Study shows the Sahara swung between lush and desert conditions every 20,000 years, in sync with monsoon activity

January 2, 2019 by Jennifer Chu, Massachusetts Institute of Technology
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. Credit: Massachusetts Institute of Technology

The Sahara desert is one of the harshest, most inhospitable places on the planet, covering much of North Africa in some 3.6 million square miles of rock and windswept dunes. But it wasn't always so desolate and parched. Primitive rock paintings and fossils excavated from the region suggest that the Sahara was once a relatively verdant oasis, where human settlements and a diversity of plants and animals thrived.

Now researchers at MIT have analyzed deposited off the coast of west Africa over the last 240,000 years, and found that the Sahara, and North Africa in general, has swung between wet and dry climates every 20,000 years. They say that this climatic pendulum is mainly driven by changes to the Earth's axis as the planet orbits the sun, which in turn affect the distribution of sunlight between seasons—every 20,000 years, the Earth swings from more sunlight in summer to less, and back again.

For North Africa, it is likely that, when the Earth is tilted to receive maximum summer sunlight with each orbit around the sun, this increased solar flux intensifies the region's monsoon activity, which in turn makes for a wetter, "greener" Sahara. When the planet's axis swings toward an angle that reduces the amount of incoming 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," says 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."

McGee and his colleagues have published their results today in Science Advances.

A puzzling pattern

Each year, winds from the northeast sweep up hundreds of millions of tons of Saharan dust, depositing much of this sediment into the Atlantic Ocean, off the coast of West Africa. Layers of this dust, built up over hundreds of thousands of years, can serve as a geologic chronicle of North Africa's climate history: Layers thick with dust may indicate arid periods, whereas those containing less dust may signal wetter eras.

Scientists have analyzed dug up from the off the coast of West Africa, for clues to the Sahara's climate history. These cores contain layers of ancient sediment deposited over millions of years. Each layer can contain traces of Saharan dust as well as the remains of life forms, such as the tiny shells of plankton.

Past analyses of these sediment cores have unearthed a puzzling pattern: It would appear that the Sahara shifts between wet and dry periods every 100,000 years—a geologic beat that scientists have linked to the Earth's ice age cycles, which seem to also come and go every 100,000 years. Layers with a larger fraction of dust seem to coincide with periods when the Earth is covered in ice, whereas less dusty layers appear during interglacial periods, such as today, when ice has largely receded.

But McGee says this interpretation of the sediment cores chafes against climate models, which show 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 says.

Beats in sync

To get to the bottom of this contradiction, the researchers used their own techniques to analyze a sediment core obtained off the coast of West Africa by colleagues from the University of Bordeaux—which was drilled only a few kilometers from cores in which others had previously identified a 100,000-year pattern.

The researchers, led by first author Charlotte Skonieczny, a former MIT postdoc and now a professor at Paris-Sud University, examined layers of sediment deposited over the last 240,000 years. They analyzed each layer for traces of dust and measured the concentrations of a rare isotope of thorium, to determine how rapidly dust was accumulating on the seafloor.

Thorium is produced at a constant rate in the ocean by very small amounts of radioactive uranium dissolved in seawater, and it quickly attaches itself to sinking sediments. As a result, scientists can use the concentration of thorium in the sediments to determine how quickly dust and other sediments were accumulating on the seafloor in the past: During times of slow accumulation, thorium is more concentrated, while at times of rapid accumulation, thorium is diluted. The pattern that emerged was very different from what others had found in the same 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 found that what emerged was primarily a new "beat," in which the Sahara vacillated between wet and dry climates every 20,000 years, in sync 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."

Explore further: Scientists find huge reduction in African dust plume led to more Saharan monsoons 11,000 years ago

More information: "Monsoon-driven Saharan dust variability over the past 240,000 years" Science Advances (2019). advances.sciencemag.org/content/5/1/eaav1887

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28 comments

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Gimp
1 / 5 (9) Jan 02, 2019
My goodness, who shall we tax to end this lunacy? Ice Ages, climate shift all of these things that happened in the past that we now refuse to believe are still happening in the present.
greenonions1
4.6 / 5 (10) Jan 02, 2019
My goodness, who shall we tax
Every science article - that supplies another piece of the jigsaw puzzle - to help us understand this incredibly complex system - is immediately followed by an inane comment from a denier goon. Who cares - Right?? Except that we have to live on the same planet as you goons - and it's discouraging. can't you go over to reddit? Smart people ignore those assholes - and like to read about science in peace.
zz5555
4.5 / 5 (8) Jan 02, 2019
Ice Ages, climate shift all of these things that happened in the past that we now refuse to believe are still happening in the present.

