Why and when the Sahara Desert was green: new research

Why and when the Sahara Desert was green: new research




Carbon emissions ACCELERATED climate change. Climate change is natural. It's the ACCELERATED change that is the problem. Not enough time for species to adjust.


Carbon emissions are moving the climate into a mode it would not have entered without human intervention.

Your argument is similar to “Everyone would have died anyway; this thermonuclear winter just sped it up. Totally not humanity’s fault we live in a bunker.”

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Even natural climate changes can be problematic, such as drier climates precipitating the fall of the Roman empire or the late bronze age collapse.


Or The Samalas eruption - which is believed to have caused the Little Ice Age - and killed 90% of Europeans & North Americans at the time (55M people).

I imagine many millions of large mammals and birds and insects also died.


See also the Year Without Summer, 1816.

Apparently Frankenstein and Dracula were both born that 'summer' at one of Byron's gatherings. Something to pass the time indoors.


Meteor strikes are also natural. That doesn’t mean they are desirable.


The meteor isn’t the problem. It’s the delta-V of the meteor which is the problem.


So with the co2 forcing function, is it possible that Canada be glaciated again And that the Sahara be green?

That would fly in the face of conventional thinking which is that Canada will be temperate and the Sahara will be more arid.


The article is saying that the green Sahara coincides with periods of greater seasonal rainfall and humidity, caused by changes to the procession of the Earth and the effect that has on the seasons. It's possible that current anthropogenic climate change could make the Sahara greener, but it's also possible that it might not. In any case, the greening would probably take hundreds of years or more to happen.


Does anyone know where one can find the estimated size per year of the Sahara. All I kind find are reductionist crap like it’s 10% bigger now than 1920. But this does not say if it’s in a growing or shrinking trend just that it’s bigger now than then. Was it even bigger in 2000? And now shrinking from a top or has it been a gradual increase?


Surprised there's no mention of Sahara Pump Theory[1] in this article.

[1] https://en.wikipedia.org/wiki/Sahara_pump_theory


I'm not a historian, archeologist, or any sort of relevant expert.

But I've long had the pet theory that the Green Sahara is the origin of the Eden Myth.

Homo Sapiens origin, lush vegetation, turns to dessert for some reason beyond our comprehension...


I've always interpreted the Eden myth as a memory of the development of agriculture.

Before agriculture, people lived as hunters and (mainly) gatherers. The life of a Hunter-gatherer is, surprisingly, mostly leisure, because of the abundance of food and the low population density. Even Kalahari Bushmen, who had been forced to live in the dessert, were found to average only about 1 1/2 to 2 1/2 hours of productive labor a day in a study [1].

In the bible, God's punishment is [2]:

  “Cursed is the ground because of you;
    through painful toil you will eat food from it
    all the days of your life.
  It will produce thorns and thistles for you,
    and you will eat the plants of the field.
  By the sweat of your brow
    you will eat your food
  until you return to the ground,
    since from it you were taken;
  for dust you are
    and to dust you will return.”
Which seems very clearly to indicate that being cast out of Eden means that people now must work in the fields and do agriculture. Agriculture ends up requiring much more labor for worse quality food, but it generates more calories. This enables classes to develop and enforces an end to nomadic life, because the fields must be tended and the harvest stored and protected.

[1] Stone Age Economics by Marshall Sahlins (https://archive.org/details/StoneAgeEconomics_201611)

[2] https://www.biblegateway.com/passage/?search=genesis%203&ver...


I've always thought that being cast out of Eden was not so much a punishment, as merely an unavoidable consequence: having eaten from the tree of knowledge, we now also have the ability to appreciate suffering, and thus no longer experience paradise. That would make paradise a state of mind, rather than a place.

After eating the fruit God lists a number of consequences, which sounds somewhat like a punishment, but that could just as easily be interpreted as "well, I didn't want this to happen to you, but now that you got to this point, you'll have to deal with the following". That includes increased pain in giving birth (a consequence of having larger brains), and the rise of agriculture. Eve is also only named after eating the fruit, not before.

