Sahara desert from space [image credit: NASA]
We’re informed that ‘scientists have identified more than 230 of these greenings occurring about every 21,000 years over the past eight million years.’ (Ice ages may account for the numerical shortfall). Climate models aren't able to simulate these greenings, or at least not their magnitude. The period they identify is the combined precession cycle, i.e. the beat period of the tropical and anomalistic orbits (years) of the Earth (when the difference between the number of each reaches 1). Inevitably it’s about the energy received from the Sun.
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Algeria’s Tassili N’Ajjer plateau is Africa’s largest national park, says The Conversation (via Phys.org).
Among its vast sandstone formations is perhaps the world’s largest art museum. Over 15,000 etchings and paintings are exhibited there, some as much as 11,000 years old according to scientific dating techniques, representing a unique ethnological and climatological record of the region.
Curiously, however, these images do not depict the arid, barren landscape that is present in the Tassili N’Ajjer today.
Instead, they portray a vibrant savannah inhabited by elephants, giraffes, rhinos and hippos. This rock art is an important record of the past environmental conditions that prevailed in the Sahara, the world’s largest hot desert.
These images depict a period approximately 6,000–11,000 years ago called the Green Sahara or North African Humid Period. There is widespread climatological evidence that during this period the Sahara supported wooded savannah ecosystems and numerous rivers and lakes in what are now Libya, Niger, Chad and Mali.
This greening of the Sahara didn’t happen once. Using marine and lake sediments, scientists have identified more than 230 of these greenings occurring about every 21,000 years over the past eight million years. These greening events provided vegetated corridors which influenced species’ distribution and evolution, including the out-of-Africa migrations of ancient humans.
These dramatic greenings would have required a large-scale reorganization of the atmospheric system to bring rains to this hyper arid region. But most climate models haven’t been able to simulate how dramatic these events were.
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Earth’s changing orbit
The fact that the wetter periods in north Africa have recurred every 21,000 years or so is a big clue about what causes them: variations in Earth’s orbit.
Due to gravitational influences from the moon and other planets in our solar system, the orbit of the Earth around the sun is not constant. It has cyclic variations on multi-thousand year timescales.
These orbital cycles are termed Milankovitch cycles; they influence the amount of energy the Earth receives from the sun.
Full article here.
Tallbloke's Talkshop 
On a closer look it becomes obvious we’ve covered this before. Why The Conversation picked it out at this time is unknown.
I assume the reason they have returned to this is because they need a distraction from all the failed climate predictions. If memory serves they were pushing the Mann made climate change when he first proposed it and all the computer games (models) have failed so far, anything to cover that failure is considered necessary.
More recently, North Africa was the breadbasket of the Roman Empire, presumably because of the Roman Warm Period. Carthage was a great city and must have been far more “green” than now in terms of farms. I suspect the ending of the warm period with its effects on bread production and the effects on steppes that pushed nomadic tribes West and South are important causes of the collapse of the Empire. My bet remaining on obliquity for these non-cyclic changes with relatively random periods.
Just to say quickly that, as well described by Ralph Ellis of Ellis and Palmer fame, you only get an interglacial warming event when the Northern hemisphere who is at perihelion to The Sun during the maximum eccentricity of the 100 Ka cycle.
So the northern hemisphere is as close to the Sun as it ever gets at interglacial time, which suggests this is why the ice caps melt at this time.
Average insolation will vary +/-40W/m^2 at this time. It is the case that eccentricity cancels out the net effects of insolation variability over one year BUT it is also the case that this is only symmetrical at the interface with space, and that the same heat will produce double the perturbation in the northern hemisphere which has half the ocean volume hence heat capacity/storage, roughly .
I’m just off to do a global podcast on geothermal heating with Tom Nelson should prove interesting. Sharing it with Wyss Yim and Arthur Viterito, oh and James Kravis, probably the heavyweights in this. I’m doing the volcanoes. BRian
Sorry, too much in hast. I should have said, of course, that the perihelion of the Earth to the Sun with the northern hemisphere closest to the Sun is a function of the 23 Ka precessionary cycle combining with the hundred Ka eccentricity cycle maximum. And the ice melts…… 130m of global sea level’s worth. It’s what the Sun is for, probably. Simples. Perhaps helped n byvolcanoes under maximum solid tides (my bit)