The Greening Cycles of the Sahara Desert

By Scot Noel

The climate of Earth includes cycles that take much longer than the span of recorded history. Among these is the greening and browning of the Sahara. This is a natural cycle that —similar to Ice Ages— is slow and immense in scale and never fully witnessed by our young civilization.

The Sahara Today

The Sahara is the largest hot desert, located in Northern Africa, spanning an area comparable to the United States. It’s mostly empty, home to around two million people who live primarily along the Nile and Niger rivers. This vast landscape covers over 2,273 billion acres or 3.5 million square miles.

Today the Sahara is expanding, spreading into the fertile lands of Sudan and Chad. This spread is worrying because it's happening in summer when these areas usually get a lot of rain to support farming. For now, that rain isn't coming, and the desert is growing instead.

Research published in the Journal of Climate suggests the growth of the Sahara is linked to a global weather pattern, and today’s climate change might be making the situation worse. Researchers, including Natalie Thomas of the University of Maryland, have looked at weather data from the past and found that a cycle of ocean temperatures can affect how much rain falls in the Sahara, and right now that cycle is responsible for increased desertification.

When the Sahara was Last Green

Thousands of years ago the Sahara was green. This was a time when the desert supported many plants and animals, and early humans lived there too. Evidence suggests that rivers, lakes, and diverse species were once common throughout the region.

Back when the Sahara was green, instead of the sea of sand dunes we see today, there were grasslands and patches of cooler forest. The weather was wetter, with regular rainfall supporting a grassy steppe.

This green landscape was home to a variety of flora, including acacia, cypress, olive, and tamarix trees. In wetter areas there would have been figs and date palms, with ferns and reeds growing alongside papyrus.

These plants supported a wide array of animals. Large herbivores like elephants and giraffes roamed the landscape, as well as predators like lions. There were also hippos, crocodiles, and fish in the rivers and lakes that dotted the landscape. As for people, the green Sahara could have supported a large number of early humans. They would have had enough water, plants, and animals to provide for their needs, from food to shelter.

What Causes the Green to Desert Cycles?

The Sahara's transformation is linked to Earth's orbital shifts, particularly a "wobble" in its axis, known as precession. This process happens every 21,000 years and affects the way the sun's energy hits the Earth, altering the strength of monsoons and the distribution of rainfall. Studies show that these green periods didn't happen during ice ages because the large ice sheets cooled the atmosphere and stopped the rains that would otherwise turn the desert green.

While it’s difficult to say how natural cycles like the greening of the Sahara can be affected by human-induced climate change, we could next expect the natural greening to return in 12,000 to 15,000 years.

Can We Make the Sahara Green Sooner?

With the current climate crisis, there are ideas about making the Sahara green before nature does. Proposals include using the desert for solar power generation or continuing the Great Green Wall project to fight desertification.

The Great Green Wall is an ongoing project to plant a wall of trees and shrubs from one side of Africa to the other, right on the edge of the Sahara. This isn't just about stopping the desert from getting bigger; it's also about helping people. Trees can help improve the soil so it's better for farming, give people food and wood, and even help take carbon out of the air. By creating this green area, the project hopes to bring back some of the green lands that used to be there and make life better for the people who live nearby.

Solar power stations could also help make parts of the Sahara Desert green. Putting up a lot of solar panels is projected to change the weather around them. The panels absorb sunlight and turn it into electricity, but they also warm up the air close to the ground. This warmer air can make clouds form and bring some rain to very dry places. If it rains more, plants could start growing around these power stations. While the solar panels make clean energy, they might also help small bits of the desert start to turn green again.

How a Green Sahara Might Affect Climate Warming

A green Sahara could help with the climate crisis by capturing and sequestering carbon dioxide in new tree growth. However, this is a two-edged sword. All that new plant life could initially make the Earth warmer by absorbing sunlight. A greener Sahara might also change the temperature of the Atlantic Ocean, leading to more unpredictable weather in places like the United States.

It may seem like humans should take a hands-off, leave good enough alone approach, but the significant changes now expected in our climate future may mean that humans have no choice but to learn something akin to climate husbandry.

Summation

As we deal with the impact of our actions on the climate, it's important to remember the long-term natural cycles of our planet. We've built our civilization during a relatively stable climate period. The Sahara's cycles remind us that there are larger patterns at play. They teach us that Earth's climate has always changed and will continue to do so, often in ways we may not expect, but in ways our growing civilization will have to account for if we are to continue to thrive.