June 23, 2024

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Thousands of years ago, the Sahara Desert was a green savannah.  What happened?

Thousands of years ago, the Sahara Desert was a green savannah. What happened?

The research reveals the complex interaction between tropical changes and ice caps in humid and arid periods in the Sahara.

Photo: Courtesy Civitates – Courtesy Civitates

On the Tassili N'Ajjer plateau Algeria, the largest national park in Africa, displays more than 15,000 engravings and paintings, some of which are up to 11,000 years old, depicting a desert very different from the one we know today. Instead of depicting hot, arid landscapes, these images reveal vibrant savannas inhabited by elephants, giraffes, rhinos and hippos. This is not the first time it has been suggested that this region of the world at some point (between 6,000 and 11,000 years ago) supported savannah, river and lake ecosystems in what are now Libya, Niger, Chad and Mali. But this is the first time a group of scientists has discovered what happened to Earth's climate to make this happen.

The study was published a few months ago in nature He developed a climate model that simulates the dramatic, large-scale reorganization of the atmospheric system that brought rain to this extremely dry region, what scientists call the greening of the Sahara. It is believed that this did not happen just once: 230 such greenings have been identified and have occurred approximately every 21,000 years over the past eight million years. Exactly 21 thousand years ago, one happened in North Africa. According to researchers, this greening was due to changes in… Earth's orbital motion.

Earth's orbital motion is a phenomenon that describes the slow but continuous change in the direction of the Earth's axis of rotation. This precession movement is similar to the swaying of the top as it rotates. Not only does the Earth rotate on its axis, but the axis of rotation is subject to conical motion, meaning its direction in space changes with time. This change in the direction of the Earth's axis results in differences in the way sunlight hits different areas of the Earth during different times of the year, which in turn affects weather and seasonal patterns.

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Orbital precession is one of the main factors affecting the Earth's climate. This phenomenon affects the intensity and distribution of seasons, as well as changes in insolation (the amount of solar radiation received) in different parts of the planet over time. In this particular case, scientists suggest that precession controls the intensity of the African monsoon system, which in turn leads to periods of humidity and increased precipitation in the Sahara region. According to their model, this led to the spread of savannas and wooded grasslands across the Sahara from the tropics to the Mediterranean, providing extensive habitat for plants and animals.

“Our results show the sensitivity of the Sahara to past climate changes. They explain how this sensitivity affects rainfall across North Africa. This is important for understanding the implications of current climate change (due to human activities),” Edward Armstrong, the study’s lead author, wrote in a column. According to The Conversation, future warmer temperatures may also strengthen the monsoons, with both local and global impacts. However, these wet periods did not occur during ice ages, when large sheets of ice covered Glacier most of the polar regions.

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In the model, these massive ice sheets cooled the atmosphere, counteracting the precession effect and suppressing the expansion of the African monsoon system. In simple terms, during these glacial periods, the presence of large masses of ice affected the climate of regions as far away as the Sahara, affecting the amount of reflected sunlight, air circulation patterns, and the location of climate systems. This led to reduced rainfall in the Sahara and contributed to droughts in that region, but also, according to Armstrong, “this highlights, for the first time, an important connection between these tropical regions and remote, high-latitude regions.”

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For its authors, the research not only reveals the complex interplay between tropical changes and ice caps in humid and arid periods in the Sahara, but also provides a tool for understanding future greening in response to climate change and its impact.

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In this sense, they say, a detailed understanding of how tropical changes and past climate conditions have affected rainfall patterns in the Sahara desert makes it possible to more accurately envision how environmental conditions in this region will change in the future. “In the future, improved models may be able to determine how climate warming will affect rainfall and vegetation in the Sahara, and the broader impacts on society,” Armstrong concludes.

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