The snake gets cursed for what it did, and so does the ground, but not Man. God even goes so far to provide them with clothes, and acknowledges their growth.


The snake always seemed to me to be a Promethean figure in this story. It brought us knowledge, like Prometheus brought us fire, and got punished for it by a god.


I dunno, I mean the myth doesn't say "Then The Lord Smote the Garden Unto Sand", it says that Eden (and the plants) persisted in the same spot as before... Humans were just being actively blocked by an angel with a flaming sword.


I do not struggle to see echoes of an oral tradition recalling receding greenery. I can quite imagine a cult feeling expelled from Eden as the productive grasslands walked away.


Hey, that's Aziraphale! Good guy.


> Many people, meeting Aziraphale for the first time, formed three impressions: that he was English, that he was intelligent, and that he was gayer than a treeful of monkeys on nitrous oxide.


I think the general consensus is Mesopotamia that at that time was a lush green area. The myths after all probably originate from the city of UR.


Somewhat off topic, but if anyone's read Sand County Almanac by Aldo Leopold, he talks about how the American Southwest, specifically Arizona, used to be green. Apparently the familiar reddish brown resulted from human impact


I went to school in Socorro, NM which was very brown on a dry year and pretty green on a wet year and my understanding is that it was a pretty grassland before white folks showed up but overgrazing caused the creosote bush to move in.

If you go up the hills to the plains of San Augustin where the VLA radio telescope that is around 7000 feet in elevation and I think a bit cooler and wetter and that is a very pretty grassland where you see big herds of antelope grazing and you might think you were in Africa.


Considering that there are archaeological sites in the American Southwest that are posited to have been abandoned due to the disappearance of their water supply, it is hard to blame all the aridness on “white folks”. Moreover, when I cycled through the Anza-Borego State Park in California, several of the information boards discussed how hardscrabble the existence of the Native American peoples in this area was, because it was already very much desert and required special strategies to survive.


This is a solid point. I was at Mesa Verde, the national park with the cliff dwellings. Apparently the people who lived there just up and left, well before Columbus showed up on the other side of the continent. One theory is that the current Hopi people are their descendants.

It's pretty clear that bad things happen, even when white people aren't around.


It's not really a "theory" as anyone normal uses that term. We know beyond any reasonable doubt that there's significant genetic, cultural, and political ancestry tying modern Puebloan groups to the the people who inhabited Mesa Verde (and other earlier pueblos). "Most" went east to the Rio Grande area, but plenty went west too. Some Hopi ancestry also comes from other groups to the north, west and south (into modern Mexico) where Puebloan groups no longer exist.

With that said, Spanish and American colonization of the Southwest has radically changed the landscape. Much of what's now low brush used to be substantially more vegetated, similar to organ pipe national monument. Mesquite was rare outside intentional cultivation. It's actually an invasive species from the separate Chihuahuan deserts in Texas that was spread alongside cattle. Another fun fact is that prior to colonization, no part of AZ+NM was more than a day's walk from water (~20mi), which was important to long distance travel and later the Apache wars. That's no longer true due to groundwater depletion.


Do you have a citation on mesquite spreading from Texas with cattle? I ask because the aforementioned information boards in the Anza-Borrego State Park speak of the local Native American population consuming mesquite, and Wikipedia, too, says the same about those Cahuilla people of southern California. From what I gather, it is only certain species that have later been invasive?


Checking back this was somewhat of a subspecies thing, but indigenous people were also cultivating mesquites outside their native ranges for food.

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What’s the proposed mechanism for that? It doesn’t seem plausible to me on the surface.


I don’t know about Arizona, but apparently, parts of the great plains were dense forest that Europeans clear cut, and they simply never recovered. Also, California used to be green in the summer, but then people introduced non-native grass that grows rapidly in spring, choking out the native plants, but dies by July. It is the reason the hills are so yellow and dusty and dry.

There is some stuff along route 66 that suggests the desert was there longer than humans though. One example is the crater they used for moon landing training. It’s basically undisturbed, and is extremely old.


