I'm gonna link this on my question thread about the Sahara.
Go nuts
The nature of chaotic shifts in systems with many stable states, such as climate, is that they are very easily perturbed during the shifts. PODs could be an increased incidence of slash-and-burn farming somewhere, slightly early or late domestication of some species, better hunting techniques or a disease wiping out large number of a species with a high ecological impact.
Or even less. The butterfly effect is originally referred to as causing a storm. A storm is enough for climate knock-on effects down the line. All of this could knock climate from one metastable state to another.
I don't think the Sahara pump can be forced permanently, but postponed and blunted should be doable.
Sadly, it doesn't matter. I tried to make a
thread on this years ago, and anthropogenic climate change is apparently ASB.
I took a look at that thread, and unfortunately, the premise does suggest that "for reason" nothing changes (although it was a brief skim). I can understand why a mod would move that to ASB. I like the ideas though, I'm going to have to be a sod and say that there would need to be a good explaination for each though (I'm not even sure I know what impact slash and burn in the South Sudan would have, besides immediate fire and desertification in the local area.)
*Cough* Well, I mean not to shill out but...
Well...
You may or may not have been a partial reason I wanted this thread to exist - more for the academic side of the idea - and not necessarily a Roman PoD (Also, it is easier to shill when you have links in your sig!)
It would be ASB to have the Sahara stay green, and it would take technology we don't have to make it green again by human impact.
I think if we knew more, we could do it without an ASB, but you're probably right based on what we can currently understand about the process.
That said, you can slow down desertification to a certain degree which wasn't done OTL. First, I think you need to butterfly Islam or stop it from expanding into North Africa, because events like the migration of the Banu Hilal sped up the ongoing desertification and push the ecosystem over the edge in many places.
Why specifically the Banu Hilal? Is this something to do with the idea that these groups brought goats across?
Second, you need a major power based around Carthage (let's call it the "Southern Roman Empire"). Since their powerbase will be drawn almost entirely from North Africa, I think they'll pay more attention to the health of the land, or at least enough to have the culture have a sense of ecology and encourage good farming practices. Third, they need to evangelise the cultures on the south side of the Sahara (which probably is inevitable given a Christian North Africa). Hopefully they'll develop a similar ethos in those cultures there. That's the basis for slowing down the desertification of the Sahara.
What if it was simply an Exarchate, or similar? I can expect the local Exarch to be interested, especially if it means he gets more money from his land. Looking at the plans for Africa in every division of the Empire, it is odd (to me) that Africa didn't get a special status. It has totally different strategic concerns that are largely disconnected from every other frontier. But I guess if money is meant to be extracted OUT of Africa, then it falls over. (Although, my favourite idea, involving canals in S.Tunisia, would be interesting as a defensive concern).
As for what they could do in later times, I think importing trees of the Acacia genus from Australia (probably no earlier than 1600, sadly) could help reduce desertification in pre-modern times. Acacias are an invasive species in mediterranean climates globally, but they could be immensely useful to cultures around the Sahara. In some West African countries OTL, they're being used both for food (their seeds can be ground into flour to make a very healthy bread) and firewood (taking the stress off of native trees). While I think their use as food would probably come later (during a famine or war, perhaps), their use for firewood, charcoal, tannins (the bark is rich in them) would be almost immediate.
If there were extra resources provided/avaliable, the Portugese could try in the 1500s IMO. Discovering Australia, and bringing back some samples of its odd flora and fauna doesn't seem unreasonable - and growing samples in Portugese territories could be useful. Would it reach Africa quickly, doubtful - but short of an earlier discovery of Australia, we can probably agree on the timescale for that.
I don't know if North Africa could ever regain the position it had as the breadbasket of the Roman Empire, but it would be able to produce much more food than OTL.
Well, it does currently produce MORE food - to the point that there are many scholars who try to dispute the idea that Africa did decline, but instead everywhere else became better. (It has legs, after all, most of the evidence for its decline comes after the application of the heavy plow, which really helped other regions flourish).
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I figure I can hardly ask without providing my own initial suggestions (not too different from the actions taken by Alternate Eagle) - Canals in N.Africa.
1) Quattara, and the Fezzan, both good and valuable ideas in their own right. Big, saline lakes - the salt industry may boom in these regions before agriculture does, but certainly provides income for the rulers of the area.
2) Chotts south of the Aures Region of Tunisia/Algeria. Whilst the first canal would need to be 21km long - it is the shorter than the Quattara and Fezzan canals. After linking Chott el Fejej and Djerid, the connection of Djerid and Melhrir afterwards creates a large chain of watery bodies - that could either form into a sea (as suggested by Alternate Eagle), or IMO more realistically a series of lake that can be dredged of salt, as they provide rainfall in the Aures Region and .other parts of the Atlas Mts (as the prevailing winds in the Sahara would blow the moisture north).
I like 2, because it creates a potentially wealthy region that ties closely to Aures, and is cheaper and easier to defend than a series of mountain passes. 250Km of watery obstruction for any invaders? Plus money? I think that would be a splendid idea. Personally I think an Exarch, or an earlier Roman govenor could be beneficial here, and would benefit from the not immediately apparent impacts - such as increased rainfall making more of the eastern Tunisian coast riverine.
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Other than introducing Acacia trees, what other ideas do people have for south of the Sahara?
Also, as an amusing point - I think we can say that the
New Valley Project is a reasonable warning sign of side effects and the futility of trying to create something from nothing. (Unless someone has a crazy idea that can help

)