Past river activity in northern Africa reveals multiple Sahara greenings

The analysis of sediment cores from the Mediterranean Sea combined with Earth system models tells the story of major environmental changes in North Africa over the last 160,000 years


Research News


Large parts of today’s Sahara Desert were green thousands of years ago. Prehistoric engravings of giraffes and crocodiles testify to this, as does a stone-age cave painting in the desert that even shows swimming humans. However, these illustrations only provide a rough picture of the living conditions. Recently, more detailed insights have been gained from sediment cores extracted from the Mediterranean Sea off the coast of Libya. An international research team examined these cores and discovered that the layers of the seafloor tell the story of major environmental changes in North Africa over the past 160,000 years. Cécile Blanchet of the German Research Centre for Geosciences GFZ and her colleagues from Germany, South Korea, the Netherlands and the USA report on this in the journal Nature Geoscience.

Together with the GEOMAR Helmholtz Centre for Ocean Research Kiel, a team of scientists organized a research cruise on the Dutch vessel Pelagia to the Gulf of Sirte in December 2011. “We suspected that when the Sahara Desert was green, the rivers that are presently dry would have been active and would have brought particles into the Gulf of Sirte”, says lead author Cécile Blanchet. Such sediments would help to better understand the timing and circumstances for the reactivation of these rivers.

Using a method called “piston coring”, the scientists were able to recover 10-meters long columns of marine mud. “One can imagine a giant hollow cylinder being pushed into the seafloor”, says co-author Anne Osborne from GEOMAR, who was onboard the research ship. “The marine mud layers contain rock fragments and plant remains transported from the nearby African continent. They are also full of shells of microorganisms that grew in seawater. Together, these sediment particles can tell us the story of past climatic changes”, explains Blanchet.

“By combining the sediment analyses with results from our computer simulation, we can now precisely understand the climatic processes at work to explain the drastic changes in North African environments over the past 160,000 years”, adds co-author Tobias Friedrich from the University of Hawai’i.

From previous work, it was already known that several rivers episodically flowed across the region, which today is one of the driest areas on Earth. The team’s unprecedented reconstruction continuously covers the last 160,000 years. It offers a comprehensive picture of when and why there was sufficient rainfall in the Central Sahara to reactivate these rivers. “We found that it is the slight changes in the Earth’s orbit and the waxing and waning of polar ice sheets that paced the alternation of humid phases with high precipitation and long periods of almost complete aridity”, explains Blanchet.

The fertile periods generally lasted five thousand years and humidity spread over North Africa up to the Mediterranean coast. For the people of that time, this resulted in drastic changes in living conditions, which probably led to large migratory movements in North Africa. “With our work we have added some essential jigsaw pieces to the picture of past Saharan landscape changes that help to better understand human evolution and migration history”, says Blanchet. “The combination of sediment data with computer-simulation results was crucial to understand what controlled the past succession of humid and arid phases in North Africa. This is particularly important because it is expected that this region will experience intense droughts as a consequence of human-induced climate change.”


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Jay Willis
January 30, 2021 2:27 am

This is particularly important because it is expected that this region will experience intense droughts as a consequence of human-induced climate change.”

Oh good grief. The stupid never stops.

Brian Jackson
Reply to  Jay Willis
January 30, 2021 3:03 am
So 160,000 years with no signigicant human production of CO2 resulted in enormous changes to the landscape and climate, desert to jungle and back again. Due to orbital changes.
So now, with further minimal additions of human produced CO2, there are going to be more massive changes, presumably unrelated to previous causal mechanisms.
What a load of utter bo – – ocks.
And people get paid for this rubbish?
I really missed my way when I went down ‘t pit wi’ mi’ dad.

Ian Magness
Reply to  Brian Jackson
January 30, 2021 3:07 am

Given the rather unpleasant nature of a small number of people who stalk sites such as this, I would remove your email address pronto. Or perhaps WUWT could do it?

John Dilks
Reply to  Ian Magness
January 30, 2021 9:46 am

Ian Magness,
I can’t figure out what you are upset about. I have read his comment several times and all I see is sarcasm.

Reply to  John Dilks
January 30, 2021 2:49 pm

He’s trying to protect Brian… his email is public in his comment… and shady ill-intention people lurk here.

Reply to  Derge
January 31, 2021 11:28 am

They don’t just lurk.

Reply to  Ian Magness
January 30, 2021 12:28 pm

Ian, I believe that Brian’s criticism (and snark) is directed, not at Charles Rottter, but at the authors of this piece, GFZ GEOFORSCHUNGSZENTRUM POTSDAM, HELMHOLTZ CENTRE.

