Briefly: earth‘s rotational axis had a different tilt, leading to completely different climate and weather systems behavior. Incidentally, before the green sahara period, the sahara was much larger than today.

There are many higher order gravitational effects which alter earths orbit and axial tilt and therefore both yearlong and seasonal solar radiation patterns.

One of these is a cyclical pattern of tens of thousands of years that caused warming and cooling somewhat less that a degree. For about six thousand years earth was half a degree warmer and weather patterns in the Sahara were different. Then there was some cooling until modern CO2 issues.

In the early twentieth century there were some very real concerns about long term climate cooling as climate science was being developed but instead greenhouse gases have blasted us very far and very fast in the other direction.