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Study reveals the driving forces behind greening in the Sahara Desert

Why was the Sahara Desert once lush and green? A recent study published in the journal Nature Communications provides new insights into periodic wet phases and greening in the Sahara over the last 800,000 years.

Earth’s orbital precession 

The primary driver behind intermittent green periods in the Sahara was identified as changes in Earth’s orbit around the Sun, notably Earth’s orbital precession. This refers to Earth’s wobbly motion on its axis, influencing the seasonal contrasts. 

The wobbles, which occur approximately every 21,000 years, determine the energy Earth receives during different seasons. This ultimately controls the strength of the African monsoon and, by extension, the spread of vegetation across the Sahara.

Ice ages

However, the researchers also found that these humid greening intervals in the Sahara did not occur during ice ages. 

Massive glacial ice sheets, blanketing much of the high latitudes, cooled the atmosphere, suppressing the growth of the African monsoon system. 

The experts report that the interconnection between these distant areas might have limited species’ movement, including early humans, out of Africa during the glacial periods spanning the past 800,000 years.

Remarkable environmental changes 

Study lead author Dr. Edward Armstrong is a climate scientist at the University of Helsinki and University of Bristol.

“The cyclic transformation of the Sahara Desert into savannah and woodland ecosystems is one of the most remarkable environmental changes on the planet,” said Dr. Armstrong.

“Our study is one of the first climate modeling studies to simulate the African Humid Periods with comparable magnitude to what the palaeoclimate observations indicate, revealing why and when these events occurred.”

Advanced climate modeling 

Many existing climate models have fallen short in simulating the amplitude of these humid periods in the Sahara.

However, the recent study utilized an advanced climate model that not only offers insights into the past shifts but could potentially improve predictions of future changes.

The results confirmed that the African Humid Periods occurred every 21,000 years and were determined by changes in Earth’s orbital precession. 

Warmer summers and increased precipitation from the intensified monsoon system drove greening across the desert. 

Significance of the study

“We are really excited about the results. Traditionally, climate models have struggled to represent the extent of the ‘greening’ of the Sahara,” said study co-author Professor Paul Valdes.

“Our revised model successfully represents past changes and also gives us confidence in their ability to understand future change.”

Study implications

There is undeniable evidence that the Sahara was home to sprawling rivers, lakes, and water-dependent animals, such as hippos, in past epochs. 

Such periods of vegetation might have played a pivotal role in creating green corridors for various species, including early humans, to disperse across the globe.

“The Sahara region is kind of a gate controlling the dispersal of species between both North and Sub-Saharan Africa, and in and out of the continent,” said study co-author Professor Miikka Tallavaara.

“The gate was open when Sahara was green and closed when deserts prevailed. This alternation of humid and arid phases had major consequences for the dispersal and evolution of species in Africa.” 

“Our ability to model North African Humid periods is a major achievement and means we are now also better able to model human distributions and understand the evolution of our genus in Africa.”

The study has the potential to reshape our understanding of early human migrations and the broader implications of global climate shifts.

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