Climate is the product of so many variables - rising and falling carbon dioxide levels, the shifts of continents, solar activity, the stately wobbles of the Milankovitch cycles - that it is as difficult to comprehend the events of the past as it is to predict those of the future. Much is simply beyond us.
Take Antarctica. For at least twenty million years after it settled over the South Pole Antarctica remained covered in plants and free of ice. That simply shouldn't have been possible.