Why and when the Sahara Desert was green: new research

https://phys.org/news/2023-09-reveals-sahara-green.html

by PaulHoule • 3 years ago

150 93 3 years ago

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

PaulHoule2• 3 years ago

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.

OfSanguineFire2• 3 years ago

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.

slashdev• 3 years ago

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

hedora3• 3 years ago

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.

mycall• 3 years ago

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

oh_sigh• 3 years ago

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.

labster2• 3 years ago

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.

theironhammer• 3 years ago

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.

alephxyz• 3 years ago

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

onlyrealcuzzo2• 3 years ago

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.

bjelkeman-again2• 3 years ago

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

labster2• 3 years ago

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

hedora3• 3 years ago

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.”

mullingitover• 3 years ago

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

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

xhkkffbf2• 3 years ago

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.

daneel_w• 3 years ago

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

seanmcdirmid• 3 years ago

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.

jazzyjackson4• 3 years ago

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

hedora3• 3 years ago

Water bunds are particularly effective:

https://justdiggit.org/what-we-do/landscape-restoration/wate...

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

eep_social• 3 years ago

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/

onthecanposting• 3 years ago

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.

https://www.earthdate.org/episodes/darwins-terraforming-expe...

DoreenMichele• 3 years ago

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.

https://news.ycombinator.com/item?id=29696251

mytailorisrich• 3 years ago

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.

tim3332• 3 years ago

They did a bit in China and there is talk of doing the Sinai https://www.theguardian.com/environment/2021/mar/20/our-bigg...

monlockandkey• 3 years ago

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

defrost3• 3 years ago

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.

kefabean• 3 years ago

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...

tim3332• 3 years ago

or without iplayer https://yandex.ru/video/preview/13243013351978795119

jazzyjackson4• 3 years ago

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

RC_ITR2• 3 years ago

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

[0]https://en.wikipedia.org/wiki/Lake_Chad

daniel-s• 3 years ago

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

jansan• 3 years ago

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

ProjectArcturis• 3 years ago

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.

jazzyjackson4• 3 years ago

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

mr_toad2• 3 years ago

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

jazzyjackson4• 3 years ago

Any recommended sources to learn more about it?

Hammershaft• 3 years ago

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.

hasmanean• 3 years ago

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.

kibwen• 3 years ago

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.

christkv2• 3 years ago

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?

bgroat• 3 years ago

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...

Terr_2• 3 years ago

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.

rokkitmensch• 3 years ago

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.

deafpolygon• 3 years ago

Hey, that's Aziraphale! Good guy.

raylad• 3 years ago

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...

agent327• 3 years ago

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.

antiatheist2• 3 years ago

That's about camels eating cactus.

christkv2• 3 years ago

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.

bennyschmidt• 3 years ago

Think it was a different green area - the Fertile Crescent, not the Sahara. Eden would have been where modern day Kuwait is. As for ancient towns in Sahara, for a long time people wondered how the desert people lived in such a harsh area - but it turns out they've just been there since it was lush. They didn't move to the desert and learn how to survive there, they're just what's left of those populations from the Sahara, and adapted to the changing conditions.

elihu• 3 years ago

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.

lettergram3• 3 years ago

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?

defrost3• 3 years ago

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:

https://www.nature.com/articles/s41467-023-41219-4/figures/1

or the full original paper:

https://www.nature.com/articles/s41467-023-41219-4

... 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.

mkl• 3 years ago

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

lettergram3• 3 years ago

Earth is currently in the ice age called Quaternary glaciation.

https://en.m.wikipedia.org/wiki/Ice_age

Basic definition is when ice covers both poles

bradley13• 3 years ago

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.

goatlover• 3 years ago

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?

ImHereToVote2• 3 years ago

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.

keiferski• 3 years ago

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...

Why and when the Sahara Desert was green: new research

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