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The Sahara Desert.

enter image description here

At 3.6 million square miles and an average high of 104 degrees Fahrenheit, this is the largest and hottest desert on the planet. Despite its sandy reputation, sand makes up only 15% of the entire desert's composition, for solid rock makes up 70% of the Sahara, the rest being mountains, oases and transition zones.

In an alternate scenario, the Sahara is still an unforgiving place to call home, but there are differences. One, sand makes up 70% of this alternate Sahara, not rock.

Two, the Sahara has lakes big enough to compare to the Great Lakes shown here in deep blue:

enter image description here

This image does not take rivers into account, which would help keep the lakes regulated and not evaporate into salt flats larger than Bonneville.

Would these two changes pose any dramatic difference to the Sahara's climate?

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closed as too broad by Aify, Brythan, Separatrix, Josh King, John Dallman Aug 27 '16 at 19:47

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ From about 7000BCE to about 3500BCE, or possibly somewhat longer on both ends of that period, the Sahara area was a savannah. It might be an idea to read up on that and formulate a new question. $\endgroup$ – John Dallman Aug 21 '16 at 17:09
  • $\begingroup$ This is an alternate Sahara, not a prehistoric Sahara, which means a prehistoric Sahara set in the 21st century. $\endgroup$ – JohnWDailey Aug 21 '16 at 17:16
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    $\begingroup$ The linked article has theories about how the Sahara switches between desert and savannah, which may inform your reasoning. $\endgroup$ – John Dallman Aug 21 '16 at 17:18
  • $\begingroup$ @JohnDallman How else would I have gotten hold of that image if I didn't know that? But this does not change the scenario of an alternate Earth, which means putting the past on the present. $\endgroup$ – JohnWDailey Aug 21 '16 at 17:28
  • $\begingroup$ I think this question is backwards. The question is not if these changes would change the climate. The question is what would have to change for the Sahara to get enough rainfall to support lakes. I.e. what is different in this world? Those changes would change the climate more than the simple presence of lakes. Look at the Salt Lake in Utah for example. How has it changed the climate in what is otherwise a desert? I think that you need to figure out the causes before you start talking about effects. Why is it sandy? How are there lakes? $\endgroup$ – Brythan Aug 27 '16 at 3:19
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They would need to be caused by "dramatic difference to the Sahara's climate". I can't really picture it all, but here are some points:

  1. much more sand means longer erosion, or faster one. So, either this region died longer ago than in our world, or winds were stronger. Or both.

  2. lakes means it is not dry there. Otherwise they would dry out. So that's something else that made it a desert and killed / prevented plant life.

  3. Lakes and a lot of sand? Why wind didn't deposit sand in lakes? Not enough wind? See point 1. please.

  4. Rivers. You know what Nile did to desert? It made fertile Egypt. Fertility is opposed to what you seem to need.

Long story short, it's either artificial or climate is really, really weird. Either way, this geography is rather a result than a cause.

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  • $\begingroup$ I'm afraid I don't follow you. $\endgroup$ – JohnWDailey Aug 21 '16 at 17:16
  • $\begingroup$ @John what you don't follow? Geography like one you described can't just happen out of nowhere. $\endgroup$ – Mołot Aug 21 '16 at 17:32
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    $\begingroup$ @JohnWDailey You got it backward. The climate makes the lakes, not the opposite. Lakes might have a local effect on climates but not on a large region like the Sahara. Without the right climate, the lakes will evaporate. $\endgroup$ – Vincent Aug 21 '16 at 17:35
  • $\begingroup$ @Mołot You're straying with the question at hand. I can't give you the entire alternate Earth in one go. $\endgroup$ – JohnWDailey Aug 21 '16 at 17:43
  • $\begingroup$ @Vincent You're straying with the question at hand. I can't give you the entire alternate Earth in one go. $\endgroup$ – JohnWDailey Aug 21 '16 at 17:44
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  1. In an alternate scenario, the Sahara is still an unforgiving place to call home, but there are differences. One, sand makes up 70% of this alternate Sahara, not rock.

    Indeed, having more sand won't change the nature of the desert. It will remain a desert, dry and hot.

  2. The Sahara has lakes big enough to compare to the Great Lakes shown here in deep blue

    The lakes are not big enough to have a large impact on the region's climate. They would have a local effect like in lake Baikal by limiting the extreme variations of temperature and somewhat increasing the precipitations, but not enough to sustain agriculture unless irrigation is used.

