6,000 years ago, the Sahara desert was tropical—what happened?

December 1, 2016, Texas A&M University
The Sahara desert was once a tropical jungle. Credit: Texas A&M University

As little as 6,000 years ago, the vast Sahara Desert was covered in grassland that received plenty of rainfall, but shifts in the world's weather patterns abruptly transformed the vegetated region into some of the driest land on Earth. A Texas A&M university researcher is trying to uncover the clues responsible for this enormous climate transformation – and the findings could lead to better rainfall predictions worldwide.

Robert Korty, associate professor in the Department of Atmospheric Sciences, along with colleague William Boos of Yale University, have had their work published in the current issue of Nature Geoscience.

The two researchers have looked into precipitation patterns of the Holocene era and compared them with present-day movements of the intertropical convergence zone, a large region of intense tropical rainfall. Using computer models and other data, the researchers found links to rainfall patterns thousands of years ago.

"The framework we developed helps us understand why the heaviest belts set up where they do," Korty explains.

"Tropical rain belts are tied to what happens elsewhere in the world through the Hadley circulation, but it won't predict changes elsewhere directly, as the chain of events is very complex. But it is a step toward that goal."

The Hadley circulation is a tropical atmospheric circulation that rises near the equator. It is linked to the subtropical trade winds, tropical rainbelts, and affects the position of severe storms, hurricanes, and the jet stream. Where it descends in the subtropics, it can create desert-like conditions. The majority of Earth's arid regions are located in areas beneath the descending parts of the Hadley circulation.

"We know that 6,000 years ago, what is now the Sahara Desert was a rainy place," Korty adds.

"It has been something of a mystery to understand how the tropical rain belt moved so far north of the equator. Our findings show that that large migrations in rainfall can occur in one part of the globe even while the belt doesn't move much elsewhere.

"This framework may also be useful in predicting the details of how tropical rain bands tend to shift during modern-day El Niño and La Niña events (the cooling or warming of waters in the central Pacific Ocean which tend to influence weather patterns around the world)."

The findings could lead to better ways to predict future rainfall patterns in parts of the world, Korty believes.

"One of the implications of this is that we can deduce how the position of the rainfall will change in response to individual forces," he says. "We were able to conclude that the variations in Earth's orbit that shifted north in Africa 6,000 years ago were by themselves insufficient to sustain the amount of rain that geologic evidence shows fell over what is now the Sahara Desert. Feedbacks between the shifts in rain and the vegetation that could exist with it are needed to get heavy rains into the Sahara."

Explore further: The climatic toll of volcanic eruptions

More information: William R. Boos et al. Regional energy budget control of the intertropical convergence zone and application to mid-Holocene rainfall, Nature Geoscience (2016). DOI: 10.1038/ngeo2833

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47 comments

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RichManJoe
1.1 / 5 (9) Dec 01, 2016
So tell me something I didn't know. I learned about why deserts form under regions where the air currents descend when I was in high school 50 years ago. I knew these circulation currents moved from reading about weather effects due to the ozone hole over the antarctic. This article did not tell me why it moved over the Sahara, as the title leads me to believe it would. Another waste of time article from phys.org.
antialias_physorg
4.3 / 5 (11) Dec 01, 2016
So tell me something I didn't know. ...

From what you post you didn't understand one word (or didn't even bother to read) the article.
Well done.

*slow clap*
Shootist
1.7 / 5 (12) Dec 01, 2016
6,000 years ago, the Sahara desert was tropical—what happened?


Climate change - doh!
antigoracle
1.3 / 5 (12) Dec 01, 2016
It was those pesky hoomans in their ess-ou-vees.
cantdrive85
1.7 / 5 (11) Dec 01, 2016
6,000 years ago, the Sahara desert was tropical—what happened?

Campfires and AGW....the science is settled!
Benni
2 / 5 (4) Dec 01, 2016
"We were able to conclude that the variations in Earth's orbit that shifted rainfall north in Africa 6,000 years ago were by themselves insufficient to sustain the amount of rain that geologic evidence shows fell over what is now the Sahara Desert."
............so how were they able to "conclude that the variations in Earth's orbit" was "insufficient"? What are they talking about, PRECESSION?

