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I'm building a world in whose history climactic changes causes a region of desert to start seeing regular rain. This causes the native culture of the desert people to also change along with it, eventually evolving to something akin to the Mongols and finally the Yuan dynasty of China. (Essentially moving from a desert nomad to steppe nomad culture, and finally becoming a full fledged settled civilisation.)

My research shows that this scenario has parallels to the African humid period, where a stronger West African monsoon caused a greening of the Sahara. But nowhere is it mentioned how long it took for the desert to turn green.

My question therefore, is this:

  1. Provided monsoon conditions of the strength we see in the Indian monsoon system, how long would it take a desert region approximately half the area of the Sahara to turn green?
  2. What would be the steps involved in this process?
  3. If a hypothetical bordering empire (with technology akin to the Chinese Tang dynasty or the Sassanids) were to conquer the region during this period of "greenification", what strategies could they use to assist in speeding up the process?

Any help would be highly appreciated!

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  • $\begingroup$ (0) The Yuan dinasty was a Chinese dynasty. A small number of Mongols were involved, but by and large "the Mongols" remained Mongols, nomaic pastoralists, they did not turn into Chinese. (1) The main factor affecting the speed of turning the desert green is how long it takes for the climate to change. If the climate changes overnight then the desert will turn green into a grassland in a surprisingly short time. Grasses are resilient plants. (2) Steps? Of what? If it rains, grass grows. (3) What do you mean by conquering a desert? Conquer it from whom? Aren't they just settling/colonizing it? $\endgroup$
    – AlexP
    Mar 12 at 9:01
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    $\begingroup$ (0) Really? The dynasty was started by Kublai Khan, a descendant of Genghis, so a Mongol. It wasn't until his great grandson that the dynasty actively sponsored the adoption of Chinese culture in its administration. But that is neither here nor there. (1) Climate change doesn't happen overnight. If I understand correctly, changes in the planetary insolation and global albedo changes caused a strengthening of the West African Monsoon, which recharged local aquifers, giving rise to lakes and rivers, eventually causing the Sahara to turn green. My question is how long this process took. . $\endgroup$ Mar 12 at 9:33
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    $\begingroup$ (2) Steps meaning changes in the local flora, moving from seasonal xerophytes to grasses and eventually trees. (3) I imagine that as precipitation in the desert increases, the local populace might thrive enough to become either raiders who are serious enough to necessitate military action, or an attractive target of conquest by any neighbouring power simply on account of greater prosperity. But yeah, you're right, there would be a certain degree of colonization involved. $\endgroup$ Mar 12 at 9:37
  • $\begingroup$ As a lower bound, consider the building of towns in irrigated areas, such as much of the western US. People build houses, add trees and other landscaping, and eventually the place begins to look like a forest. So about 50-100 years, depending on how fast your tree species grow. Cottonwoods can grow to large size in less than 50 years. $\endgroup$
    – jamesqf
    Mar 13 at 19:33
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Areas of the Sahara desert do turn somewhat green after heavy thunder storms which happen occasionally. Within days the desert is in flower.

But with massive quantities of regular rain there would be plants everywhere before long.

Initially there would be plenty of low growing desert flowering plants, cacti and shrubs limited by the amount of nutrients available. But this would attract birds, insects and other animals from peripheral areas creating a thicker cover of vegetation starting from the peripheral areas and working in towards the deeper areas of desert.

This all assumes vast quantities of rain. With less rain less regularly the process might not get started at all or could take any amount of time you desire depending on quantity and frequency.

But with regular heavy rain the main limiting factor would be the availability of nutrients. It would take a long time to establish trees, but within decades the more mobile seeded species could start to appear everywhere carried but birds and the wind.

Concerning speeding up the process, this would depend on the new people being aware of what was happening and the exact amount of rain and its distribution. Assuming they were aware by some means and the rain was less than Amazonian then standard arid land management should be effective.

This involves channeling water run off into a series of plant growing areas with one overflowing into another and by capturing water below ground by use of many small barriers and dams can help prevent rapid water run off. Measures to prevent people chopping down trees for firewood would be needed as well as growing the right crops.

edit: It really does depend massively on how much rain there is both in volume and frequency. With constant regular rain it could tend towards the Amazon encouraging rapid forestation with a massive river system(s) developing from the get go.

