Ice Ages
U-shaped valleys, moraines, and grooves abraded into bedrock are direct evidence that glaciers have come and gone. They are likewise evidence that Earth’s climate has been dramatically different in the past. Consider that during 90% of the last 600 million years, world temperatures have averaged 72°F; today they average 58°F, significantly cooler, right? Yet, during ice ages, world temperatures were cooler still, probably below 50°F.

The last time Earth’s climate was dramatically different from what it is today was during the last glaciation, an interval when Earth’s global-ice cover greatly exceeded what it is now. The last glaciation, which culminated about 20,000 years ago, was the most recent of a succession of glaciations, or ice ages, that punctuate our geologic past. But what exactly constitutes an ice age? Why do they happen?

Select a topic from the menu at the left to learn more about ice ages.