Evidence from the geologic record shows that ice ages have occurred relatively few times in Earth’s history, with the first known large-scale ice age taking place approximately 2.3 billion years ago. (In the last about 670 million years, ice ages have occurred less than 1 percent of the time.) Geologists believe there have been five major ice ages over geologic time. The following lists the occurrences:

• 1.7 to 2.3 billion years ago (Huronian Era, Precambrian)

• 850 million years ago (Cryogenian Period)

• 670 million years ago (Proterozoic Era, Precambrian)

• 420 million years ago (Paleozoic Era, between the Ordovician and Silurian Periods)

• 290 million years ago (Paleozoic Era, between late Carboniferous and early Permian Periods)

• 1.7 million years ago (Cenozoic Era, Quaternary Period, Pleistocene epoch)