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Apologies if I sound like a madman ahead of time.

Me and a friend have been worldbuilding this alternate timeline for awhile now, and I've recently become interested in the idea of one of the factions in this world performing large desert greening projects in the Sahara and Australian outback.

We were thinking that the Libyan aquifers could potentially be pumped to the surface to create an inland sea in on of the Saharan depressions or even to expand Lake Chad.

I'm also curious if the Nile, Congo or Niger rivers would play a part in this.

My main concerns with greening the Sahara are the potential negative effects to the Amazon rainforest, the African monsoon cycle and Earths albedo.

I'd love to hear every idea/potential project, no matter how expensive it may be. I do want to note, that this alt-timelines politics are vastly different from real life.

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  • $\begingroup$ Old egyptians already did it with the Nile. In fact, channels and dams are built for that purpose. Your limit is the water available in the rivers. en.wikipedia.org/wiki/Bahr_Yussef $\endgroup$
    – Santiago
    Commented Dec 29, 2023 at 16:45
  • $\begingroup$ It wasn't obvious to me why the Amazon would be effected. Are you referring to fertilisation of the Amazon rainforest by Saharan dust? $\endgroup$
    – Martin Smith
    Commented Dec 29, 2023 at 18:59
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    $\begingroup$ @MartinSmith I suppose making Sahara green would affect ocean currents, which would also affect climate in general, including Amazon? $\endgroup$
    – Yksisarvinen
    Commented Dec 30, 2023 at 1:11
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    $\begingroup$ I'm guessing that the Amazon rainforest is shorthand for all the ways you could screw up the biosphere if you upset some balance. Wetting the Sahara is guaranteed to mess something up. Probably several somethings. The thing is, we've already wiped out 64% of the world's rainforests. It just isn't that hard. $\endgroup$
    – Robert Rapplean
    Commented Dec 30, 2023 at 5:58

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Slowly

If you simply dump a load of water into a pond in the middle of a desert it will simply evaporate.

The project you need to look into is the Great Green Wall, the general principle being to plant a forest across the width of Africa to stop the expansion of, and partially push back the Sahara.

Once you start going into the details you realise that there is no fast solution to this, no magic waving of large quantities of water. You have to work on the climate, you have to build something that will sustain itself, fundamentally improve the soil and let nature build on it.

The joy of this project is the massive level of international cooperation behind it, along with local support and involvement. It looks like something that will work.

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  • $\begingroup$ Just look into existing permaculture projects and literature for demonstrations of how adding trees changes local climates, assuming the trees survive long enough to accomplish the goal. It's definitely not fast, but it does the job. suddenly thinking of Robo from Chrono Trigger $\endgroup$
    – coppereyecat
    Commented Dec 29, 2023 at 1:05
  • $\begingroup$ I really recommend you reading The Man Who Planted Trees by Jean Giono. $\endgroup$
    – byMaelstromer
    Commented Sep 6 at 5:44
  • $\begingroup$ @byMaelstromer unfortunately he's fictional, but there are a couple of real examples, a couple in the US who slowed the water flow through their land with lots of loose rock dams and a chap in ?Brazil who's been replanting rainforest on a very small scale using the high density model. $\endgroup$
    – Separatrix
    Commented Sep 6 at 7:59
  • $\begingroup$ @Separatrix I know he is fictional. It is just great read on how perseverance, humbleness and pure intent changes lives of those around you for better and it incidentally touching on similar topic. That is why I recommended it. $\endgroup$
    – byMaelstromer
    Commented Sep 6 at 8:21
  • $\begingroup$ @Separatrix the guy in Brazil is Sebastião Salgado? I looked him up and what he and his wife managed to do makes me emotional. $\endgroup$
    – byMaelstromer
    Commented Sep 6 at 8:24
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Plant weeds

There are a lot of project plans for the conversion of desert zones into grassland. The trick to them isn't water or trees. It's grass. You need something to hold the soil down for long enough for an ecosystem to dig in.

In the American plains, we have an entire class of plant that is well adapted to this purpose. They root fast, grow quickly, and spread their seeds prolifically. These plants are colloquially called "weeds." No, not weed. Jimson weed, dandelion, thistle, tanglevine, and many more. They move in and take over an area that would otherwise turn to dust before the grass and trees could get a foothold.

A lot of people hear this argument and ask, "why would I want weeds?" If that's you, then you're thinking backwards. You don't want weeds. The soil and the bugs want weeds. The American Dust Bowl demonstrated what happens when you pull up the existing plants and plant just what you want. If you want plants to grow, you need an ecosystem, which means biodiversity.

You seed an area with weeds, then populate it with animals and bugs that will eat the weeds. Once the weeds have properly tacked down the topsoil, you introduce plants with longer life cycles. The grass, trees, and bushes have competitive advantages that will make them outcompete the weeds in the long term, turning a weedy lot into a grassland or forest.