MR166
1.9 / 5 (12) Dec 01, 2016
So 6000 years ago the Sahara was grasslands but today even the slightest variation from weather norms is due to AGW. Mark my words, we are just one or two cold winters away from the claim that CO2 causes cooling.
Telekinetic
4.2 / 5 (10) Dec 01, 2016
@MR166:

This year, 2016, has been declared the hottest year on record. It's not a biased opinion, it's a fact of record, Since the industrial age of unmitigated chugging smokestacks and fossil fuel energy production, the steady climb of the earth's overheating over the past 200 years is traceable by very credible scientific measurement. Maybe you think that we live on a flat earth and the numbers are skewed to fit a round earth model.
wjr321
4.1 / 5 (7) Dec 01, 2016
Goats. Surface reflectivity is influenced by color. Goats and their overgrazing destroy top soil. The result is rocky soil from erosion and a change in reflective color with the result that local air temperature and the consequent thermals change.

It's not just the Sahara but also the much of the Middle East -- look at the area of the dead cities in Syria. Famous ancient cities whose ruins stand in a wasteland.

Nonlinear dynamical systems are a bitch when you change the initial conditions.
MR166
2.1 / 5 (7) Dec 02, 2016
"This year, 2016, has been declared the hottest year on record. It's not a biased opinion, it's a fact of record,"

The key word in your sentence is "Declared"! since about 96 there has been no statistically significant increase in temperatures. Now that El Nino is over and a new solar minimum is upon us cooling could become a real concern.
howhot3
3.4 / 5 (5) Dec 02, 2016
Following @antigoracle lead; the article does bring up a question... Isn't the Earth only 6000 yrs old? So how the hell can it have been all green and pretty 6000 yars ago? Wasn't weez alls covered in the great flood at that time? And wasn't the planet saved by Noa. DId Noa really capture all that bacteria(s) two by two and wouldn't that make him a bio-tearist now? These is important questions uze climate skeptics haz answers for right?
TheGhostofOtto1923
1 / 5 (2) Dec 02, 2016
Goats. Surface reflectivity is influenced by color. Goats and their overgrazing destroy top soil. The result is rocky soil from erosion and a change in reflective color with the result that local air temperature and the consequent thermals change.

It's not just the Sahara but also the much of the Middle East -- look at the area of the dead cities in Syria. Famous ancient cities whose ruins stand in a wasteland
I was going to say this very thing. But not just the ME - a belt stretching from the Sahel to the gobi - overgrazed, overirrigated, saltified, desertified. The results of overpopulation. Swine are the last - they chew the bark off trees and dig up the roots. This is probably why they are verboten to moslems and jews.

Nebucchadnezzar had a grand plan to strip the top few feet of soil from the entire euphrates valley as it had become saltified due to irrigation. He gave up.
howhot3
1 / 5 (2) Dec 02, 2016
GOATS! Why didn't I think of that?!? Surely the white fleece of a goat would reflect so much sunlight back to space as to effect the albedo of the middle-east and cool it! So that is why 6000 years ago, the Sahara was wet, green and luscious, Lots of goats. Ok and the Pig idea eating the bark... I give up.
antigoracle
1.5 / 5 (8) Dec 02, 2016
GOATS! Why didn't I think of that?!?

Perhaps because you have your head so far up your man crush, Al's rear end, that no one can tell where he ends and you begin. Then again, you are just plain stupid. Now, stick your head back up there.
https://www.googl...ypocrisy
Phys1
4 / 5 (4) Dec 03, 2016
@phys.org
How can you allow this antigoracle filth?
Don't you read English ?
Phys1
4 / 5 (4) Dec 03, 2016
GOATS! Why didn't I think of that?!? Surely the white fleece of a goat would reflect so much sunlight back to space as to effect the albedo of the middle-east and cool it! So that is why 6000 years ago, the Sahara was wet, green and luscious, Lots of goats. Ok and the Pig idea eating the bark... I give up.

Overgrazing can contribute to desertification.
Now you know.
https://en.wikipe...rgrazing
billpress11
5 / 5 (2) Dec 03, 2016
The thawing of the ice age changed the climate over many parts of earth.
antigoracle
Dec 03, 2016
This comment has been removed by a moderator.
Phys1
1 / 5 (1) Dec 03, 2016
@insanicle
You brought this up several times already.
You seem to like the image of babies being dropped with their head on the floor.
Obviously you are a very sick person.

Phys.org, why do you put up with this sicko ? Is he paying you ?
TheGhostofOtto1923
not rated yet Dec 03, 2016
@insanicle
You brought this up several times already.
You seem to like the image of babies being dropped with their head on the floor.
Obviously you are a very sick person.

Phys.org, why do you put up with this sicko ? Is he paying you ?
I suspect sinister influences.
https://en.wikipe...i/Moloch

-And what of the vast cedar forests of Lebanon? Deforestation.
https://en.wikipe...s_of_God
baudrunner
not rated yet Dec 03, 2016
None of any of this explains where all that sand came from.
billpress11
5 / 5 (3) Dec 03, 2016
My guess is the sand was there all along. Over thousands of years the dry dusty top soil was blown away leaving behind the larger heavier sand particles. Even today dust and lighter sand particles are being blown all the way to America.
TheGhostofOtto1923
3 / 5 (4) Dec 03, 2016
Why guess when Google?

