Gotta throw another out here.
There are shells on the deserts of Northern Africa and Oil deposits caused by Jungles underneath. Science wanted to know when this drastic climate change happened.
Since the wind blows the sands, seeds and other telltale indicators out to Sea they went out to Sea and drilled deep borrings to establish the time between the Jungle and the Desert.
They were shocked to find that about every 20,000 years there is a shift between lush jungle and barren Desert with waterways arriving during the Jungle periods.
Seems that this has happened time and time again over many hundreds of thousands of years.
Natural climate changes------.
Cannot recall if 20,000 is the opposite timetable or the gross of both extremes. If its the opposite then the gross change would take 40,000 years.
Too bad cause I would have liked to see the change-lol.