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    $\begingroup$ A daffodil in an onion patch is a weed, and a dangerous one at that because daffodil bulbs are poisonous. You don't want weeds, you want endemic native plants, as soon as you want them they're not weeds. $\endgroup$
    – Separatrix
    Commented Dec 30, 2023 at 11:36
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    $\begingroup$ @Separatrix, Granted, that is the correct definition of the term "weed." Better terms for what I'm describing are pioneering, opportunistic, early successional, or colonizing. My thought was that the term "weed" would be more effective at conveying my intent, even if it isn't technically accurate. $\endgroup$
    – Robert Rapplean
    Commented Dec 30, 2023 at 21:04
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An iteration of the 'Sahara Sea Project'.

The basic idea was and is that large depressions in the Sahara could be flooded using sea water channeled through canals usually from the Mediterranean but the Atlantic has also been suggested. The most likely target for such a project is the Qattara Depression which has a average depth of about 60 meters and an area of almost 20,000 square kilometers albeit it does not automatically follow that the entire basin would or could be flooded.

If the project was supported by Egypt the result would be a large permanent inland salt water 'sea' that could cool the local climate dramatically depending on evaporation rates and wind direction. Solar powered desalination projects around the shores of the lake could also support large scale agriculture and forestry projects around its shores that would further reinforce localized cooling effects and raise atmospheric moisture levels further.

The completion of the Qattara sea project would not in and of itself change the climate enough to 'green' all of the Sahara but could in theory have an impact on a large portion of it. Other, smaller 'sea' projects could also be constructed further east but using only these projects while you'd probably end up greening a large part of the region you would still not 'green' all of it.

(There's a Wikipedia Post on the Saharan Sea if you want more details on the topic.)

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    $\begingroup$ There is the actual real 440,000 square km (170,000 square miles) Red Sea between the Sahara and the Arabian Desert. Doesn't help at all. Not even a little bit. $\endgroup$
    – AlexP
    Commented Dec 29, 2023 at 2:00
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    $\begingroup$ Geography, topography and seasonal wind patterns matter a lot. Beyond that the Sahara Sea projects were semi-seriously considered on the basis that it would decrease aridity locally. Finally I did mention the conversion of sea water to fresh water for large scale agricultural and forestry projects, something not yet tried along the Red Sea coast. $\endgroup$
    – Mon
    Commented Dec 29, 2023 at 6:57
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Diverting a bunch of water to deserts might help but before we start hauling tons of water about how about we look to the past when much of our current deserts were greener for inspiration.

The big difference I'm aware of between then and now is more CO2 in the atmosphere.

Which makes sense.. as more CO2 stimulates more plant growth, just ask any comercial grower that uses greenhouses, pumping CO2 in to encourage growth is a thing that's done.

It also causes the pores plants use to take up gasses for photosynthesis to constrict, or perhaps remain open less often? .. either way the side effect of that is less water lost to evaporation which allows plants to survive and thrive in hotter dryer conditions than otherwise.

So increase the CO2 content of the atmosphere might seem like one way to go?

[Maybe not just that, but it should definitely help as part of a range of measures]

Burning coal and oil to pump all the old CO2 (that was previously taken out of the atmosphere by plants to make all that coal and oil) back into it until we have CO2 levels back to where they were back when our deserts were on average greener would seem the simplest way to go about it.

I can't see a direct negative effect on the Amazon from that, after all, it was around and doing just fine when the CO2 was higher and the deserts greener (or smaller if you prefer) last time wasn't it?

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    $\begingroup$ [twiddles thumbs while he waits for the 'green' activist backlash and outrage 😁] $\endgroup$
    – Pelinore
    Commented Dec 29, 2023 at 0:45
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    $\begingroup$ "The big difference between then and now": Milankovitch cycles. For the application to the humid and dry African periods, see Edward Armstrong et al., "North African humid periods over the past 800,000 years", in Nature Communications vol. 14 (2023). $\endgroup$
    – AlexP
    Commented Dec 29, 2023 at 1:12
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    $\begingroup$ The greening and drying out is periodical and the level of carbon dioxide in the air just doesn't do such wild oscillations with such a short period. On the other hand, plain old celestial mechanics provides for a simpler explanation for an oscillation of the amount of energy supplied by the sun in northern hemisphere summer, which creates a stronger or weaker monsoon. Long story short, everything else being equal we can expect the Sahara to re-become a lush savannah, even maybe forested, with lakes and rivers, some 4,000 or 5,000 years from now, and to dry out again some 10,000 years later. $\endgroup$
    – AlexP
    Commented Dec 29, 2023 at 1:33
  • $\begingroup$ Yes @AlexP there is a cyclical thing that happens, that's one thing with it's own causes, another separate thing with different causes has been a progression .. your cyclical thing sits on top of my progression. $\endgroup$
    – Pelinore
    Commented Dec 29, 2023 at 1:45
  • $\begingroup$ Increased CO2 does help some plants, namely the C4 ones; C3 plants are built to use current-day CO2 levels and won't benefit (but would suffer from less CO2 in the air). It's not going to help the OP, unless he wants a scenario with a green sahara but no technological civilisation; also, higher temperatures won't automatically irrigate the Sahara. $\endgroup$
    – toolforger
    Commented Dec 30, 2023 at 12:39
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You can't drain an aquifer without consequences

There are locations throughout the world today where people are suffering from declining access to water because aquifers are drained faster than they can recharge. The idea of greening the Sahara by, effectively, draining an aquifer might result in a green Sahara... but it will create deserts elsewhere.