I heartily concur with his assessment- human induced climate change is going to cause a drought in the Sahara Desert?

That is rich, indeed.

Bob Hoye
Reply to  Ian Magness
January 30, 2021 7:46 pm

Ian–CO2 has absolutely nothing to do with real climate change.
It should be taught in Geology 101.

Ed Zuiderwijk
Reply to  Jay Willis
January 30, 2021 4:07 am

That is the compulsory sop to get the paper past the censors. It doesn’t mean a thing in the context of the paper.

Reply to  Ed Zuiderwijk
January 30, 2021 4:24 am

Precisely! This is the way we pass ideas when some sort of censorship in underway. Nowadays I cannot pass any project related with environment If it does not have some phrase of this sort.

Reply to  J N
January 30, 2021 9:47 am

Yeah, politics. I wrote requests for research funding from DARPA from mid 70s (Ford) – early ’90s (Clinton) and included unrelated “buzz” words in the introductions and executive summaries just to get attention. The buzz changed with each administration and went cuckoo with AGore’s reinvention of the government.

Dave Fair
Reply to  czechlist
January 30, 2021 12:17 pm

After 11 years of significant advancements in government employment, I quit the Feds over this sort of crap.

Reply to  Dave Fair
January 31, 2021 4:43 am

Me too. I did nearly six years of research for the Federal government before my self-respect got the better of me.

Reply to  czechlist
January 30, 2021 12:29 pm

I absolutely believe you.

Smart Rock
Reply to  Ed Zuiderwijk
January 30, 2021 4:38 pm

Exactly, Ed. I remember reading English translations of papers from the old Soviet Union, when I was a student in the 1960s. They very often had a reference in the introduction to the latest pronouncements of the Comrade General Secretary. Then they went on to present their data. It just meant that they were good party members, and that’s all.

Reply to  Jay Willis
January 30, 2021 5:49 am

I actually find this excerpt quite interesting. I found the original paper but it was pay walled. To be honest I would probably not be able to understand it anyway as I have a technical but not scientific background. I have no idea of what Cecile Blanchete’s climate beliefs are, but I believe it is impossible for a young scientist to obtain funding without throwing in a few lines about anthropological climate change. This of course distorts all science. To stay sane I have to believe some young scientists are tongue in cheek when they throw these lines in. I know, I know. I should take off my rose colored glasses.

John Tillman
Reply to  Mariner
January 30, 2021 11:24 am
Dave Fair
Reply to  John Tillman
January 30, 2021 12:21 pm

Socialist (Marxist?) to the core. Their reasoning would lead to a world with no private property. Sure, that will work out well.

Last edited 1 month ago by Dave Fair
Reply to  John Tillman
January 30, 2021 2:09 pm

Thanks John, I didn’t find that. I guess they have all been corrupted. I am apprehensive for the future but at 73 sorry I will (probably) not see the collapse.

Dave Fair
Reply to  Jay Willis
January 30, 2021 12:13 pm

Somehow, institutions and journals have to start making CliSci practitioners justify their throw-away statements about human-induced climate change. Citations must be made mandatory. The same for ‘climate emergency,’ etc.

Tom Abbott
Reply to  Jay Willis
January 31, 2021 5:58 am

“Oh good grief. The stupid never stops.”

They have to get that Human-caused Climate Change mention in there in order to get paid.

While reading, I was wondering when the climate change angle would come in, and there it was, in the last sentence.

Other than that, I thought this quote in the article was the most important:

“We found that it is the slight changes in the Earth’s orbit and the waxing and waning of polar ice sheets that paced the alternation of humid phases with high precipitation and long periods of almost complete aridity”, explains Blanchet.”

end excerpt

So the real news from this article is the changes in the Sahara are related to orbital changes, not Human-caused Climate Change.

Ron Long
January 30, 2021 2:43 am

So, natural cycles and oscillations for 160,000 years, climate changes producing drought and greening and rivers, but now anything that happens is CAGW? Nothing more than a pitifully obvious attempt at more research funding. Some good science gone astray.

Rod Evans
January 30, 2021 3:12 am

Reading this article, culminating in the very last line of crazy logic reminds me of the old joke.
A man (it’s always a man by the way) who has been confined in the severe mental institution for many years, has been demonstrating a marked transition towards normal behaviour. The assessing psychiatrist is most impressed with the rational responses he has received at the final interview before a decision is made on the patients future.
As the doctor is exiting the door, he is hit on the back of the head by a heavy book thrown by the patient under review. He hears the fateful words, “Don’t forget to tell em I am normal now doc”
The absolute need to throw in the fateful AGW message never leaves the minds of the on message scientists does it.
This is particularly important because it is expected that this region will experience intense droughts as a consequence of human-induced climate change.”

