Many years ago, the German engineer Willy Lea proposed that a canal be cut through the Libyan desrt, and mediterranean Ocean water be pumped through it into the Quatarra depression (about 300 ft. below sea level). This flooding would create an inland sea, and in the desrt heat, evaporation would be rapid…perhaps enough would take place to allow heavy raifall in the eastern sahara…perhaps this region would then be farmable.
In addition, the Middle east , which is now critically short of water, would probably experience substantially higher rainfall.
So, would it be feasible to turn the Sahara into a green, arable region? Seems like a LOT less work that trying to regenerate dried-up mars!
Well, the thing about terraforming is that it’s global in effect. On Mars, no one really cares exactly what the effects are on a specific area because you’re aiming to transform the whole ball of sand.
But in the case you posit, what are the side-effects of creating an inland sea in the Sahara? Water flows from the Med and I assume more water then flows * into * the Med to replace it, likely from the Atlantic. Does the temperature of the Mediterranean sea lower? How is the climate affected? Do you have enough water flow to avoid just ending up with a big salt lick in the Sahara? What will additional rain do to the crops of the Middle Eastern countries? Will there be floods?
So – maybe it’s a good idea to modify the Sahara. Sounds like one. But until you can predict the * international * effects, you might expect some opposition.
(Plus, who’s gonna pay for it?)
Y’all* have never been to Phoenix Arzona have you?
The Arizona Canal Project brought much needed water from the Colorado river into the Phoenix Valley, coupled with the Roosevelt Reservior, Apache Lake and canyon lake it brought no significant climactic change, granted that was not the initial intent, but still these are huge bodies of water in the middle of the Sonoran Desert and it is still a desert.
That said, since urban sprawl is at an all time high in Phoenix and it’s surrounding areas…the man made lakes, the constant sprinklers going and everything else, I hear the humidity level in Phoenix is on the rise, ever so slowly…
In all, even a diversion of water from the Mediterranean would do little in the African Desert. And I seriously doubt (if the project actually happened) that there would be any significant reduction in water level in that sea…The oceans are quite large folks.
*IANA redneck
Also, the sand in the Sahara is not as conducive to plant growth as you might think.
Just adding water will not replenish all the things (microbes, minerals, worms, etc…)
that make soil a good environment for plants. It would take tons of terraforming of
the sand to create a lush jungle.
Plus, in human factors, do the desert peoples there actually want the desert turned
to a jungle, a plain, a steppe or whatever?
Would this change the social and economic situations of those countries for the
better?
First of all, let’s be realistic; any terraforming of Mars, if it ever happens, is several centuries away, and would take many millenia to complete.
But as to the question, why put so much effort into terraforming distant Mars and not “improving” the Sahara? Reason number one: Consequences. If we successfully terraform Mars, we get a whole new planet’s worth of land to live on. If we totally screw it up … well, who cares? Aside from the wasted effort, we’re no worse off than when we started. Mars is a dead planet. It can’t get deader.
The earth, on the other hand … If we somehow succeed in making the Sahara a lush jungle, what are the global consequences? Heck, anything we do to the Sahara will have global consequences, even if we fail. And this is the only planet where we know we can live. I’d rather not risk it.
I forgot to mention that there’s at least a theory that we’ve already terraformed the Sahara (http://encyclopedia.thefreedictionary.com/Sahara%20Desert%20(ecoregion)), if by “terraform”, you mean “converted it to desert by overgrazing”.
I’m not sure if it’s known whether the desertification of the Sahara was a natural phenomenon, or the product of bad agricultural and grazing practices in a fairly sensitive and naturally arid area. However it’s possible that the desert could be reclaimed more cheaply than by digging canals, simply by planting and irrigating a whole bunch of trees over a period of many years.
Well, that’s the point, isn’t it? The the proposal would have to be on a massive scale to change rainfall patterns over an entire continent. On the order of a significant percentage of the size of the Mediterranian. That would lower the sea level in the Med, causing a large influx of Atlantic water to flow in. Your Phoenix example, well, yeah. The man-made lakes don’t impact significantly the rainfall because they aren’t (although quite large) large enough. So, what we’re talking about here is something much bigger.
But that said, even if such a body of water is constructed, the results can be unpredictable. The Sahara is, afterall, already surrounded on three sides by water. Much of it equitorial water that evaporates at a fantastic rate, filling the air with moisture. The problem is that water all goes elsewhere. Nearby water is no guarantee of rain. The Namib Desert and the Atacama Desert are two of the driest places on Earth and are nestled right up against the Atlantic and Pacific oceans respectively.
According to a lecture I attended by a USC geology professor, desertification is mostly a product of the current structure of the Earth. The current pattern of Earth’s atmospheric convection draws air up from the Equator and at 60 degrees north and south, and down at about 30 degrees.
Because the equator is mostly water, the warm air that rises from there is saturated with moisture. By the time it reaches the top of the convection cycle, it has cooled and the moisture has condensed out of it as rainfall. The air that comes down at 30 degrees, then is very dry. That’s why, if you look at a climate map of the world, all the major deserts are more or less centered around those latitudes, and pretty much nowhere else. (Rotation eastward puts the deserts mostly at the western ends of large landmasses as well.) Heating of the air by the ground only increases the “desertification” effect.
No doubt careful land use can alleviate the effects, but nothing short of a complete change in the map is going to keep the atmosphere from circulating dry air down to the Sahara. It will always tend to be very dry there.
http://www.death-valley.us/desert2.html
http://phoenix.liu.edu/~divenere/notes/atmospheric_convection.htm
This is going to sound callous, but unless you’re the power in that particular country and have a vested interest in maintaining power over someone, why bother? In a strictly pragmatic sense, nuking the place to ensure that no one survives or otherwise committing genocide is the cheaper approach that solves the goal of providing for those that choose to live there. Of course, relocation is another option that’s probably cheaper. If someone chooses to stay in a nasty place despite the offer of a free resettlement in Fiji, well, then, they’re happy and don’t care about the lack of rain.