    As I said in a previous comment, without a large increase in precipitations, the lakes will evaporate over the course or a few decades and everything will be back as it is now. Except the sand.

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Ok, here is a second attempt at your question. Taking into account your related questions for Lake Makgadigadi and the Indian Monsoon.

I'm assuming all your questions are inter-related. If not, your multitasking abilities are impressive.

Please note, I drew up these very basic maps, in paint, under the impression that the East coast of Africa was a shallow and warm continental shelf, not a deep subductive zone.

alternate ocean alternate winds

From your comments and questions, I have deduced that you have the Himalaya's blocking the passage of air causing traditional monsoons to form.

  • I don't think the extra height of the mountains will effect the strength of the monsoon too much. The monsoon is formed by a barrier at a certain height. If the barrier is higher or lower than that threshold shouldn't matter. Although a lower barrier will allow some overflow into the interior, so in your scenario Tibet is going to be even dryer than real life!

  • The steepness of the barrier might effect the speed that the monsoon forms at, as the air will be pushed higher quicker, causing precipitation sooner. This might actually cause more of the moisture to fall immediately to the ground as snow. So the steepness, could possible weaken your monsoon winds.

But you asked about the Sahara, why on earth am I talking about the Monsoon in India? well, because you removed the barrier between Asia and Africa and created a open channel of water for moisture to flow into. I believe your monsoon, what ever strength it is, will be able to flow Westwards over the Tethys Sea into Africa through Arabia, Sudan and Egypt.

Arabia will no longer be such a stark desert, the fertile crescent region will be, well, fertile. Presumably for the entire year, with a small decrease during the dry months. This moisture laden air will then flow into Africa itself.

The Red and Dead sea, will probably create another large lake region as all the rain runs off the Arabian landscape. This will give the somewhat depleted moist air, a extra boost, allowing it to penetrate further into North East Africa.

Once in Africa, your monsoon inspired winds, will be able to provide your 'extra source' of rain that everyone has been mentioning. This will be able to supply your Lake Chad and Lake Congo.

With such moisture in the Eastern Air, the sand in this region will not be loose and flowing which you kinda forgot to mention. It will be held down by lush vegetation. You will have undulating hills (composed of loose minerals gathered around rocky outcrops). The further westward you get, the more likely the hills will be undulating hills of tall grasses (think Dothraki hordes). Yes, you can have your Sahara Savannah.

The grassy savannah will then dry out as the air looses it's moisture, before becoming the sandy desert we all know in the middle to far west. This also means, that the Egyptian pyramids probably either were never built or are situated in a very green environment (I'm not saying the Pyramids have to be built in a sandy desert, but I believe the entire religion revolved around the Pharaoh's ability to commune with the Gods and guarantee a good annual flood. If your civilisation isn't relying on an annual event for survival, they are hardly likely to create a religion around it)

The western mountain range will do two things to your interior:

  • it will stop any existing loose sand from blowing into the Atlantic Ocean, and therefore stop any iron fertilisation, carbon sequestering and ocean temperature changes (which will affect the air temperatures) and climates across the globe. Unfortunately this also means, that any ocean life in this region will be lacking in a food source. Fishing opportunities will decrease in this region. There will be increased iron fertilisation occurring north and south of the western mountain range where the sand can possibly spill around the edges, so increased fishing communities can form in those regions instead.

  • It will also, allow more orographic uplift. However, this air will be a lot dryer at the start so it will not be as impressive as the Andes and the Amazon etc. The air to be uplifted, may be further fed by moisture from the warmish Mediterranean in the north and the warmish Atlantic Ocean in the South. This precipitation will not be able to travel back east once formed, so will have to fall in the mountains themselves. Rivers should be able to flow from the western mountains back into the Eastern Interior. Again I have to stress, while there will be rivers, it will not be similar to the Amazon. Maybe a Mississippi or Colorado River. The far western interior back to the East to the Savannah will most likely be stony desert or something close.

The western flank of the western mountain range will be very dry, similar to the real-life Chilian and Peruvian Western Mountain climates. It won't be similar to the Real-life South African West Coast, as there is only a small mountain range in the very south, and the rest is just a gentle slope until Namibia. The elevation will change the dynamic.