Makes you wonder what the conditions of the Sahara have been this past few thousand years if drought is to be expected soon eh?

Jim Gorman
Reply to  Rod Evans
January 30, 2021 4:28 am

I agree, what in blazes has the Sahara been experiencing for the last what, several millennia. Does a CAGW drought look different from that? How do these people or their funders not realize how stupid “intense droughts” in an already existing desert sounds? Is it only going to get 1 inch of rain instead of 2 inches?

Dave Fair
Reply to  Rod Evans
January 30, 2021 12:24 pm

CO2 ‘greening’ must have missed her review.

January 30, 2021 3:30 am

So what did they actually discover. I don’t see any results whatsoever. Where’s my jigsaw pieces? I want to see them.

Ben Vorlich
January 30, 2021 3:53 am

I’d like to know when was the last time if changed from green to desert and does this coincide with anything in the historical/Archaeological record?

Reply to  Ben Vorlich
January 30, 2021 4:23 am

The Sirte Basin, being a major oil area , has also a lot of water :
Groundwater recharge and palaeoclimate in the Sirte and Kufra basins, Libya

“his water with an age of ± 7800 yr. BP is chemically and isotopically distinct from the regional groundwaters and provides direct evidence of a significant recharge event during the Holocene.”
I read that as around 7800BP, which would match the change in the Nile civilization known as the “egyptians”.

Reply to  Ben Vorlich
January 30, 2021 4:26 am

About 7,000 years ago, IIR

Ewin Barnett
January 30, 2021 4:23 am

Where careful and thoughtfully conducted original research is used to support the broader contemporary lie, a lie intended to support imposing the socialist utopia upon humanity, a utopia that cannot allow truthful science.

January 30, 2021 5:00 am

Northern Africa, the Arabian peninsula and parts eastward have been experiencing drought for a very long time without the presence of mobs of Hoomans, she has to throw that idiocy into the mix to get past the censors?

Okay, sure. Whatever, just get the cash, right?

dodgy geezer
January 30, 2021 6:16 am

I can see no great advantage in listing papers which indicate that the Catastrophic AGW hypothesis is not correct.

Such papers might be of interest to dispassionate scientists. But I don’t think there are any of those left working in the Climate speciality. And the establishment grandees and politicians will either ignore or actively suppress such papers.

I don’t know where we need to be fighting, but it’s sure as hell not in the area of proper science…

Peta of Newark
January 30, 2021 6:32 am

Hun, where did all the sand come from?
What sort of Weather or Climate phenomenon can dump pure silica sand over such a large area, to depths of 10’s of metres?
Clue: It was there already, it always was there….

You assert that (or does your computer assert, it patently is The Brains in all this)
“”slight changes in the Earth’s orbit””
Maybe yes, maybe not

What would be the change in incident Solar Energy/Power because of that?
Serious question hun, You say that’s what happened, tell us what did happen. Put some numbers to it.
Show me pictures or It didn’t happen
If you put them behind a paywall, or in fact any wall, what are you embarrassed about, what are you hiding?

Meanwhile, let’s do our own calculation.
Lets say that peak noon-time solar energy on the Saharan Region is 1,200 Watts per square metre.
To get a daily average and assuming Sol follows a roughly sinusoidal path, we need half of root 2 of that value –
I get 424 Watts per sqm. = Average solar input over 24 hours.

Now, lets put a Rainforest on the ground and we’ll allow it an albedo of 0.45
Thus the forest absorbs 233 watts per sqm or just over 20 mega Joule daily

If, by ‘some unknown catastrophe’ that forest burned to the ground, it would present a rather black face toward El Sol, lets say an albedo of 0.1
Thus absorbing 380 Joules per second per square metre.
Just shy of 33 mega Joule daily.
That’s a lot innit….

How do/did Earth’s Orbital Variations change the energy input compared to those figures?
While we’re waiting. we’ll assume they don’t match very well, we’ll start wondering ‘What Happened There’

Is it beyond the bounds of reality that some ingenious little critters, looking for a bite to eat, caused The Forest to ‘go black’
What sort of critters might do that?

Howzabout critters that cannot very well climb trees or critters that cannot fly or critters that find it difficult just to move about among the trees and undergrowth.
Would not The Removal Of The Trees be of significant help to those critters?

Did Climate come to help those critters remove the trees OR – did they help themselves to a spot of lumber-jacking?

Wonder who or what they were?
Wonder if they left any trace of themselves?

So many questions………..