The truth is that if we account only for current technology, each and every attempt to radically redistribute a water source has resulted in bad things. Take, for example, the Aral Sea. Formerly the fourth largest lake in the world and once the breadbasket for a region with a strong economy that included everything from food to recreation, the Soviets diverted almost all of the water from its sources to irrigate (aka, "green") Uzbekistan. The irrigation project succeeded! But it drained the lake to about 10% of its former glory and caused all kinds of problems with toxic salinity.

A Worldbuilding Solution

There is plenty of water to do everything humanity can imagine... if you can just get the salt out of it and transport it efficiently. The end result might be a much cloudier planet... which means less sunlight on the ground... which might actually kill everything (that's be just our luck...). But let's ignore all that.

Invention of the day!

I propose a practical fission/fusion/antimatter/whatever power plant big enough to provide energy for disintegration-level recycling and in need of a ferocious amount of water to keep it both cool and under control.

Sea water... and a honking lot of it... is brought in to cool the system. The process wonderfully results in desalinated hot water, which is pumped to massive cooling lakes (not ponds... lakes...) creating an entirely new ecosystem with tons of available water.

Place these power plants strategically so that the water is captured efficiently as lakes, creating freshwater rivers and streams back to the ocean.

And thus we have a green Sahara. Oh, it would take forever to get the Sahara to green up. The world is full of lakes and reservoirs (e.g., Utah's Strawberry Reservoir) that are full of water and surrounded by nothing more green than sagebrush... (After fifty years there's still nothing more green than sagebrush around Strawberry Reservoir.) The point I'm making is that water is only one of a bunch of things you need to get grass and trees — and deserts lack pretty much all of them.

But that's the subject of another question.

Ocean water + cheap desalination = green Sahara... eventually.

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  • $\begingroup$ I like this answer, except for the idea that water will take long to make something green. If the Utah reservoir isn't greening up the environment, then something else is preventing plant growth - too high salinity or something else with the soil quality, I'd guess. Most Sahara regions instantly green up as soon as there's water, that's how oases work. In fact many Sahara states have done pretty successful irrigation projects, the main problem there has been long-term water supply. $\endgroup$
    – toolforger
    Commented Mar 6 at 8:52
  • $\begingroup$ @toolforger You're not wrong, but the soil pre-existed the reservoir and we have that same situation here. Ask anyone who lives in a desert climate whether or not simply adding water is enough - they'll tell you no (I've lived in one, so...). The organic material that makes the difference between sand and soil is gone and must be replenished. Given water alone, that takes a very (very) long time. Those successful irrigation projects? They included fertilizers and other soil enhancements. They included the mass of planting seeds and the remainder mass after harvest. $\endgroup$
    – JBH
    Commented Mar 7 at 3:31
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Sahara's "Greening" probably wouldn't have no effect on Amazon. 6000 years ago to 12,000 years ago, Sahara wasn't a desert, but a subpluvial grassland with modest desert at it's tips. Lakes size US states, lake Chad was 50x it's size. At height of Holocene climatic optimum where average temperatures were WARMER than present. enter image description here

Global Warming made Africa Humid. That all went away when planet got colder.

But I digress, not political aspect. Terraforming Sahara into habitable permanent environment.

enter image description here Stage 1: Process begins with reintroduction of water (Desalination) and pumping into it's major Aquifers. Saturation of water into the depressions of former lakes.

Stage 2: Processing of the desert soil into organic containing soil biome. Organic material, compost, coconut husks, any water loving organic material is mixed into soil over time. Certain trees may be added in clusters. using a landscaping technique called "Zai" filling them with manure and compost to provide plant nutrients. The manure attracts termites, whose tunnels help further break up the soil and introduce more organic material deeper and deeper. enter image description here

Stage 3: Tree planting. Water efficient but habitat worthy trees like Acacia and various native plantings would be best candidates. However GRASS, especially dominant in African ecosystems is better available choice. Over time the soil will be colonized with microorganisms with the constant introduction of organic materials, once tree's establish a tap root, they can thrive on natural rainfall. And the constant additional humidity from stage 1 will saturate soil to build a suitable biosphere.

Stage 4: Animals: Dung beetles, worms, insects like ants and termites will be first candidates for introduction.

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