Huh? Why bother turning an enormous barren wasteland into productive farming and grazing land? On a continent where starvation is endemic and poverty is widespread?
I’m reasonably sure you didn’t think your post through carefully.
Isn’t it cheaper to kill people anywhere* than to take care of them? How about this idiots who live along the east cost who always getting hit by hurricanes? Or the folks in Tornado Alley? Or those dumb asses who live within 10 miles of the Mississippi?
We better get busy, there’s a shitload o’ killin to do!
Back to the OP…
The Salton Sea/Sink in SE California is an example of an answer of what you’re seeking. Although there is no “heavy rainfall” from the recent geological existence of the inland sea, there is farming from the irrigation that takes place because of it.
Regarding the salton Sea: it is drying up because there is no input of water into the basin. if the Quattara Depression were flooded (with salt water from the Mediterranean), a vast inland sea, possibly thousands of square miles, would be created. The evaporation from this sea would be very significant…maybe enough to cause a rain-forest type climate in Irael and Syria.
Lets do it!
Why terraform Mars, instead of the Sahara Desert?
Because Mars lacks crazy people with guns who are trying to kill you, for one thing.
My thinking was more along the lines of Balthisar’s comment. If people are making money off of a disadvantage, in this case the lack of water and arable land, it’ll take awhile before they set about trying to fix anything. 
This would make sense for a geologist to say, since it outlines factors which make certain regions more susceptible to desertification. However a desert is not a geological feature; it’s an ecological structure. Desertification is mostly due to ecological factors (though geological factors play an important role.
The “global circulation” argument would apply at all times, yet the geological, paleontological, archeological and even historical record simply doesn’t bear it out. Continental drift and the fossil record allow us observe the effects of many biomes and geographic structures as they slowly crawled through the “desert danger zone” you describe, and desertification was NOT the rule. Even in historical times, most of what is currently the Sahara was not desert.
Indeed, the growth of the Sahara, at rates of up to 20 miles/year has been well mapped over the 20th century, and its causes well explored. Human destruction of forests, disruption of grasslands, etc. led to a positive feedback system: as the land ‘died’ it was less capable of supporting life. Look up the boundaries of the Sahara in 1900 (or better yet, one of the animations of its growth over the century); if “hard geological facts” define its extent, why was it a fraction of its current size in the memory of current humans? Why does it cover regions that had long been fertile according to historical records?
Similarly, though it is popular to think of the American Indians as “living in harmony with the land” there is a substantial body of evidence that their deforestation practices led to the creation of the grassy Great Plains and the contributed to expanding the deserts of the American Southwest.
I don’t want to minimize the magnitude of geological effects, but as great as they are, they merely influence the biological system. It is almost impossible to overstate the effect of biology on climate and weather. Our nitrogen-oxygen atmosphere is a completely biological creation. Soil is a completely biological creation–geology can only create ground stone). The Earth’s convection patterns, local and global, would be quite different without life: the key circulation patterns over South America (probably the closest available comparison to Africa) are almost entirely driven by patterns of local vegetation.
In fact, it’s well documented that human cities create unique microclimates. In the past ten years, some areas (e.g. DeKalb County, Atlanta, GA) have actively encouraged planting of ornamentals and shade cover, with quite dramatic effects -five degrees in the first few years!- on summer temperature, humidity, and thunderstorm patterns as far as 50-100 miles away. We’re not talking about massive reforestation, or hugely intrusive government programs; we’re talking about public education, community landscaping projects, and businesses planting shade trees to reduce air conditioning costs.
Although the Qattara Depression is fairly large - 7,000 sq mi, or a bit smaller than Lake Ontario - it is very very small compared to the size of the Sahara, and it is also very close to the Mediterranean. I think it’s virtually impossible that flooding it would have any significant effect on the climate of the Sahara in general, or even a widespread effect on the eastern Sahara. There could possibly be some effects on northern Egypt, but even that is doubtful. After all, how much impact has the Aswan High Dam and Lake Nasser (different sources give its area as anywhere from 1,500-2,600 sq mi) had on Egypt’s climate?
Okay, assuming the powers that be decided that it would be valuable to create a new inland sea, how long would it take? Presumably the channel to the sea would be how large? How large a basin to fill? How deep? I think it would be interesting to try out.
When dams are built, how long does it take to typically top out the backwater? Obviously the size of the damn is important – how long did it take Lake Mead to fill, for example? Has Lake Mead appreciably altered the climate around Hoover Damn? It still looks like desert to me, but Lake Mead’s not huge like you’re proposing.
Also wouldn’t there have to be outflow, too, to prevent the ocean from dying from over-salinity?
You mean they planted enough vegetation that the temperature and humidity dropped in and around the city? Does that mean the plants took up enough moisture from the ground and air to reduce humidity and relfected and absorbed enough sunlight to take down the temperature? Cool 
In roman times, the Sahara was what fed most of the roman empire. The sahara is a rather interesting case in that it constantly oscillates from lush forest to dead desert. Even a little vegetation will be enough to capture the moisture rich air flowing over it which increases rainfall which consequently increases vegetation and moisture retention until you have quite productive land. On the other hand, a slight reduction in vegetation reduces the rainfall which has a flow on effect into turning it into arid desert. Most of this research is quite recent and we still don’t know many of the details which just highlights how careful we should be going into large climate changing projects as the effect could be quite dramatic.