With a very wet season and very dry season you could end up something like southern Madagascar with huge rivers such as the Mandrare river (a 400m wide winterbourne) gallery forests around the rivers and drought tolerant thorny forest (or equivalent) elsewhere.

Mandrare river curtesy Rainbow tours

With less regular and/or more modest rain fall many other climatic types would be possible for example any climate type from the bottom of this diagram: https://www.climate.gov/file/holdridge-lifezonespng

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  • $\begingroup$ Hey, thanks! My research indicates it took about 1-2 centuries for the West African Monsoon system to become regular over the Sahara. Would the desert turn green at the same pace, or would it be behind by a few decades or centuries? Also, how long would it take for rivers to appear? $\endgroup$ Mar 12 at 9:59
  • $\begingroup$ won't many desert dwelling plants drown if theres too much water or if it happens to quickly? $\endgroup$
    – zackit
    Mar 12 at 13:49
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    $\begingroup$ I imagine that the desert would not green at the same rate. some areas would remain semi desert (rock based areas), some areas would quickly green up (periphery and areas expanding around existing oasis). Rivers would depend entirely on rainfall. If there was enough rain rivers would develop very quickly once any ground aquifers became fully charged and might be winterbourne. I suggest you need to think carefully what you mean by "turn green" (Savanah to Amazonia), "regular" and "Sahara" (3.6 million square miles of semi desert, desert, mountains, rocky flats and dunes etc) $\endgroup$
    – Slarty
    Mar 12 at 13:54
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    $\begingroup$ Some desert plants would not do well in wetter conditions others would adapt. winds and birds would bring in new species and sandy soils / high temperatures in many areas would help prevent waterlogging $\endgroup$
    – Slarty
    Mar 12 at 13:58
  • $\begingroup$ @zackit nature doesn't care; other plants will quickly move in. $\endgroup$
    – RonJohn
    Mar 13 at 1:47
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I want to expand a bit on Slarty's answer.

First, I'd like to second his emphasis on just how much rainfall you'd need; it's much more than you'd think. Something that most people don't realize is just how much water there actually is in soil. One of the primary reasons that deserts get so hot in the day, and so cold at night is actually because there is no water in the soil. It's just like how being close the the ocean moderates the climate of a region, because the immense thermal mass of the ocean slows down the temperature fluctuation. Well, the large amount of water in the soil actually does the same thing; although not quite as strong as the ocean. The thermal mass of water is huge, compared to most materials. Water has a specific heat capacity of 4,182 J/Kg C, while quartz sand is only 830 J/Kg C. Check out this video of a gardener reviving dry soil; he's not quite starting with completely dead sand, but it should give you a better idea of just how much water is needed.

Secondly, as far as what humans can do to accelerate the de-desertification, again, the primary objective should be retaining water. Extremely dry soil, actually has the counterintuitive property of being hydrophobic (i.e. it repels water). So, in areas with very dry soil, you may get a monsoon, but the extreme vast majority of that water will just skim across the surface, and virtually none of it will get absorbed. What you need is either frequent soft rains, or something to get the water to stagnate (or at least slow down) over the land, to give it a chance to soak through the initial hydrophobic property of the dust, and get into the soil. And even once you have gotten some water into the soil, you need to keep it there. If the air is very dry, it will suck the water out of the soil, and all that water will evaporate and be lost again. Biochar is a very effective method of retaining water in soil, and it can even be done with extremely primitive technology (ex: clay or dirt mounds on top of a wood pile). Basically, you just burn or heat up wood, in a container with zero or minimal oxygen, and it turns into charcoal, but the microscopic structure of the wood is retained, which produces tiny little pockets with immense surface area. You grind the charcoal up into the consistency of sand or smaller, and mix it in with the soil. All these tiny particles act like little sponges, and protect the water from evaporation, thus allowing soil to retain water far longer than normal. Also, biochar has profound benefits for soil health, as it gives micro organisms a safe place to hide, and it can retain soil nutrients to prevent them from being washed out of the soil. When combined with things like rock dust (either natural, as would be present in dessert dust, or manufactured), it can significantly increase the long term health & productivity of the soil, as well as the speed at which a soil can regrow fertility. There are several ancient sites across the world, where peoples were using biochar thousands of years ago. You don't necessarily have to know the science in order to discover that it works.