The Great Plains have been grassland since the LGM, that’s one thing you can’t pin on European settlers. East of the Mississippi though, a squirrel in the 17th century could have walked from Maine to Georgia and back without ever touching land.


Models help test test our understanding of past events. If you can create a model that reproduces past events, there is a chance that you have understood something about those events. Equally, however, there is a chance that you have created a model that only coincidentally works, because you forced it to do so.

When "scientific" articles write that a model "confirms" some conclusion, they are just wrong. Given enough free variables, you can construct a model that does anything you want. Models confirm nothing.

"The results confirm the North African Humid Periods occurred every 21,000 years"

No, they don't. Did the Sahara green every 21000 years? except when there were ice sheets? Only physical tests can confirm that. Do these greening phases correspond with orbital precession? Statistics can confirm that correlation. Running a model can test hypotheses of causality, but it doesn't "confirm" anything, least of all physical facts.


It's like how the model for a snow slab doesn't prove there was a snow slab that slipped over the Dyatlov Pass tent that night, despite what the media claimed about the case being solved. It only shows that it could have, assuming the data and assumptions are correct for that specific location.

Or models showing some solution to the Fermi Paradox, and proponents claiming that the mystery is finally solved. How do they know the model is accurate?


Because the only thing you can ever have is a theory. The strongest theory is the one that gets chosen. Theories vary in strength over time.

Nothing is ever solved or proven. Theories get stronger and stronger evidence. Believing in a theory doesn't in any way help you make sense of the world. We just do it for expediency of communication.


Theories being ultimately all we have, there is (or should be) no “chosen” one. People are free to believe whatever—this is a pre-requisite for theories to evolve.

There may be a theory that enjoys better predictive success for a while, but if everyone always believed in a singular “chosen” theory then we’d get nowhere: to evolve a better theory, you have to come up with and believe in something other than the preexisting one. Thus, it is normal for multiple theories to coexist at any given time.

Crucially, a theory is all map no territory, always and necessarily. It describes something we do not understand in terms of something that we do understand. At its core, it is a metaphor—and more than one metaphor can be useful, each in a slightly different context.


There is no need to believe a theory, but when it comes time to make an irreversible choice; you better pick the strongest theory. No belief necessary. Sometimes you do have to choose, and you have to accept that your choice will have a degree of uncertainty.


The point is that there is no “strongest” theory. Different theories are different maps. There is no perfect map, at that point it is no longer a map but territory.

Better not use a bad map, but there can be more than one good map depending on what you need it for.


The chosen explanation should be held to be contingent and corrigible but that doesn’t mean all explanations should be held to the same regard particularly when some can be harmful as well as contradictory.


Many currently accepted theories contradict one another. For instance in relativity the rate of change of time is dynamic and changeable value, whereas in quantum mechanics it's fixed value. Both theories work exceptionally well in their own bubbles and have abundant support. Yet when you put them side by side, they say fundamentally different things on many topics. And a theory being "harmful" does not make much sense. Because theories may be right or wrong, but they are constructions for how the universe works - not actions.


It is not abnormal for multiple theories or hypotheses to contradict each other. In strictly natural sciences it is usually possible to experimentally rule out some of them, but not always so (for example, sometimes the technology is not there). In philosophy it is often not the case (no experimental means available where concepts like “objective reality” are involved).


That's what religious people say in response to discoveries threatening their revelations. What's your religion, Climate Change [1]?

[1]: https://churchofclimatechange.org/


> The results confirm the North African Humid Periods occurred every 21,000 years and were determined by changes in Earth's orbital precession. This caused warmer summers in the Northern Hemisphere, which intensified the strength of the West African Monsoon system and increased Saharan precipitation, resulting in the spread of savannah-type vegetation across the desert.

So… as we finally exit the current ice age we should expect a green Sahara?


> So… as we finally exit the current ice age ...

If by "Ice Age" you mean period of glaciation - the most recent one ended ~ 11,500 years ago after starting some 120,000 years ago.

