How the Sahara is going to turn green again

In ancient Greek mythology, there exists a tale called "Phaethon and the Sun." This narrative follows Phaethon, the young son of Helios, the god of the sun. Each day, Phaethon would embark on a journey across the sky in his chariot, bringing warmth and vitality to the Earth. Driven by a deep desire to prove his father's divinity, Phaethon visits Helios' temple. After confirming their familial relationship, Helios, wanting to grant his son a favour, allows Phaethon to make a request. Acted impulsively, Phaethon boldly asks to personally take control of his father's chariot and guide it across the heavens. Despite Helios' attempts to dissuade him, his promise compels him to relinquish control to the determined Phaethon. However, as soon as Phaethon takes charge, he loses control of the chariot, causing it to veer off course. Recklessly, the chariot hurtles through the sky, descending dangerously close to the ground and scorching the Earth's surface. According to Greek mythology, this catastrophic event resulted in the bleaching of Libya's hills into desert, the retreat of the Nile, and the darkening of the Ethiopian people's skin, Although this tale is purely mythical, it subtly suggests a more significant truth: the deserts of Libya and other Saharan countries were not always the desolate wastelands they are now.

The Earth is a highly dynamic planet, with constant changes occurring in its tectonic plates, weather patterns, and seasons. Additionally, its orientation in space is not fixed due to its slightly oblate shape and the gravitational pull from other planets in our solar system. Over time, the Earth's orbit, tilt, and wobble experience continuous fluctuations. Every 100,000 years, the Earth's orbit transitions between a nearly circular shape and a slightly elliptical shape. The tilt of the Earth's axis also fluctuates between 22.1 and 24.5 degrees every 41,000 years. Furthermore, the Earth's axis wobbles around one complete cycle every 26,000 years. These three fluctuations are commonly known as the Milinkovic cycles, named after Serbian mathematician Milotin Milankovic, who extensively studied them in the early 20th century. Milankovic discovered that these cycles significantly impact the long-term climate of the Earth by influencing the amount and location of sunlight reaching our planet. For instance, a more eccentric orbit leads to greater disparities in season lengths, while a greater tilt results in more extreme seasons. However, the most intriguing relationship arises from the interaction between the Earth's wobble and its orbit.

Nowadays, the Earth reaches its perihelion, which is the point when it is closest to the sun, in early January. This results in the Earth receiving a little extra sunlight during the winter in the northern hemisphere and the summer in the southern hemisphere. However, around 13,000 years ago, when the Earth's wobble was in the opposite phase of its cycle, this situation was reversed. The Earth received a little extra sunlight during the summer in the northern hemisphere and the winter in the southern hemisphere. As a consequence, the radiation during the summer over North Africa was seven percent higher compared to the present day. Although this difference may appear small, it had a profound impact on the climate of the region. With more energy reaching the surface, the air became hotter and rose higher, creating a low-pressure zone and resulting in convection. This convection drew moist air from the surrounding Atlantic, which then rose and cooled down, condensing into rain clouds that brought showers to the Sahara.

Climate models have demonstrated that this process amplified rainfall in North Africa by 17 to 50 percent, compared to present-day conditions. This increased precipitation facilitated the growth of vegetation along the coastlines and southern boundaries of the Sahara. The presence of vegetation, which absorbs more energy due to its darker color, aided in transferring underground moisture into the atmosphere through transpiration. Consequently, this generated further rainfall, fostering a positive feedback loop that led to the proliferation of plant life across North Africa. Simultaneously, the surplus rainfall began to accumulate in geographic depressions within the region, forming lakes. Remarkably, some of these lakes, such as Lakes Mega Fezzan and Annette in modern-day Libya and Algeria, and Lake Mega chad, once covered an extensive area of 360,000 square kilometres. At the time, Lake Mega chad stood as the largest freshwater lake globally, slightly smaller than the Caspian Sea. These lakes, alongside now-extinct river networks, trapped water within the Sahara's interior, enabling the sustained presence of vegetation throughout the year. As a result, nearly the entire Sahara Desert underwent a transformation into a verdant and flourishing region, a mere 10,000 years ago. Despite its incredibility, abundant evidence supports this phenomenon, visible across the Sahara. Ancient rock art depicts intricate depictions of various animals, including gazelles, elephants, rhinos, hippos, and giraffes, in the Tenere Desert of Niger. It is noteworthy that this desert, which now lacks any hospitable conditions for life, was once a thriving ecosystem.

Archaeologists have discovered numerous human graves amidst the remains of land animals, large fish, and crocodiles in Libya and Algeria. They have also unearthed ancient riverbeds providing evidence of human habitation, as well as indications of the ancient shores of Lake Mega Chad through sand spits and beach ridges in Chad. However, the most compelling evidence comes from studying sampled cores of underwater sediment off the coast of Africa. These cores are used to analyze the Saharan dust flux, which refers to the amount of sand blown off the African continent and into the ocean. The analysis of these cores revealed that between 10 and 5,000 years ago, significantly less sand was blowing off Africa compared to today. This reduction in sand can be linked to increased moisture levels and plant life. Furthermore, the pollen trapped in these cores indicates a rise in plant species such as grasses and sedges during that specific time period.

Scientists have determined, based on substantial evidence, that the Sahara experienced a dramatic transformation between 12 and 5,000 years ago. During this period, the region was characterized by lush grasslands, waterways, forests, and abundant animal life, as well as ancient human settlements. However, approximately 6,000 years ago, this verdant environment rapidly gave way to the arid wasteland that we know today. This shift was primarily attributed to orbital cycles that reversed a feedback loop, which had initially contributed to the Sahara's greenery. Additionally, human practices such as overgrazing by cattle and the use of fire for land management likely exacerbated ecological strain in the region. Within a few centuries, lakes dried up, plant life retreated southward, and both animals and humans followed suit. Despite the current state of the Sahara, it is highly probable that another humid period will occur in the future. In fact, evidence spanning millions of years suggests that the Sahara has experienced over 230 of these green periods, coinciding with fluctuations in the Earth's wobble. Consequently, researchers anticipate that in approximately 13,000 years, when the Earth's wobble reverses once again, the Sahara will undergo a transformation, turning green. If human society survives and thrives until then, this development will have significant implications for global geopolitics. The barren deserts of Algeria, Libya, Egypt, Mauritania, Mali, Niger, Chad, and Sudan will become fertile, presenting new opportunities for settlement and likely resulting in substantial population growth. Rivers will flow, and lakes will replenish, transforming North Africa into a new global hotspot. New cities may emerge, and potential new countries could be established. However, these changes are far off in the future, and it is uncertain whether human civilization will still exist and prosper at that time.

To address this time constraint, researchers have proposed methods to artificially restore vegetation in the Sahara, potentially within the next few centuries. One such method is already underway. Unlike the gradual climate fluctuations caused by Milankovitch cycles, the Earth's current climate is rapidly transforming due to human-induced greenhouse gas emissions. These emissions are trapping more energy in the Earth's atmosphere and on its surface. Models have indicated that this increase in energy could activate the same processes that led to greater rainfall in North Africa 10,000 years ago, resulting in expanded vegetation across the region. While this is undoubtedly exciting, it may not be the optimal solution to warm the entire Earth for the sake of more plants in the Sahara. There are alternative, less harmful approaches to achieve the same objective. One of the most promising alternatives could also contribute to combating climate change. Models have shown that by implementing large-scale solar and wind farms in the Sahara, which increase surface roughness and darken the terrain, more energy would be absorbed, and precipitation would increase. This approach allows us to simultaneously harness renewable energy and trigger a positive feedback loop towards restoring the green Sahara.