"This sand was washed in by rivers or streams in distant, less arid times – often before the area became a desert. Once a region becomes arid, there's no vegetation or water to hold the soil down. Then the wind takes over and blows away the finer particles of clay and dried organic matter. What's left is desert sand... Wind slammed sand into rocks, wearing those rocks away, which created more sand. Hot days and freezing nights subjected rock outcroppings to extreme temperature changes. This daily expansion and contraction eventually shattered mineral crystals in the rocks so that the cliffs broke into boulders, boulders shattered into pebbles, and pebbles crumbled into sand."

-And why wouldn't you think geologists would already know this?
BurnBabyBurn
3 / 5 (2) Dec 03, 2016
Phys1 5 / 5 (2) 12 hours ago
@phys.org
How can you allow this antigoracle filth?
Don't you read English ?



No, they read hit counts. Your noting him is money in their pocket. He'll soon be sorted, though.
BurnBabyBurn
1 / 5 (3) Dec 03, 2016

TheGhostofOtto1923 not rated yet 1 hour ago
Why guess when Google?


"Wherever two or three are gathered in My name, there inane speculation is in their midst!"

BurnBabyBurn
2.3 / 5 (3) Dec 03, 2016
Phys1 not rated yet 7 hours ago
@insanicle
You brought this up several times already.
You seem to like the image of babies being dropped with their head on the floor.
Obviously you are a very sick person.


Reality check: you're trying to suss out the psychology of someone online that is terrified of Al Gore. Terrified of Al Gore. Think about that. It's December, 2016, and his whole online identity is based on terror of Al Gore. Send the slob a "Feckless from Birth" tee.


Phys.org, why do you put up with this sicko ? Is he paying you ?


Already answered.
howhot3
4 / 5 (4) Dec 03, 2016
From my sarcasm abour someones comment that goats caused the Great Sahara:
GOATS! Why didn't I think of that?!?
To which @antigoracle says;
Perhaps because you have your head so far up your man crush, Al's rear end, that no one can tell where he ends and you begin. Then again, you are just plain stupid. Now, stick your head back up there.

While I admire Al Gore for his political positions and activism on the environment and global warming, I don't have what @antigoracle calls a "Man Crush". The rest of what @antigoracle says is basically rude and I take offence of.

That said, @antigoracle maybe projecting his own issues with homophobia and gay-ness. So @antigoracle, I'm sorry you have issues concerning your manliness (or machismo in spanish). Was it your concern of goats that brought this out?

MR166
3 / 5 (4) Dec 03, 2016
"While I admire Al Gore for his political positions and activism on the environment and global warming........"

How can you possibly admire someone who's only claim to fame is based on wild exaggerations and fraudulent misrepresentations?
howhot3
1 / 5 (1) Dec 03, 2016
"While I admire Al Gore for his political positions and activism on the environment and global warming........"

How can you possibly admire someone who's only claim to fame is based on wild exaggerations and fraudulent misrepresentations?

Maybe you are projecting too much of your climate denier nonsense on him. He doesn't exaggerate nor are his claim fraudulent. It comes down to who is credible and there isn't a single one you can site that has more brains than a toad stool or is just a flat out liar. But you choose to believe that shit, so please have at it my friend. It's all on you.
Zzzzzzzz
3 / 5 (2) Dec 03, 2016
Antigoracle is an obvious shit eater who has been on my ignore list for a long time. I also don't understand why a putrid pile of filth like this is tolerated, but the fact is you don't have to actually see his stench or smell his retchedness. I for one do not become contaminated by his diseased breath. The ignore list is a usefully thing.
antigoracle
1 / 5 (5) Dec 03, 2016
While I admire Al Gore for his political positions and activism on the environment and global warming, I don't have what @antigoracle calls a "Man Crush". The rest of what @antigoracle says is basically rude and I take offence of.

That said, @antigoracle maybe projecting his own issues with homophobia and gay-ness. So @antigoracle, I'm sorry you have issues concerning your manliness (or machismo in spanish). Was it your concern of goats that brought this out?

