The Sahara desert is known for its undulating dunes and unforgiving Sun, but it wasn’t always that way. Scientists believe it was once green and wet. But what caused it to turn from a verdant savannah to a dry, sandy wasteland? Scientists have been trying to answer this question for a while, but now, a new study has unveiled insights into the periodic greening of the Sahara.
The research, published in Nature Communications, reveals that the Sahara’s climatic pendulum swings between wet and dry climates are driven by changes in Earth’s orbit around the Sun, primarily the planet’s wobbling on its axis during its annual cycle. This, in turn, affects how much sunlight the region receives during different seasons and ultimately influences the strength of the African monsoon. This process, which occurs every 20,000 years, is called precession. During these periods, warmer summers in the Northern Hemisphere juice up the African monsoon system and fuel the spreading of savannah-type vegetation.
To understand this, researchers used a combination of observations from satellites and ground-based weather stations and climate simulations that looked to both the past and future. They found that the Sahara’s frequent wet phases were due to the interplay between these orbital changes and the ebb and flow of distant, high-latitude ice sheets in the Northern Hemisphere.
However, these Green Sahara periods were suppressed during ice ages, which skewed the cyclical effects of the climate system. As a result, they lasted for fewer years and were shorter than during non-ice age intervals.
In addition, researchers observed that these periods of the Green Sahara ended erratically—in some places, it took only a few decades for the landscape to ultimately turn into a desert. In contrast, in others, it was far more rapid. They suspect that human behavior played a vital role in this. As early humans expanded west from the Nile River 8,000 years ago, they brought sheep, goats, and cattle that gobbled up, mowed, and trampled native vegetation. This, in turn, reduced the amount of atmospheric moisture sparked by plants and enhanced how much sunlight was reflected off the ground.
As a result, the Sahara turned back into a desert more quickly than would have been the case without humans. Fortunately, there is hope that this trend will reverse as global greenhouse gas emissions slow down—though even this won’t push the planet back into a Green Sahara for some time. To see if that happens, researchers are currently looking at the fossilized remains of ancient savannahs preserved in marine sediment cores off the coast of Africa. Using this data, they can determine the timing and extent of these historic Green Sahara events. It could also give us clues as to how long it will take for today’s Sahara to revert to a lush, natural habitat again.