Between 11,000 and 5,000 years ago, after the end of the last ice age, the Sahara desert transformed. Green vegetation grew atop the sand dunes and increased rainfall turned the arid caves into lakes. About 3.5 million square miles (9 million square kilometers) of North Africa have turned green, attracting animals such as hippos, antelope, elephants and aurochs (wild ancestors of domesticated cattle), who ate on its lush grasses and shrubs. This lush paradise is long gone, but could it ever return?
In short, the answer is yes. The Green Sahara, also known as the African Wet Period, was caused by the Earth’s constantly changing orbital rotation around its axis, a pattern that repeats every 23,000 years, according to Kathleen Johnson, an associate professor of earth systems at the university. of California Irvine.
However, due to a wildcard, man-made greenhouse gas emissions which led to the escape climate change – it is not clear when the Sahara, currently the largest hot desert in the world, will turn a new green leaf.
Related: Has the Earth ever been this hot before?
The green change of the Sahara happened because the tilt of the Earth changed. About 8,000 years ago, the inclination began to shift from about 24.1 degrees to the current 23.5 degrees, Space.com, a sister site of Live Science, previously reported. That change in tilt made a big difference; at this time, the Northern Hemisphere is closest to the sun during the winter months. (This may seem counterintuitive, but due to the current tilt, the Northern Hemisphere is tilted away from the sun during the winter season.) During the Green Sahara, however, the Northern Hemisphere was closest to the sun during the summer. .
This has led to an increase in solar radiation (in other words, heat) in the northern hemisphere of the Earth during the summer months. The increase in solar radiation amplified the African monsoon, a seasonal wind shift over the region caused by temperature differences between the land and the ocean. The increased heat in the Sahara created a low-pressure system that introduced moisture from the Atlantic Ocean into the arid desert. (Usually, the wind blows from the mainland towards the Atlantic, spreading dust that fertilizes the Amazon rainforest and builds beaches in the Caribbean, Live Science previously reported.)
This increased humidity has transformed the former sandy Sahara into a steppe covered with grass and shrubs, according to the National Oceanic and Atmospheric Administration (NOAA). As animals thrived, humans did too, eventually taming buffaloes and goats and even creating an early symbolic art system in the region, NOAA reported.
But why did the tilt of the Earth change in the first place? To understand this monumental shift, scientists looked to Earth’s neighbors in the solar system.
“The axial rotation of the Earth is perturbed by gravitational interactions with the moon and more massive planets which together induce periodic changes in the Earth’s orbit,” said Peter de Menocal, director of the Center for Climate and Life at Lamont-Doherty Earth Observatory at Columbia. New York University, wrote in Nature. One such change is a “wobble” in the Earth’s axis, he wrote.
That wobble is what positions the northern hemisphere closest to the sun in summer – what researchers call a northern hemisphere summer sunshine maximum – every 23,000 years. Based on research first published in the journal Science in 1981, scholars estimate that the Northern Hemisphere experienced a 7% increase in solar radiation during the Green Sahara compared to today. This increase may have intensified African monsoon rainfall from 17% to 50%, according to a 1997 study published in the journal Science.
Related: Why does the rain smell good?
What’s interesting to climate scientists about the Green Sahara is how abruptly appeared and disappeared. The end of the Green Sahara took only 200 years, Johnson said. The change in solar radiation was gradual, but the landscape changed suddenly. “It’s an example of sudden climate change on a scale that humans would notice,” he said.
“Ocean sediment records show [that the Green Sahara] happens repeatedly, “Johnson told LiveScience. The next peak of summer insolation in the Northern Hemisphere, when the green Sahara may reappear, is predicted to happen again about 10,000 years from now in AD 12,000 or AD 13,000. what scientists can’t predict is how greenhouse gases will affect this natural climate cycle.
Paleoclimatic research “provides unequivocal proof of what [humans] they are doing is quite unprecedented, “Johnson said. Even if humans stopped emitting greenhouse gases today, those gases would still be high by 12,000.” Climate change will be superimposed on the Earth’s natural climate cycles, “he said.
That said, there is geological evidence from ocean sediments that these orbitally paced Green Sahara events have been occurring as far back as the Miocene era (23 million to 5 million years ago), including times when atmospheric carbon dioxide was similar, and perhaps higher, than today’s levels. Hence, a future event in the green Sahara is still very likely in the distant future. Today’s increase in greenhouse gases may even have its own greening effect on the Sahara, albeit not at the level of forced orbital changes, according to a March review published in One Earth magazine. But this idea is far from certain, due to the limitations of the climate model.
In the meantime, there is another way to transform parts of the Sahara into a green landscape; if massive solar and wind farms were installed there, rainfall could increase in the Sahara and its southern neighbor, the semi-arid Sahel, according to a 2018 study published in the journal Science.
Wind and solar farms can increase heat and humidity in surrounding areas, Live Science previously reported. An increase in rainfall, in turn, could lead to vegetation growth, creating a positive feedback loop, the researchers of that study said. However, this massive feat has yet to be tested in the Sahara Desert, so until such a project receives funding, humans may have to wait until the year 12,000 or more to see if the Sahara will turn green again.
Originally published in Live Science.