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So 5-6 million years ago, the strait of Gibraltar closed due to plate tectonics. Because of this the Mediterranean dried up (maybe leaving a couple small very salty lakes). This would have made Europe much drier, maybe even gave it a desert. This event is expected to happen again in the future.

So 5-10 thousand years ago, a combination of Milankovitch cycles stuff, topography, and albedo-by-vegetation feedback loops caused the Sahara desert to become a vast grassland. This event will also happen in the future, as the Greening and Deserting of the Sahara happens in a 41000 year cycle.

I am thinking of a world in the future where both of these events happen at the same time. However, these conditions are obviously contradictory. The Salinity Crisis happened mainly because the strait of Gibraltar closed, which prevented the Mediterranean from being able to receive water from the Atlantic (The Mediterranean evaporates much of its water, and Atlantic Ocean is it's main source of water.) Green Sahara happens due to Axial Changes over time, the Northern Hemisphere would receive more energy, which means more evaporation, which means more rainfall, which is the seeds for plants to grow. Plants draw out groundwater and most of it is evaporated off leaves, more water. The plants increase the albedo which means more energy which means more rainfall and- oop we have got a feedback loop.

As you can see, with a grassy Sahara that receives rainfall (and the seasonal monsoon), it seems a little weird that there could be a very dry Mediterranean Basin next door. So what I want to ask is, would it be plausible for a Green Sahara to co-exist right next to a dried up Mediterranean Sea (with the exception of some small lakes of brine)?

Notes:

  • There must be a Green Sahara in the Sahara (with its rainfall and its monsoon)
  • There must be a desiccated (dried up) Mediterranean Basin (going through something similar to the Messinian Salinty Crisis), with the exception of some small very very salty lakes collected at the deepest parts.
  • To be more specific about the Mediterranean Basin, it should not be receiving much rainfall, however the Sahara must receive a nice dose of it, for it to be grassland. The Mediterranean sea can permit a small amount of it, as evaporation will take care of it
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  • $\begingroup$ How can Mediterranean dry up if it has so great influx of fresh water from numerous European rivers? Look at Caspian sea, it's located close to a desert, does not have a drain but does have a single great income of Volga river that keeps it filled. Mediterranean has several rivers flowing into it of similar capacity, so I doubt that it would dry up. $\endgroup$
    – Vesper
    Apr 23 at 5:00
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    $\begingroup$ @Vesper: The Mediterranean has a large water deficit, that is, very much more water evaporates than comes in from the Rhone, the Nile and the Black Sea. If the Strait of Gibraltar were to close, cutting off the massive influx of water from the ocean, the Med would dry out in a few centuries, at most one measly millennium. This is why the Med is considerably saltier than the ocean, and why the eastern Mediterranean is about one meter (3 feet) below the notional word-wide sea level. And there is no room for doubt. We know it dried out, and it dried out several times. $\endgroup$
    – AlexP
    Apr 23 at 7:34
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    $\begingroup$ @Vesper: The Caspian has much smaller surface area and is situated in a much less warm climate. In addition, the Volga is a very large river. It carries more than twice the amount of water carried by all the rivers which empty in the Mediterranean and the Black Sea combined. $\endgroup$
    – AlexP
    Apr 23 at 7:43

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It would be not just plausible, but likely.

For Sahara to be green, you need the Intertropical Convergence Zone (ITCZ) to move further northwards during the summer than it does today. Milankovich cycle and other mechanisms cause that periodically, as you described. ITCZ effectively collects moisture from all over the tropics into a narrow(ish) band, providing plenty of rain as a result. So this is the easy bit.

The even easier bit is that the bottom of a dried out Mediterranean will always be dry, precisely because it lies at such a low elevation. Any air reaching it would necessarily have to descend, and will become hot and dry as it does that. The exact mechanism is that as the air descends, its pressure increases (because there is then more air above it, weighing down on it); the increased pressure causes the air's temperature to rise; and the warmer air can hold ever more water vapor as it keeps descending, which prevents cloud formation and therefore rain. It's the same mechanism that keeps Death Valley brutally hot and dry, but on a much larger scale in all dimensions.

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