Perhaps unsurprisingly, it's those that oppose science, like Gimp above, that are refusing to accept that these things still happen. Anyone that accepts science understands that both natural and anthropogenic forcings continue to affect the climate (http://www.realcl...atement/ ). The natural forcings are tending to cool the climate since ~1950 and anthropogenic forcings are warming it. So AGW causes > 100% of the warming since ~1950.
clans_estine
1 / 5 (10) Jan 02, 2019
My goodness, who shall we tax
Every science article - that supplies another piece of the jigsaw puzzle - to help us understand this incredibly complex system - is immediately followed by an inane comment from a denier goon. Who cares - Right?? Except that we have to live on the same planet as you goons - and it's discouraging. can't you go over to reddit? Smart people ignore those assholes - and like to read about science in peace.


You should be more respectful of others opinions. Trolling and offensive arguments will get you nowhere.
Ojorf
3.5 / 5 (11) Jan 02, 2019
You should be more respectful of others opinions. Trolling and offensive arguments will get you nowhere.


And you should get your facts straight, opinions are worthless on a science site.
AGW is a fact not an opinion.

greenonions1
4.3 / 5 (6) Jan 03, 2019
You should be more respectful of others opinions
I disagree. I think one of the strategies of the anti science trolls - is to demand respectful discourse. But it is not respectful to jump on every internet science site - and turn science into politics. zz5555 above outlines the scientific response to the post. But it does not change anything - cuz the trolls are not interested in facts, or truth. So I think it is fine - to at least expose the trolling. Look at how many 1's the troll (and you) receive. Clearly I am not the only frustrated one.
Scroofinator
1 / 5 (5) Jan 03, 2019
AGW is a fact not an opinion

Have you ever heard Mann speak? Watch the 2013 senate hearing which he uses the term opinion numerous times.

It's still a hypothesis, per the science definition. Political definition on the other hand...
Captain Stumpy
4.3 / 5 (6) Jan 03, 2019
@clans_estine
You should be more respectful of others opinions
except that the noted "opinion" is from a person who has a history of denying factual evidence

when you specifically target an aspect of science that you disagree with because you don't want to accept reality, then you choose to post nonsensical or delusional claims on a science site where the literate are capable of comprehending the science, then the poster is *literally* trolling to elicit a response and gain attention for whatever purpose

in this case, the "Trolling and offensive arguments" are the poster who argues the political opinion as a justification for denying factual science

.

.

@Scroof
It's still a hypothesis
no, it's not
It's been repeatedly validated by nations that can't agree on much of anything
Scroofinator
1 / 5 (3) Jan 03, 2019
Until AGW can accurately predict clouds and the supposed water vapor positive feedback to even a respectable amount, it can't be considered validated.

As of now, all they have done is fit models to prior data, and claim they *know* how sensitive climate is.
Captain Stumpy
4.2 / 5 (5) Jan 03, 2019
@scroof
Until AGW can accurately predict clouds and the supposed water vapor positive feedback to even a respectable amount, it can't be considered validated
you don't understand what validated means in science

moreover, your argument "betrays a misunderstanding of the difference between weather, which is chaotic and unpredictable, and climate which is weather averaged out over time. While you can't predict with certainty whether a coin will land heads or tails, you can predict the statistical results of a large number of coin tosses. In weather terms, you can't predict the exact route a storm will take but the average temperature and precipitation over the whole region is the same regardless of the route." - LarryM
all they have done is fit models to prior data
testing with prior data helps prediction power and accuracy

you don't randomly shoot a rifle and hope you hit your target - you have to sight it in