Last edited 1 month ago by Peta of Newark
Reply to  Peta of Newark
January 30, 2021 9:38 am

This short video will suggest some reasonable answers to Peta’s questions. There are others, perhaps more interesting in total but generally longer. Once, relatively recently in geological terms, much of the Sahara was a shallow sea.

Reply to  Peta of Newark
January 30, 2021 10:19 am

I am not here to argue but just to add to the questions you solicit. Isn’t desertification a result of cold temps rather than hot? Cold kills vegetation and dries the air. Dry winds erode rock …

Mumbles McGuirck
January 30, 2021 6:39 am

I wish we could see their results without having to pay for them. It seems that the last greening of the desert was near the Holocene Optimum, when the atmosphere was warmer. It seems that if the atmosphere is warming (human induced or not) then the Sahara should be greening again, not headed for more droughts as they contend.

January 30, 2021 6:52 am

A warmer world has more rainfall. But about 3 C before there is enough rainfall to green North Africa. And the increased cloud cover that goes along with the increased rainfall fights against you every degree. So greening of the Sahara is a long time away.

January 30, 2021 7:12 am

A chart of the past would be helpful. That’s a nice giraffe rock carving….the oldest art goes back 45,500 years ago to an island in Indonesia – a pig in a cave. I seem to remember seeing some aerial or satellite or Lidar pictures of the Sahara showing meandering river outlines. The last greening was not rainforest but more grasslands / wetlands type.

Last edited 1 month ago by T. C. Clark
Joseph Zorzin
January 30, 2021 7:16 am

I’m no geologist but I thought I read once that the wetter periods in north Africa occured when the glaciers advanced over northern Europe- pushing Europe’s weather to the south. So, a slight warming trend without glaciers advancing in Europe shouldn’t result in a new wet Sahara.

John Tillman
Reply to  Joseph Zorzin
January 30, 2021 11:28 am

The ice sheets were gone during the Holocene Optimum after about 8 Ka, except for those still extant, on Greenland and Antarctica.

January 30, 2021 8:09 am

Just a couple of days ago Bova et al managed to adjust Holocene temperatures down to match the climate model.

The authors claim the Holocene Optimum was not as warm as previously thought.

The interesting bit is they seem to have discovered a way of using foraminifera as a proxy with seasonal resolution whereas all previous studies could only achieve 4-year resolution.


John Tillman
Reply to  Redge
January 30, 2021 11:42 am

“We need to get rid of the Holocene Climate Optimum.”

Never mind that sea level was higher.

Dave Fair
Reply to  John Tillman
January 30, 2021 1:13 pm

CliSci leads to pall review or vice versa.

Dave Fair
Reply to  Redge
January 30, 2021 1:08 pm

“We further demonstrate that global mean annual sea surface temperatures have been steadily increasing since the start of the Holocene (about 12,000 years ago), first in response to retreating ice sheets (12 to 6.5 thousand years ago), and then as a result of rising greenhouse gas concentrations (0.25 ± 0.21 degrees Celsius over the past 6,500 years or so). 0.25 +/- 0.21 over 6,500 years says it all. Not to mention the fact that CO2 follows temperatures.

Last edited 1 month ago by Dave Fair
Dave Fair
Reply to  Redge
January 30, 2021 1:32 pm

This substantial model–data discrepancy undermines confidence in both proxy reconstructions and climate models, and inhibits a mechanistic understanding of recent climate change.” So, lets ignore problems with the unvalidated models and put our CliSci microscope on proxy reconstructions; dick around with methods and assumptions until we get what our ideology tells us to be true.

Reply to  DSchmitt
January 30, 2021 11:09 am

Thirty years ago when my brother was digging wells in northern Ghana, the nascent greening was a ‘good’ thing – more food, less destructive agriculture. I guess the peoples of the Sahel do not matter as long as the politicians get UN cash.

Mike Dubrasich
January 30, 2021 12:44 pm

Gripe not, science fans. The study cited above builds on many previous studies of past Saharan greening, many of which are not behind pay walls. For instance, Hoffmann et al (2016) used cave formations to date N. African wet periods, which they attributed to Milankovic cycles:

Hoffmann, D., Rogerson, M., Spötl, C. et al. Timing and causes of North African wet phases during the last glacial period and implications for modern human migration. Sci Rep 6, 36367 (2016).

…the first speleothem-derived central North Africa rainfall record for the last glacial period. The record reveals three main wet periods at 65-61 ka, 52.5-50.5 ka and 37.5-33 ka that lead obliquity maxima and precession minima. We find additional minor wet episodes that are synchronous with Greenland interstadials. Our results demonstrate that sub-tropical hydrology is forced by both orbital cyclicity and North Atlantic moisture sources. …

Marine sediments have been sampled before, too. Here is a paper with a graph showing the findings together with orbital insolation cycles:

Skonieczny, C., Paillou, P., Bory, A. et al. African humid periods triggered the reactivation of a large river system in Western Sahara. Nat Commun 6, 8751 (2015).