We're not currently in an "Ice Age" .. although by general expectation of this cycle we should be slowly moving into one . . . save for this recent rapid rise in C02 and atmospheric insulation increase caused by the most recent century of accelerated human activity.

See the enlarged graphic from the Nature article this thread is about:


or the full original paper:


> ... we should expect a green Sahara?

Well, as noted we've rather changed the system parameters a fair bit, it's difficult to know what to expect where anymore other than a general increase in the amount of trapped energy at the lowest sea | land layers globally.

It's enough energy to eventually tip the otherwise periodically stable climate cells into all manner of not seen before new redistributions of climate activity.


We are still in an ice age, just an interglacial part of it: https://en.wikipedia.org/wiki/Interglacial, https://en.wikipedia.org/wiki/Ice_age


Let's not play the dull game of pedantic definitions.

The GP to whom I replied referred to the ~ 21 Kyear glaciation cycles associated with humidity changes in the Sahara, I specifically asked about what they meant by "ice age" and pointed out that now other parameters have been altered (the insulative properties of the atmosphere) we can't reliably predict how specific regions will change in the future on the basis of the past.

Your own link clearly differentiaties between geological "Ice Age" (Quaternary glaciation of the past 2.5 million years) and the popular notion of "Ice Age" (glacial cycles during that time).

Hence why I carefully wrote:

> If by "Ice Age" you mean ..

Do by all means contribute to the discussion, that is so much better than just playing the game of "gotcha's" (or whatever it is you're doing in your comment above).


It’s my understanding that the Amazon is dependent on minerals from the Sahara blowing across the ocean, and if we “re-greened” the Sahara, the Amazon would be negatively affected.

Is this the case, and if so, does that mean the Amazon is only as old as the desert version of the Sahara?

Edit: bit of an answer here for anyone curious: https://earthscience.stackexchange.com/questions/2566/what-w...


I've heard about some efforts to regreen the desert by careful planting and geoforming. Anyone know anything about this? It would be cool if it could work, even partially.


You might as well be referring to permaculture generally. It’s exactly about careful, intentional planting and working with the environment rather than trying to tame it. Geoff Lawton’s Greening the Desert Project comes to mind in particular: https://www.greeningthedesertproject.org/


Darwin and Hooker did something like this in the 19th century to Ascension Island. Few know this. Deliberate modification of an ecosystem couldn't be done lawfully in the US. However, that's an administrative obstacle rather than a technical one.



Check out the work being done by https://justdiggit.org/


Water bunds are particularly effective:


(That site breaks the url bar though; I had to get that via a search engine. Wtf.)


I think the key is the lack of water.

In the Sahara and the Middle East, which are sunny and warm, if you add water plants grow fast.

We can imagine future scenarios in which fusion or solar are used to power large-scale desalination and irrigation to represent deserts from the coasts.


There are projects going on in both China and on the African continent. Search for "Great Green Wall".


China has gone through various tries of this for the last couple of decades. The first tries failed horribly because of lack of water and they sent the wrong trees to be planted for the ecology they were going into. I'm not sure how their current try is going, we will just have to see when/if the dust storms stop coming in the spring.


You might find histories of Fresno County, California interesting.

When settlers showed up, it was essentially river running through a desert landscape. They dug canals to support agriculture and the landscape greened up substantially.

A lot of water law and patented water development tech happened in Fresno County. It's essentially a modern "hanging gardens of Babylon" situation and no one seems to talk about it.



Sahara could be green again in the future once the East African Rift becomes a new ocean 10 million years from now.


Would it though? The trade winds converge over central Africa, which is already quite lush, so presumably any extra moisture would just be deposited there instead of a few thousand miles to the north.


Probably yes, but I’d need to see a model to be sure. Changes to the general circulation of the oceans had massive effects on paleoclimates. Recent examples are the closing of the Panama strait, and the opening of a new strait between Australia and Antarctica.

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I think it was in Michael Palin's show Sahara, he's talking to some locals living in a village in the middle of the desert, and while I'm wondering why anyone would ever settle out there, the person being interviewed relayed that many more people used to live there before it became a desert. It wasn't that they moved out there, they were all that was left. Kind of post-apocalyptic.