Scroofinator
1 / 5 (3) Jan 03, 2019
If you think cloud cover and water vapor don't have to do with the climate, then it makes sense why you would believe it's settled...
Captain Stumpy
4.2 / 5 (5) Jan 03, 2019
@scroof the illiterate
If you think cloud cover and water vapor don't have to do with the climate, then it makes sense why you would believe it's settled...
well, at least you've demonstrated one particular point why you can't comprehend AGW - you're incapable of comprehending what has been written to you

also note: I know WV has a part in AGW, but I also understand the relationship it has with CO2, which, apparently, you don't
greenonions1
4.2 / 5 (5) Jan 03, 2019
If you think cloud cover and water vapor don't have to do with the climate
Captain did not say that. In fact Captain gave a very careful explanation of the difference between weather and climate - and that definition in no way implied that clouds and water vapor have nothing to do with climate.
Until AGW can accurately predict clouds
It is interesting to watch the arbitrary lines that deniers keep throwing down - as some kind of turing test for the validity of science. Weather is very difficult to predict. It is chaotic. Meteorologists have trouble telling us how much snow a storm is going to drop on a given location. I still watch the Weather channel. Gravity is not chaotic. I can predict with great accuracy how long a weight will take to drop to the ground. Every time. Weather is different, and climate is weather averaged - which is certainly more predictable - but the big question is - to what degree of accuracy - or granularity. You really don't understand.
Scroofinator
1 / 5 (3) Jan 03, 2019
You guys are trying to deny the water cycle is climatic? The biggest factor of AGW models is water vapor infrared absorption. Global cloud coverage is not weather, it's climate.

And i'm changing the goal posts...
greenonions1
3.7 / 5 (3) Jan 03, 2019
The biggest factor of AGW models is water vapor infrared absorption
Can you give us a reference that supports saying that water vapor infrared absorption is the "biggest' factor?"
Global cloud coverage is not weather, it's climate
That statement is meaningless. Clouds are of course a part of the weather picture (short term) - No clouds, no rain right?- and thus by extension - part of the climate picture. Clouds reflect energy back into space - but water vapor absorbs energy.
Here is a pretty good discussion of the issue of radiative balance (or lack of) related to the Earth's energy balance. https://www.acs.o...ect.html
Scroofinator
1 / 5 (1) Jan 03, 2019
https://www.nasa....ing.html

Andrew Dessler and colleagues from Texas A&M University in College Station confirmed that the heat-amplifying effect of water vapor is potent enough to double the climate warming caused by increased levels of carbon dioxide in the atmosphere


https://www.princ...ud-cycle

Porporato and first author Jun Yin, a postdoctoral research associate in civil and environmental engineering, found that not accurately capturing the daily cloud cycle has models showing the sun bombarding Earth with an extra one or two watts of energy per square meter. The increased carbon dioxide in the atmosphere since the start of the Industrial Age is estimated to produce an extra 3.7 watts of energy per square meter. "The error here is half of that, so in that sense it becomes substantial," Porporato said.
Bert_Halls
2 / 5 (4) Jan 03, 2019
My goodness, who shall we tax
Every science article - that supplies another piece of the jigsaw puzzle - to help us understand this incredibly complex system - is immediately followed by an inane comment from a denier goon. Who cares - Right?? Except that we have to live on the same planet as you goons - and it's discouraging. can't you go over to reddit? Smart people ignore those assholes - and like to read about science in peace.


You should be more respectful of others opinions. Trolling and offensive arguments will get you nowhere.


Eat shit, you piece of rotten cum adhering to an elderly prostitute's ass.
Scroofinator
1 / 5 (3) Jan 03, 2019
If you guys really cared about the science, you would've already known what I've linked. Instead, you care about the politics of "consensus" and "facts"...

