reorganization of the atmospheric system to bring rain to this ultra-arid region. However, most climate models have been unable to simulate how dramatic these events were.
As a team of climate modelers and anthropologists, we overcame this obstacle. We have developed a climate model that more accurately simulates the influence of vegetation on atmospheric circulation and rainfall in the Sahara Desert.
We determined why North Africa greens up approximately every 21,000 years for the past 8 million years.This was caused by changes in the Earth’s orbit. precession— A slight wobble of a rotating planet. This brings the northern hemisphere closer to the sun during the summer.
This results in warmer summers in the Northern Hemisphere, and warmer air allows it to hold more moisture. This strengthened the West African monsoon system and shifted Africa’s rain belt northward. This increased rainfall in the Sahara, resulting in the spread of savannas and wooded grasslands across desert regions from the tropics to the Mediterranean, providing vast habitats for plants and animals.
Our results show that the Sahara Desert is sensitive to past climate changes. They explain how this sensitivity affects rainfall across North Africa. This is important for understanding the effects of current climate change (caused by human activities). Future increases in temperature could increase the strength of monsoons, with both local and global impacts.
Changes in Earth’s orbit
The fact that the North African Wet Period repeats approximately every 21,000 years provides a major clue as to its cause: changes in the Earth’s orbit. The Earth’s orbit around the Sun is not constant due to the gravitational influence from the Moon and other planets in the solar system. There are periodic fluctuations on timescales of thousands of years. These orbital periods are called: milankovitch cycle; they affect the amount of energy the Earth receives from the Sun.
With a period of 100,000 years, the Earth’s orbit (or eccentricity) moves between circular and elliptical shapes, and the tilt of the Earth’s axis changes every 41,000 years ( inclination). Cycles of eccentricity and obliquity are what caused the ice ages of the past 2.4 million years.
The third Milankovitch cycle is precession. This is about the wobble of the Earth’s axis changing over a 21,000 year timescale. The similarities between the period of precession and the timing of wet periods indicate that precession is their primary driver. Precession affects seasonal contrast, increasing contrast in one hemisphere and decreasing it in another. Warmer summers in the Northern Hemisphere would have resulted in increased summer rainfall in North Africa and the onset of a wet period, resulting in the spread of vegetation across the region.
Eccentricity and ice sheets
Our study also confirmed that wet periods did not occur during ice ages, when large glacial ice sheets covered much of the polar regions. This is because these vast ice sheets cooled the atmosphere. Cooling counteracted the effects of precession and suppressed the expansion of the African monsoon system.
Ice ages are caused by eccentricity cycles, which determine how circular Earth’s orbit around the sun is. Therefore, our findings indicate that eccentricity indirectly affects the magnitude of the wet period via its impact on the ice sheet. This highlights for the first time a major link between these distant high latitudes and tropical regions.
The Sahara desert acts as a gate. It controls the dispersal of species between North Africa and sub-Saharan Africa, and within and across the continent. When the Sahara Desert was green, the gates were open, but when the desert expanded, the gates were closed. Our results revealed the sensitivity of this gate to the Earth’s orbit around the Sun. They also show that high-latitude ice sheets may have limited species dispersal during the past 800,000 years of ice age.
Being able to model wet periods in Africa will help understand the alternation between wet and dry periods. This had a major impact on the dispersal and evolution of species, including humans, within and outside of Africa. Additionally, it provides tools for understanding future greening in response to climate change and its environmental impacts.
Sophisticated models may one day be able to determine how climate warming will affect rainfall and vegetation in the Sahara region, as well as the broader impacts on society.