Edit: found the mention, he's talking to folks from the dogon tribe in mali, [0]@29min - 30min says that the area was heavily forested 500 years ago. The location is the bandiagara escarpment, old cliff dwellings [1]. All the episode of Sahara are on youtube, I highly recommend the series.

[0] https://youtu.be/oZHR16GN1EU

[1] https://en.wikipedia.org/wiki/Bandiagara_Escarpment


The Earth's precession cycles driving Sahara rainfall have a period of about 23,000 years. Whatever caused deforestation there over the past 500 years likely was distinct from the Green Sahara cycles.


I was mistaken in saying "the middle of the desert", the area of mali being toured is decidedly on the edge of the desert.


Great reminder to all that we know we are effectively killing Lake Chad, but there's not much anyone is really doing about it.



How exactly are "we" killing Lake Chad and what would be an easy solution to save it? Also, who is "we"?


Humans. How do you want me to segment us to minimize the problem?

And we are killing it by diverting water from it.


Segmenting maximizes rather than minimizes.

When the problem is caused by all humans in the world, then no one feels pressured to do anything.

If you could instead name people individually, they would have to take a stand to actually do something about the issue.


Why would they have to take a stand? Ah because other people would make them take a stand. Why would these other people make the first group of people take a stand? Because they care. What if they don't care? Somebody else should pressure them into caring so they can pressure the ones who can do something about it. Etc etc

My point is that this is what it's usually meant by "we are not doing anything about it". It generally means that there is not enough collective awareness about an issue for the people involved to have enough incentives to do the right thing.

Some people observe that the issue is under somebody else's direct responsibility but draw the conclusion that the responsibility chain ends there.

Other people instead think it's their responsibility to apply some sort of pressure from outside and failure to even attempt to do so extends the responsibility of the inaction to them too.

Some of those people extend their feeling of responsibility to also shame people for not shaming other people.

This is all natural and human and as with all human things it functions well until it fails catastrophically


Same story for the Aral sea. https://en.m.wikipedia.org/wiki/Aral_Sea


Timbuktu’s population dropped by 4/5ths since the Middle Ages, from a near legendary city to a nearly forgotten one.


Any recommended sources to learn more about it?


Not of Timbuktu specifically. There are books on the history of West Africa, or Mansa Musa.


Paul MM Cooper's podcasts and video essays on YT are excellent. There's info on Mansa Musa in this episode: https://www.youtube.com/watch?v=GfUT6LhBBYs


The Fall of Civilization series: The Songhai Empire - Africa's Age of Gold https://www.youtube.com/watch?v=GfUT6LhBBYs


Pretty amazing clip, sent me down a rabbit hole. I can't help but think what extreme climate niches our descendants 500 years from now might be surviving in, that were once fertile & lush.




When was this research established? I've heard about this a few years ago but when was it found that Arabia was green historically?


Periodic desertfication of the broad north African region north of the Sahel is a notion that dates back to at least the early 1980s (and probably decades before) and is based on layering in river systems and other strata evidence.

This specific research tightens up the timing and driving factors for the specific Sahara region (Libya, Egypt, Sudan) and doesn't relate to the Arabian Peninsula which is an almost seperate landmass more than three quarters surrounded by sea water.

On that landmass the Arabia Felix region (modern Yeman (mostly)) is known to have been greener in the past.


It obviously doesn't cover the why of Sahara's greening, but for anyone who has access to BBC iPlayer, the following programme [1] has a nice summary of humanity's journey out of Africa.

It's amazing that the majority of the world's population is descended from the couple of hundred individuals that left during one of these greening periods.

[1] https://www.bbc.co.uk/iplayer/episode/b00kfqps/the-incredibl...


This is an interesting video I came across awhile back: https://www.youtube.com/watch?v=jjOW6kLEckg

Apparently Libya has a huge ancient aquifer (or several aquifers really) that they built pipelines to access, and now that's their main source of fresh water.

The aquifers, being left over from a previous era when Libya got a lot of rain, are not being refilled by anything.