https://www.natur...ure18273
Clouds substantially affect Earth's energy budget by reflecting solar radiation back to space and by restricting emission of thermal radiation to space. They are perhaps the largest uncertainty in our understanding of climate change, owing to disagreement among climate models and observational datasets over what cloud changes have occurred during recent decades and will occur in response to global warming
greenonions1
4 / 5 (4) Jan 03, 2019
Andrew Dessler and colleagues from Texas A&M University in College Station confirmed that the heat-amplifying effect of water vapor is potent enough to double the climate warming caused by increased levels of carbon dioxide in the atmosphere
Does that not imply they are basically equal? If I have a heat amplifying effect of X - caused by C02, and then I include water vapor in the equation and double the effect to 2X - they are actually equal.
However - now we are debating the details of climate change. Here is a good look at the relationship between C02 and water vapor - https://www.forbe...e95c3238
Does this mean you are now on board with discussing the science of climate change - and no longer declaring that it is all nonsense - by biased - dishonest - money grubbing scientists?
greenonions1
4 / 5 (4) Jan 03, 2019
you care about the politics of "consensus" and "facts"
I think we care about both. The science is the science. Is the Earth warming? What are the mechanisms of that warming? etc. All pretty factual.
The politics part gets into how we should respond as a society to the facts presented to us by the science. You are the one who plays politics - by declaring that the science is distorted by dishonest scientists. We accept the information presented to us by the thousands of highly trained/educated/hard working scientists. I hope we will employ the precautionary principle - look for win/win responses - and keep funding the science. Our survival as a species very probably depends on how we respond in the coming decades.
Scroofinator
1 / 5 (3) Jan 03, 2019
Does this mean you are now on board with discussing the science of climate change

You are delusional, i have been trying to discuss the actual science. I bring up the two biggest AGW admitted issues in climate models, and not only do you guys try to deny the issue but then accuse me of playing politics because i bring up Mann's use of the term opinion..

Keep eating from your trough, i'm outta here.

rrwillsj
5 / 5 (2) Jan 03, 2019
hey scrooloose, since only you have the valid science? You present the evidence that is not based on comicbooks & Ouija boards. A repeatable experiment?. A verifiable prediction? A peer review that doesn't come out of a police blotter of known bunco artists?

Whydening Gyre
5 / 5 (4) Jan 03, 2019
I must have missed this in the article. Is the cycle periodicity still on track? And if so, where are we in that period?
greenonions1
4.2 / 5 (5) Jan 03, 2019
not only do you guys try to deny the issue
We don't try to deny anything in terms of the science. For example - you said that
all they have done is fit models to prior data
Well that's a lie. They do hindcast the models - and Captain addressed that issue. Did you even understand? So here is a good discussion of the validity of climate models. https://www.yalec...warming/

Yes it is you who has to make it all political - and wants to disagree with thousands of trained/educated/hard working scientists - and of course any time we ask for your qualifications for undertaking such a task - crickets chirping. I hope you are outta here. Head on over to Wattsup - you will be welcome over there.
Captain Stumpy
5 / 5 (4) Jan 03, 2019
@scroof
You are delusional, i have been trying to discuss the actual science
calling bullsh*t again. Usually, you ignore the science

if you want to talk about WV, why not include CO2 as it's relevant? (see: Lacis et al)
and not only do you guys try to deny the issue
where did I deny the issue?
where?

Ah right! now do you see why no one wants to talk science with you?

you've lied about me at least twice now in this thread alone
i'm outta here.
ah, so if we demonstrate where your opinion is wrong, we're not playing fair, but you can flat out lie, ignore evidence and then produce opinion and state it's equivalent to science and then get mad when this is pointed out?

ROTFLMFAO

you'll be back
you are either being paid to post on climate threads or you're an obsessive delusional who can't help but post on them

you're no different than rc in that regard
NeMaTo
5 / 5 (5) Jan 04, 2019
I must have missed this in the article. Is the cycle periodicity still on track? And if so, where are we in that period?


Yes, and we are in a low-northern-summer-insolation period, with the predicted dry Sahara. The source article has a nice graph (search for "Eccentricity-tilt-precession (ETP) parameter"). http://advances.s...eaav1887

rrwillsj
1 / 5 (1) Jan 04, 2019
This is one of the things that bug me on these issues. That so many commentators want one single magical explanation. Chiseled in unchanging stone.
To explain Natural Phenomena.

A wobbly Earth seems to be the foundation of desertification, However, it is not the only cause.

Especially the last however many hundreds of thousands of years, With our ancestral Hominids liberally using fire to alter their environment.

Foreshadowing the last ten thousand or so years. When our ancestral Humans compounding the Anthropocentric damage with overgrazing of domestic animals.
SURFIN85
not rated yet Jan 11, 2019
Made the mistake commenting. Great article

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