Drastic changes in climate have occurred many times in Earth's history. There is direct fossil evidence of global cooling that took place 35 million years ago, when a tropical climate shifted to a temperate one.
Natural World: Goshen Fossil Bed

Drastic changes in climate have occurred many times in Earth's history. There is direct fossil evidence of global cooling that took place 35 million years ago, when a tropical climate shifted to a temperate one.