"Our results suggest the story of North African climate is dominantly this 20,000-year beat, going back and forth between a green and dry Sahara."
Dust deposited on the coasts of West Africa during the last 240,000 years reveals that the Sahara – the largest desert in the world – has oscillated between humid and dry climates every 20,000 years or so.
It’s almost poetical in nature, showing how the desert breathes through its long cycles.
Researchers from the Massachusetts Institute of Technology (MIT) say that this climatic pendulum is mainly caused thanks to changes in the Earth’s axis as the planet orbits around the Sun. This phenomenon, in turn, affects the distribution of sunlight between seasons: every 20,000 years, the Earth changes from more sunlight in summer to less, and vice versa.
This causes the Sahara to blossom and dry out cyclically.
For North Africa, experts say it is likely that, when the Earth is tilted to receive the maximum summer sunlight with each orbit around the Sun, this greater solar flux intensifies the monsoon activity of the region, which in turn makes the Sahara wetter and “greener”.
When the planet’s axis rotates at an angle that reduces the amount of summer sunlight, monsoon activity weakens, producing a drier climate similar to what we see today.
“Our results suggest the story of North African climate is dominantly this 20,000-year beat, going back and forth between a green and dry Sahara,” explained David McGee, an associate professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences.
“We feel this is a useful time series to examine in order to understand the history of the Sahara desert and what times could have been good for humans to settle the Sahara desert and cross it to disperse out of Africa, versus times that would be inhospitable like today.”
The new findings have been published in Science Advances.
Scientists have discovered that every year, the northeasterly winds sweep hundreds of millions of tons of Saharan dust, depositing much of this sediment in the Atlantic Ocean, off the coast of West Africa.
The layers of this dust, accumulated over hundreds of thousands of years, can serve as a geological chronicle of the climatic history of North Africa: thick layers of dust can indicate arid periods, while those containing less dust can indicate wetter seasons, experts have found.
To understand what’s going on, scientists have analyzed the sediment cores extracted from the ocean floor of the West African coast, in search of clues about the climate history of the Sahara.
These cores contain layers of ancient sediments deposited over millions of years.
Each layer may contain traces of Saharan dust, as well as remnants of life forms, such as the tiny plankton shells.
Previous studies revealed surprising results.
Analysis of sediment cores previously shown how the Sahara changes between wet and dry periods every 100,000 years, a geologic feature that scientists have related to the cycles of the ice ages on Earth. Earth’s ice ages are thought to cycle every 100,000 years or so.
The layers with a larger fraction of dust seem to coincide with the periods when the Earth is covered with ice, while the less dusty layers appear during the interglacial periods, like today, when the ice has receded greatly.
However, scientists from MIT believe this interpretation of the sediment cores goes against climate models, which demonstrate that Saharan climate should be driven by the region’s monsoon season, the strength of which is determined by the tilt of the Earth’s axis and the amount of sunlight that can fuel monsoons in the summer.
“We were puzzled by the fact that this 20,000-year beat of local summer insolation seems like it should be the dominant thing controlling monsoon strength, and yet in dust records, you see ice age cycles of 100,000 years,” McGee explained.
Therefore, researchers used their very own techniques and studied a sediment core taken off the coast of Africa by fellow researchers from the University of Bordeaux.
This sediment core was drilled a few kilometers away from the cores where scientists previously had identified the 100,000-year pattern.
Scientists went on and analyzed layers of sediment that had been deposited over the last 240,000 years. They measured each layer for traces of dust and looked for concentrations of a rare isotope of thorium, which allowed them to understand how rapidly dust accumulate on the ocean floor.
Thorium is produced at a constant rate in the ocean thanks to small amounts of uranium dissolved in seawater, and it quickly attaches itself to sinking sediments.
Using the concentration of Thorium inside the sediments, scientists have the ability to find out how fast dust and other sediments accumulated on the ocean floor throughout history.
As explained by MIT, during times of slow accumulation, thorium is more concentrated, while at times of rapid accumulation, thorium is diluted.
The pattern that eventually emerged was very different from what other studies had found in the same sediment cores.
“What we found was that some of the peaks of dust in the cores were due to increases in dust deposition in the ocean, but other peaks were simply because of carbonate dissolution and the fact that during ice ages, in this region of the ocean, the ocean was more acidic and corrosive to calcium carbonate,” McGee says.
“It might look like there’s more dust deposited in the ocean when really, there isn’t.”
Once the researchers removed this confounding effect, they discovered that what emerged was primarily a new “beat,” in which the Sahara Desert vacillated between wet and dry climates every 20,000 years, in synchronization with the region’s monsoon activity and the periodic tilting of the Earth.
“We can now produce a record that sees through the biases of these older records, and so doing, tells a different story,” McGee says. “We’ve assumed that ice ages have been the key thing in making the Sahara dry versus wet. Now we show that it’s primarily these cyclic changes in Earth’s orbit that have driven wet versus dry periods. It seems like such an impenetrable, inhospitable landscape, and yet it’s come and gone many times, and shifted between grasslands and a much wetter environment, and back to dry climates, even over the last quarter million years.”