A further source of moisture for your savannah, will be your large mountain ranges themselves. Increased elevations, means one thing in mountains. SNOW. Thankfully you will be on the interior side of the mountains, so you should experience any of the potential heavy snowstorms that could occur. When the snow melts it will allow spring and summer melt to flow into the interior of Africa into your Lake C&C.

I'm not 100% certain off the internal wind dynamics, so the ITCZ is just a best guestimate. But from what I can figure out, the convergence zone of the Hadley cell would be a lot further north during the northern hemisphere winter. This should increase the rainfall over the Sahara allowing an almost year round rainfall pattern to occur. I think. I may be wrong here. This will also affect the climate in Europe, but that is beyond the scope of this, or any of your related, questions.

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  • $\begingroup$ Yes, they are interrelated. Where can I find you so that I can show you my entire alternate Earth? $\endgroup$ – JohnWDailey Aug 24 '16 at 0:08
  • $\begingroup$ I was going to suggest you edit your question and just add your world map but see that you have added all the info as a separate question. I thought it would just be a single map.... You have put a lot of work into this worldbuilding.stackexchange.com/questions/52805/… $\endgroup$ – EveryBitHelps Aug 25 '16 at 6:46
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70% sand instead of 15% sand!! OK. I can do this.

Ignoring how it all got there (which is kinda important but I'll go with the flow).

Sand. Is loose. It blows in the wind. Into the Atlantic Ocean.

It's called iron fertilisation. It will impact the creation of phytoplankton blooms downwind. With larger blooms at the surface, this will change the solar reflection/absorption properties of the ocean water. More carbon dioxide will be stored in the ocean (plants absorb carbon dioxide and release oxygen). Sequestered. That's the word I was looking for.

More Saharan dust in the atmosphere will impact more than just the weather in the Sahara. Living in the UK, I constantly hear weather reports about how the Saharan dust is responsible for this or that heat wave. It gets into the atmosphere and blocks the sunlight from exiting back into space. So it increases the greenhouse effect. So your Sahara will be hotter!

How your lakes survive these large amounts of flowing sand and higher temperatures is difficult. Wind in that area generally flows towards the west. So more of the sand will already have been scoured away from Egypt and probably have massive sand dunes in the east and Morroco and Algeria. Hey, at least you will be able to find all the pyramids. Hmm, what were the pyramids made of if there was less stone? Would they have made such huge monuments for one man (not a god) with such scarce resources?

What was I on about? oh the lakes. Your lakes would have to be fed by large underground aquifers. Any rivers above ground will be constantly clogged up with moving sand. (hmm, some areas could become more swampy during the wet rainy season).

And I think you will need to create some mountains to the east of the lakes, or at least raised ridges, to act as windbreaks. With no mountain ridges, lakes will probably be orientated parallel with the wind direction so east to west, rather than in your image where they are more north south. It would be too easy for the wind to clog up a narrow section of lake with sand if it were north to south.

That's it for now. But ya, I don't know how such a situation would arise naturally. But your Sahara would be hotter and sandier. You didn't mention anything about it being windier or having more precipitation, just that you had rivers and lakes (where is the increased water source coming from?). I think, your loose sand will make things drier. Unless you make it rain differently, I don't see how the Sahara savannahs could return.

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  • $\begingroup$ Any idea how to link an image in the comments? $\endgroup$ – JohnWDailey Aug 21 '16 at 23:45
  • $\begingroup$ Nope sorry. The best I can do in comments is just provide the url. $\endgroup$ – EveryBitHelps Aug 21 '16 at 23:59
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I'm pretty sure your two changes will cancel each other out. Or clash terribly, or something.

Water follows the path of least resistance. It runs from high places to low ones, and generally heads towards the sea. Lakes are places where there is a depression in the ground, and water collects there for a while, either evaporating water out as more flow in, or eventually rising high enough for the water to spill over and seek lower ground again.

Your desert is 70% sand. Sand is very mobile, very fluid in its motion. Like water, it will follow the path of least resistance and tends to flow downwards. It is a serious question why, in a desert that is so very mobile, you still have depressions big enough to collect water in. I would honestly expect your desert to be generally about as flat as a pancake, since the sand would fill in as many gaps, nooks and crannies in the underlying bedrock as it possibly could - and at 70%, it can fill and smooth over a lot of variation unless your rock formations are pretty vertical. Maybe a little would pile up here or there, because of wind movement and the shelter of higher rock formations, maybe the sand layer would slant a bit as it gradually moves with the wind or towards the ocean (high to low, just like water) - but I would expect the difference to really be the top layer of sand. Maybe you can take a container of sand and use your fingers to mound and carve it, but shake it out even a bit at a time - and the sand flattens itself again and again over time.

