That's "may have delayed", of course. There's not too much detail here.
As the Earth's orbit slowly changed, the rain started to reduce, and the vegetation started to die back. Around 5,500 years ago, the ecosystem in the Sahara went into a terminal decline towards the desert we have today.
Pastoralism (nomadic or semi-nomadic cattle-herders) blossomed in the Sahara from around 1000 years before that collapse. Previous studies have put the blame for the collapse of the 'Green Sahara' onto these nomads who have often been marginalised in history, but this latest studies dispels that myth.
The study uses a novel climate-vegetation model to determine whether the end of the African Humid Period occurred earlier than expected. The model keeps track of variables such as vegetation and rainfall, and other processes such as the amount of energy coming from the sun, and the amount of carbon dioxide in the atmosphere. The model found that the 'Green Sahara' should have collapsed earlier than it did. This suggests that pastoralists lasted longer than expected and the techniques they used helped them to adapt to the environmental changes.
https://phys.org/news/2018-10-humans-onset-sahara-years.html
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