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Around 20,000 years ago, the world was cold.

Temperatures around the world averaged 10 degrees Fahrenheit cooler than they are today.

And most of North America was covered in ice.

Humans were surviving, though, mastering fire and making friends with wolves.

There's places where the ice is a kilometer thick sitting on top of North America, the northern U.S. and Canada.

That's Frankie Pavia, a geochemist at the University of Washington.

He says because the Earth's climate was so different during that time, the winds and oceans move differently than they do now.

And the sort of background state of Earth's climate, right, is just different as a result, both of these changes to the Earth's surface and, right, because there's a hundred parts

per million less carbon dioxide in the atmosphere during this time.

But in the couple thousand years to follow, carbon dioxide levels in the atmosphere and

...