The Sahara Desert Was a Green Savanna: Research Explains Why

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of Algeria Tashili N’Ajjar Plateau It is the largest national park in Africa. Its vast sandstone formations may house the world’s largest art museum. More than 15,000 carvings and paintings are on display there, some 11,000 years old according to scientific dating techniques, representing the unique ethnographic and climatic record of the region.

Interestingly, however, these images do not depict the dry, barren landscape that exists in Tassili N’Ajjar today. Instead, they depict a vibrant savannah inhabited by elephants, giraffes, rhinos and hippos. This rock art is an important record of past environmental conditions in the world’s largest Sahara. Hot desert.

These images date back to around 6,000-11,000 years ago Green Sahara or North African humid period. It has a wide range of climates Evidence During this period the Sahara supported tree-rich savanna ecosystems and numerous rivers and lakes in what is now Libya, Niger, Chad and Mali.

This greening of the Sahara did not happen once. Using ocean and lake sediments, there are scientists Identified More than 230 extinctions have occurred every 21,000 years over the past eight million years. These greening events provided vegetation corridors that influenced the spread and evolution of species, including the out-of-Africa migration of ancient humans.

This dramatic greening would require a large-scale reorganization of the atmospheric system to bring rain to this extremely arid region. But most climate models are unable to simulate how dramatic these events were.

As a team of climate modelers and anthropologists, We have crossed this hurdle. We have developed a climate model that more accurately simulates the effects of vegetation on atmospheric circulation and precipitation over the Sahara.

We discovered why North Africa turned green about once every 21,000 years during the last eight million years. This was caused by changes in the Earth’s orbit Pioneer– Slight wobble of the planet while rotating. This moves the Northern Hemisphere closer to the Sun during the summer months.

This resulted in warmer summers in the Northern Hemisphere, and warmer air held more moisture. This intensified the strength of the West African Monsoon System and moved the African Rain Zone northwards. This increased the Saharan rainfall, resulting in the spread of savannah and wooded grasslands across the desert from the tropics to the Mediterranean, providing a vast habitat for plants and animals.

Our results demonstrate the sensitivity of the Sahara Desert to past climate changes. They explain how this sensitivity affects rainfall across North Africa. This is important for understanding the impacts of climate change (driven by human activities) today. Warmer temperatures in the future may increase the strength of monsoons with local and global impacts.

Earth’s orbit will change

The fact that wet periods in North Africa recur every 21,000 years or so is a big clue to their occurrence: variations in Earth’s orbit. Due to the gravitational influences of the Moon and other planets in our solar system, the Earth’s orbit around the Sun is not constant. It has cyclical variations on multi-millennial time scales. These orbital cycles are called cycles Milankovitch cycles; They affect the amount of energy the Earth receives from the Sun.

In 100,000 year cycles, the shape of Earth’s orbit (or eccentricity) alternates between circular and oval, and the tilt of Earth’s axis varies over 41,000 year cycles. slope) eccentricity and tilt cycles are responsible for driving ice ages over the past 2.4 million years.

Third Milankovitch cycle Pioneer. It’s about the Earth’s wobble on its axis, which varies over a 21,000-year period. The similarity between the timing of precessional circulation and wet periods indicates that forecasting is their dominant driver. The forecast affects seasonal differences, increasing them in one hemisphere and decreasing them in the other. During warmer Northern Hemisphere summers, increased North African summer rains would have initiated a humid phase, resulting in the spread of vegetation across the region.

Weird and icy

Our study also found that wetter periods did not occur during ice ages when large glacial ice sheets covered much of the polar regions. Because these vast ice sheets cooled the atmosphere. The cooling counteracted the frontal influence and suppressed the expansion of the African monsoon system.

Ice ages are driven by eccentricity, which determines how circular the Earth’s orbit is around the Sun. Our findings therefore show that it indirectly affects the extent of wet periods through its influence on ice sheets. This highlights, for the first time, an important connection between these distant high-latitude and tropical regions.

Sahara acts as a gateway. It restricts the distribution of species to northern and sub-Saharan Africa, both within and outside the continent. The door was open when the Sahara was green and closed when deserts prevailed. Our results reveal the sensitivity of this gate to the Earth’s orbit around the Sun. They also show that high-latitude ice sheets may have restricted the distribution of species during the last 800,000 years of ice ages.

Our ability to model African wet periods helps us understand the transition between wet and dry conditions. This had major consequences for the spread and evolution of species, including humans, both within and outside of Africa. Furthermore, it provides a tool for understanding future greening in response to climate change and its environmental impact.

Refined models can identify, in the future, how climate warming will affect rainfall and vegetation in the sub-Saharan region and the broader implications for society.

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Quotation: Sahara Desert Was a Green Savannah: Research Explains Why (2023, December 16) Retrieved 16 December 2023 from https://phys.org/news/2023-12-sahara-green-savannah.html

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