So, your lakes? Will have to be deep. Really, really deep. Because they have to survive all the sand getting dropped in them, from wind and sandstorms and carried by your rivers and flash floods. You will be spending a fair chunk of your 30% rock to line your lakes, and raise the lip of the depressions high enough above sand level that your lakes stand a half a chance. Because I don't know of any mechanism that will get than much sand out of the depression again, to keep it a depression where water can collect instead of an under-sand rock formation. And even if each occurrence is only inches of sand - that sand will be building up storm by storm, flood by flood, by wind and rivers, year after year after year - lots and lots of sand.

Maybe they started as deep cenotes, underground caverns filled with water, and grew to shallower lakes as the caverns filled and sand competed with water for space - which would also, after a while, cut of that connection between the source of moisture and the surface. Maybe your rivers propped up the cenote-lakes, and kept them from evaporating completely, and swept some of the sand out - this would delay, not prevent, the loss of your lakes to burial under the sheer volume of sand, but maybe it's enough for you? And it would take a lot of precipitation to make your system work (make it "upstream" of your desert if you want it to remain a desert), because your rivers will have to be quite big, and carry a fair amount of water, at a rapid pace, to refill the lakes faster than the sun can evaporate them or the sand fill them. Your lakes would tend to collect sand, since the water slows there - which fills the depression in. And the force of the water still coming from the incoming river would carve a channel, and your lake is replaced by a fast moving, eroding river that can keep the sand moving with it.

Of course, if your lakes survive by sweeping the sand out with water means the sand keeps getting swept towards the sea. And sand is fluid, and like water will seek the lowest level. I expect the desert will lose sand into the sea at a relatively rapid rate - it doesn't have to wear rock down into sand, which takes a lot of time and erosion, just let the sand keep moving seaward. Sand will keep getting swept into the depression, and out along the rivers to the sea, until the lip of the rock formations is well above the sand level, and the force of water necessary will also carve deeply into the rock and erode that way, and into the sea. Water can recycle through evaporation and precipitation, but there is no getting sand (or rock) back onto higher ground. So, at the relatively rapid rate at which sand can blow and flow and deposit into the depressions and the water, it will deposit back out into the sea, sweeping a fair amount of that 70% sand out in a geologically short period of time, until there's a high enough proportion of rock again to serve as windbreaks and structural support and stuff to make the land not-flat.

It will take another fair chunk of your 30% rock to line the edges of your desert, to keep the sand from just sweeping into the ocean - and even so, I would expect your continent to be bordered by fairly shallow seas, as the loose sand keeps getting swept to sea, filling the depression, seeking the lower level, and raising the sea floor (a much larger scale version of the sand's attempt to bury your lakes). Again, it will take time for the sand to move, since it will take time for local movement to translate to movement across the whole desert - but every storm, and flood, and wind will slowly work to equalize the whole area.

So, by the end of it all, I would expect that your water system will be inundated with loose sand, and to survive run forcefully and carve deeply to sweep the sand out to sea, until there is much less loose sand and much less capacity for storing water (depressions filled flat, so no lakes, rivers erode deep to deal with water volume quickly instead). Either half the mass of sand will be lost to the ocean, making shallow seas and lots of flat shallow floodplains, and/or the volume of water needed to keep up the lakes will carve deeply into the desert, and you'll end up with deep canyons and cliffs and possibly fragment the desert (depending on the height of bedrock vs sea level vs depths of river carving). Not much like the original Sahara, unless I miss my guess.

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  • $\begingroup$ The only way to make the Sahara as flat as a pancake is to make it an abyssal plain, and after what I've read on Mythic Scribes, habitats that flat can't thrive above sea level. $\endgroup$ – JohnWDailey Aug 22 '16 at 3:29
  • $\begingroup$ @JohnWDailey - Yeah, I don't think it can thrive, which is kinda the point. Its just, sand is fluid enough it won't support elevated structures or depressions, not big differences and not over time unless something else is helping it - and with so much sand, the rock will be like islands in an ocean... not really enough to shape or contain it. Then the water falls of the edge of the world, er, I mean the sand falls into the ocean, and boom - the surviving results will be very different. $\endgroup$ – Megha Aug 22 '16 at 3:52

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