After water, the biggest thing is getting organic material back into the "soil". A desert has been bone dry for so long, that basically all of the organic matter has turned to dust, and either blown away or broken down into sand and trace minerals. Most plants don't like growing in sand; they need the protection & symbiotic environment of soil, which is largely defined by the presence of organic matter. When you're doing this on a large scale, you really want to get nature to do most of the work for you, so you want to set up the conditions for the ecosystem to build the soil & organic matter for you. One way to do this, is planting trees that are hardy enough to survive the initial harsh environment. If you can get them to survive, their roots will push sugars into the soil to cultivate fungus & bacteria who in turn process the raw minerals and make them available to the root systems. Once you have enough water in the soil, these fungus & bacteria can begin breaking down raw organic matter into soil. China is currently using this strategy to stop "the yellow dragon", a region of desertification that is spreading rapidly across China.

To help this process along, you can bring in organic matter to jump start this process. There are 2 ways to do this:

  1. On a small scale, you can bring in compost and mix it in with the native dirt. When combined with the introduction of water (and optionally biochar) this has the benefit of instantly boosting the fertility of the soil to that of semi-high grade soil (depending upon how much compost you put in), that is immediately ready for growing growing crops or high-grade vegetation. This could be appropriate for outposts, or oasis scattered throughout the area to serve as introduction sites for worms & animal species, which will eventually be required for full restoration, to speed up their migration into the new ecosystem. However, compost takes time & manpower to produce, so a cheaper alternative would be to mix raw manure into the native soil; as seen in the gardener video I referenced earlier. This has the same benefits of boosting organic matter, and speeding up the restoration process, however, especially if you're mixing it with the raw sand of a desert, it will require time to decompose (with sufficient hydration & bacterial life), before it becomes high-grade soil.
  2. On large scales this amount of work becomes infeasible, unless you have the cultural incentive, financial capacity, or dictatorial authority to command the necessary manpower. An alternative would be widespread mulching. Covering the surface of the ground in wood chips or straw around 1 foot deep. This acts in two ways: First, it acts as a form of water retention, as the top layer of mulch protects the bottom layer from sunlight, and partially insulates it from heat & wind which would accelerate evaporation. Second, the mulch acts as a source of organic matter. With sufficient water & fungal spores, the mulch will rot into a good quality soil. But to maintain this soil, you either require consistent application of mulch until the ecosystem is stabilized enough for planting, or biochar & hardy vegetation to stabilize it. Also, on large scales like this, of trying to transform an entire desert, none of this stuff has to be perfect. So, just going through vast areas, and spreading a 1 inch layer of biochar, with as much manure as you have access to, scattered over it, and covering that with a 1 or 2 foot thick layer of mulch (wood chips, straw, wheat stalks, etc.), with a grid of trees planted every 10 or 20 feet, would be fine, and would drastically accelerate restoration of the area. The fungal mycelium, and worms, will take care of spreading everything out & mixing it up.

Finally, another thing to keep in mind, is that once you have done this on a large enough scale, the presence of plants (especially trees), actually starts to produce more rain.

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    $\begingroup$ This actually gives me a lot of ideas. Thanks a lot! $\endgroup$ Mar 13 at 3:06
  • $\begingroup$ You're welcome. Good luck. $\endgroup$
    – Yurelle
    Mar 13 at 5:08
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Just a tidbit that might be helpful: beavers can green a landscape !

Beavers protect themselves from predators by modifying their environment. By damming small watercourses, they create large pools of water that they can hide in to reach further food sources. They also dig channels at the bottom of these pools as they repeatedly swim the same route. All of this creates deep reservoirs of water and wetlands which can weather droughts where a seasonal stream would dry up, allowing more vegetation year-round. This has the potential to create a chain reaction in a watershed. The new ecosystems would also present new food opportunities for cultures in the vicinity.

I can't find the exact video I saw on the subject that really made it click for me, but here's a nice outline of beaver lodge construction and a vid on beavers' effect on desert.

If your world has the same animals as earth, you could consider adding in beavers as a passive regreening process. Or, you could even draw up some other fauna that have similar effects.

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