Posted in | Climate Change

Study Reveals the Sahara Desert Swung Between Wet and Dry Climates Every 20,000 Years

Known as one of the most inhospitable and harshest places on earth, the Sahara desert covers most of North Africa in approximately 3.6 million square miles of windswept dunes and rock.

A new analysis of African dust reveals the Sahara swung between green and desert conditions every 20,000 years, in sync with changes in the Earth’s tilt. (Image credit: MIT)

However, the desert was not always parched and desolate like what is seen today. Fossils and primitive rock paintings unearthed from the area indicate that the Sahara was once a rather verdant oasis, in which human settlements and many different plants and animals flourished.

Now, MIT researchers have examined the dust deposited off the West African coast over the last 240,000 years, and discovered that every 20,000 years, the Sahara, and North Africa in general, has fluctuated between dry and wet climates. According to them, this climatic pendulum is predominantly caused by variations to the Earth’s axis as the planet rotates around the sun, which consecutively impact the distribution of sunlight between seasons—in summer, the Earth changes from more sunlight to less and back again, every 20,000 years.

With regards to North Africa, it is possible that when the Earth is tilted to get the highest summer sunlight with each rotation around the sun, this elevated solar flux increases the monsoon activity of the region and consecutively makes the Sahara wetter and “greener.”

Monsoon activity tends to weaken when the Earth’s axis moves toward an angle that lowers the amount of incoming summer sunlight; this results in a drier climate similar to what is seen 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. 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.

David McGee, Associate Professor, Department of Earth, Atmospheric, and Planetary Sciences, MIT.

McGee and his coworkers have reported their results in Science Advances.

A puzzling pattern

The northeast winds sweep up innumerable tons of Saharan dust every year and deposit most of this sediment into the Atlantic Ocean, off the West African coast. Layers of this dust, accumulated over hundreds of thousands of years, can act as a geologic chronicle of the climate history of North Africa: Thick layers of dust may suggest arid periods, while those containing less dust may indicate wetter periods.

For clues to the Sahara’s climate history, researchers have studied the sediment cores dug up from the bottom of the ocean off the West African coast. These cores include ancient sediment layers deposited over a countless number of years. Traces of Saharan dust and also the remains of life forms, like the tiny shells of plankton, can be present in each layer.

Previous analyses of these sediment cores have exposed a perplexing pattern: It would seem that every 100,000 years, the Sahara swings between dry and wet periods—a geologic beat that researchers have associated with the planet’s ice age cycles, which also appear to come and go every 100,000 years. The larger fraction of dust layers appears to coincide with eras when the Earth is covered in ice, while less dusty layers emerge at the time of interglacial periods, like today, when ice has considerably receded.

However, according to McGee, this understanding of the sediment cores chafes against climate models, which reveal that climate in the Sahara should be fueled by the region’s monsoon season, whose strength is established by the amount of sunlight that can drive monsoons in the summer and the tilt of the Earth’s axis.

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,” stated McGee.

Beats in sync

In order to solve this paradox, the scientists applied their own methods to inspect a sediment core attained off the West African coast by coworkers from the University of Bordeaux—which was drilled just several kilometers from cores in which other researchers had earlier detected a 100,000-year pattern.

Headed by first author Charlotte Skonieczny, a former MIT postdoc and currently a professor at Paris-Sud University, the researchers inspected sediment layers deposited over the last 240,000 years. They examined each layer for traces of dust and determined the concentrations of a rare thorium isotope, to find out how quickly the dust was piling up on the seafloor.

By using very small quantities of radioactive uranium dissolved in seawater, thorium is created at a steady rate in the ocean and this radioactive metal rapidly attaches itself to the sinking sediments. Consequently, thorium concentration in the sediments can be used by scientists to establish how rapidly dust and other sediments were piling up on the seafloor in the earlier days: thorium is more concentrated at times of gradual accumulation, while it is diluted during times of fast rapid accumulation. The pattern that materialized was quite different from what others had discovered 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. It might look like there’s more dust deposited in the ocean, when really, there isn’t.

David McGee, Associate Professor, Department of Earth, Atmospheric, and Planetary Sciences, MIT.

After removing this confounding effect, the researchers observed that what emerged was mainly a new “beat,” in which the Sahara swung between dry and wet climates every 20,000 years, in sync with the periodic tilting of the planet and the monsoon activity of the region.

We can now produce a record that sees through the biases of these older records, and so doing, tells a different story,” stated McGee. “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.”

The study was partly funded by the National Science Foundation.

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