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Note: edited based on helpful comments.

In Walking with Cavemen, Robert Winston claims an African rainforest existed 8 mya, but by 4 mya it was gone. He attributes this to newly Himalayas-triggered Indian monsoons, which prevented rainfall from reaching Africa. However, it's my understanding that India collided with the rest of Asia circa 50 mya. So why did the monsoons take so long?

Multiple comments have noted that Winston should have credited the monsoons to the Tibetan plateau. Either way my assumption is that, until the mountains had been pushed up far enough, they didn't threaten the forest's rain supply. On a linear approximation, by 8 mya the Himalayas' mean height would be approximately 85% of its present value, which bounds the "critical value" of the altitude as a percentage of its current one. A similar calculation applies for the Tibetan case.

Why did the motion of India lead to a multi-stage mountain formation? How did the timing of these effects influence Africa?

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  • $\begingroup$ The Tibetian plateau developed later and maybe in two steps see Wang et al (2012) around 10-15 Ma. The India-Asia collision might also have happened in two phases, but much earlier, see von Hinsbergen et al (2012) $\endgroup$ – Tactopoda May 9 '17 at 10:51
  • $\begingroup$ I wrote a pretty detailed related answer with paper citations on Worldbuilding.SE: worldbuilding.stackexchange.com/questions/73842/…. Conclusion, the main line Himalayas that we see today were build in three periods, 21-17 mya, 11-7 mya, and starting 2mya to present. However, for your specific question, the monsoon is driven by the Tibetan plateau, not the Himalayas. To answer your question I'll have to look into when the plateau got as big and high as it is now. $\endgroup$ – kingledion May 9 '17 at 12:58
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    $\begingroup$ It seems dubious, but consistent with television, to attribute climate change in east Africa 3 to 8 Mya to one single cause. There have been several significant geological events in the last 50 Mya beside the formation of the Himalaya and the Tibetan plateau. The isolation of Antarctica by the formation of the Drake Passage and Tasmanian Gateway between 30 and 50 Mya is very big. So is the formation of the Isthmus of Panama 2.6 to 15 Mya. A number of people attribute the Quaternary glaciation (2.8 Mya to present) being due in part to the formation of the Isthmus of Panama. $\endgroup$ – David Hammen May 9 '17 at 18:23
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Drying out of North Africa is likely the result of something far more local. I've never heard of the either the Tibetan Plateau or the growing Himalaya as causing changes to Africa's monsoons and always understood the connection between Milankovich cycles and the consequences of glaciation. According to this study, which modelled weather patterns of the time:

Aridification of the Sahara desert caused by Tethys Sea shrinkage during the Late Miocene

The drying of Africa started much earlier and was likely related to orbital cycles of glaciation-induced drying of the Mediterranean region.

Through a set of experiments, we demonstrate that the African summer monsoon was drastically weakened by Tethys Sea shrinkage, allowing arid, desert conditions to expand across north Africa. Not only did the Tethys shrinkage alter the mean climate of the region, it also enhanced the sensitivity of the African monsoon to orbital forcing, which subsequently became the major driver of Sahara extent fluctuations.

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