…Marine sediment records from the Mediterranean and Atlantic margins have provided consistent evidence of monsoon variability in northern Africa since the middle of the Pleistocene. The succession of past African humid periods (AHPs) is probably best documented by the deposition of organic-rich sediment layers (sapropels) in the Eastern Mediterranean basin, which has been linked to periods of enhanced discharge from the Nile River back to the Pliocene. The most recent AHP, during the early Holocene, spans from ~11,700 to 5,000 years BP and is well recorded in a number of marine sedimentary archives from the Gulf of Guinea to the Northeastern Tropical Atlantic Ocean, the Mediterranean margin and the east of Africa. …

Caption for Fig 2 below: (a) Sapropels record from core ODP Leg 160 (site 967, Eastern Mediterranean Sea)38 together with summer insolation (June, July and August) at 25 °N (ref. 39). The AHPs—identified using the sapropels1 (except for the early Holocene AHP)—are highlighted in green. (b) Continental humidity index from grain-size measurements of core GeoB7920 (ref. 11; 20.75 °N;18.58 °W), (c) Terrigenous Flux6 of ODP658 site (20.75 °N; 18.58 °W), (d) the estimated latitudinal position of the sedimentary Sahara–Sahel boundary24 (black line) with its uncertainty (grey dashed lines). (e) Lake level status in East and North African basins.

Last edited 1 month ago by Mike Dubrasich
Wim Röst
Reply to  Mike Dubrasich
January 31, 2021 2:24 am

Mike Dubraisch, very good comment, in line with what Javier wrote before. I will repeat Javier’s important comment below:

 November 12, 2015 4:47 pm

Is what happened to North Africa / Sahara a consequence of Earths cooling in the past? Less heat, less rain?

No. The African Humid Periods are a recurrent phenomena about every 23,000 years linked to the precession cycle, modulated by the eccentricity cycle (Milankovitch) and reflected in the Mediterranean sediments sapropels (dark organic layers). They are caused by the northern migration of the Inter-Tropical Convergence Zone (ITCZ) due to the high latitudes receiving more insolation.
Each dark layer corresponds to an African Humid Period and their grouping reflects the interaction between the precessional and eccentricity cycles.
We now happen to be near a minimum in the precession cycle (bottom in about 1000 years). In about 8,000 years the Sahara will be green again. Some predictions about future climate can be done with high confidence.

WR: I can only add that more CO2 will make it easier for the Sahara to green: plants need less moisture because they use fewer stomata to inhale CO2 and in this way, they lose less H2O. More plants/forests that will grow with the same precipitation means that better soils (that keep more moisture) will develop and that air over the Sahara will be more humid. More convection will develop and by high convection more humid air from the tropical Atlantic will be attracted to the Sahara. Monsoons will develop earlier, will be stronger and will perform more inland. More CO2 would be a blessing for the Sahel and for all dry parts of Africa (and elsewhere in the (sub)tropics). Desserts will earlier become green than they did in the cold dry and CO2 poor Pleistocene era of the past 3 million years.

January 30, 2021 3:41 pm

This is particularly important because it is expected that this region will experience intense droughts as a consequence of human-induced climate change.”

What could be more devastating than a drought in the Sahara desert!

That has to be worse than we thought.
Or worse than someone thought?

Meanwhile greening and rains increase in the Sahara region as the ITCZ extends north with no help from obliquity but plenty from CO2.

Bob Hoye
January 30, 2021 7:44 pm

Given the scope of time, the author’s use of “drastic” change seems innapropoiate.

Andre Den Tandt
January 30, 2021 8:19 pm

The North-African origin of Rome’s food supply at the time of the emperors is well documented (Tacitus, Suetonius). Hundreds of ships brought wheat from areas that are now, Morocco, Tunisia, Libya and Egypt. North-Africa was Rome’s wheat-belt.

January 31, 2021 4:09 am

There is oil under the sand. That means there was plant growth there at one time. Nothing new here.

February 4, 2021 8:58 am

I believe that climate change in North and East Africa played a key role in the evolution of human language and sociability. Our ancestors lived on shorelines and spurred by changes in ecologies the small and competing populations were an evolutionary fastbreader reactor – like the Galapagos Islands that Darwin wrote about.

It is unfortunate that more people don’t know about the discoveries of frontline researchers. I write about this in my just published book

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