@howshat
LOL
http://www.urband...%20Crush
Now, with that response I know you are gay for Al.
Now go stick your head back up there before he dumps you.
howhot3
3 / 5 (2) Dec 03, 2016
@antigoracle; I could say the same thing for you, but sub trump for gore, but I won't. Instead, this from Dante's Inferno seem to have meaning here; "Love insists the loved loves back".
HeloMenelo
3.9 / 5 (7) Dec 04, 2016
So tell me something I didn't know. I learned about why deserts form under regions where the air currents descend when I was in high school 50 years ago. I knew these circulation currents moved from reading about weather effects due to the ozone hole over the antarctic. This article did not tell me why it moved over the Sahara, as the title leads me to believe it would. Another waste of time article from phys.org.


socks of antiscience gorilla can't learn, dunno why, must be their baboonish instinct that overrides logic and sense
HeloMenelo
3.3 / 5 (7) Dec 04, 2016
Overgrazing can contribute to desertification.
Now you know.

Being repeatedly dropped on the head as a baby can contribute to retardation.
You are living proof.


Monkey gorilla was dropped out of the tree on his head and that even did not help to get him to understand science, he just can't break that 1 out of 5 barrier not even after 15 years LOL
Phys1
3.7 / 5 (3) Dec 04, 2016
How can phys.org allow this ?
How come we don't see phishing or prostitution oriented spam, or extremist views? It seems that only filth that defames science is let through by phys.org.
Does anybody have a conspiracy theory on that?
Phys1
3 / 5 (2) Dec 04, 2016
While I admire Al Gore for his political positions and activism on the environment and global warming, I don't have what @antigoracle calls a "Man Crush".

Only perverts have this in their vocabulary and the unfortunates like me who have read his filth and understood the misanthropist that secreted it.
Phys1
4.2 / 5 (5) Dec 04, 2016
Could antigoracle be a member of the editorial board?
That could explain why he's still around.
Is this blog run by perverts ?
antigoracle
1 / 5 (4) Dec 04, 2016
Was physRetard dropped as a baby, or is he retarded because he's been practicing.
That could explain why he's retarded.
Do retards run in his family?
Caliban
4.3 / 5 (6) Dec 04, 2016
For a long time, now this issue has been theorized to be a combination of the two things:

1. The shifting of atmospheric circulation following the end of the last ice age, shifting of rain towards the equator, and the consequent aridification of the lower lattitudes as a result. The process takes quite a long time.

and

2. The overgrazing of the Med. basin, N Africa, the M East, W Asia and C Asia following the domestication of goats, sheep, and other foraging animals by humans. Overgrazing, combined with a drying climate spells environmental disaster. Add large-scale agriculture as icing on the cake.

This study is focused upon atmospheric aspects of the transformation, and so doesn't consider other contributing factors.
Phys1
3 / 5 (2) Dec 04, 2016
Was physRetard dropped as a baby, or is he retarded because he's been practicing.
That could explain why he's retarded.
Do retards run in his family?

This is what I predicted, that insanicle would degenerate into using "retard" in each and every sentence.
Soon he will end up in a straightjacket.
Phys1
4 / 5 (4) Dec 04, 2016
The increased level of perversion in his posts is another sign that his condition is rapidly deteriorating. Loss of decorum is a clear sign of dementia as well.
promytius1
5 / 5 (2) Dec 05, 2016
I had a thought while reading the article about the flexibility of perceptions of time. Then I read the comments. Won't waste my time here, no science going on in the hate-you-comments that have nothing to do with science and only expose the less admirable aspects of our own human nature.
My pearls are not for you swine.
barakn
5 / 5 (3) Dec 05, 2016
None of any of this explains where all that sand came from.

The Sahara has been a desert intermittently for the last 7 million years.
cantdrive85
2.3 / 5 (3) Dec 06, 2016
Loss of decorum is a clear sign of dementia as well.

That would explain Cap'n Stoopid's posts...
Phys1
3.7 / 5 (3) Dec 06, 2016
@cd
Are you trying to distract attention form the unspeakable posts of lunaticle ?
Captain Stumpy
5 / 5 (4) Dec 07, 2016
@eu pseudoscience cult
That would explain Cap'n
funny thing... all i ever ask for was peer reviewed evidence that can be verified. something that isn't already falsified... something that adheres to the scientific method and isn't argument from opinion.

i follow the evidence... but all you've ever provided was pseudoscience, argument from rhetoric or continuously repeated false claims

who is the dementia patient?
the evidence clearly points to you...
antigoracle
1 / 5 (1) Dec 08, 2016
funny thing... all i ever ask for was peer reviewed evidence that can be verified. something that isn't already falsified... something that adheres to the scientific method and isn't argument from opinion.

i follow the evidence... but all you've ever provided was pseudoscience, argument from rhetoric or continuously repeated false claims

funny thing... well...not really...Stumpid asking for that which is beyond his comprehension.

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