The recurring deep freezes of the last 2.6 million years, when ice sheets kilometers thick advanced over the northern continents and the seas fell hundreds of feet — dozens of cold-warm cycles paced by slow wobbles in Earth’s orbit and amplified by feedbacks in ice and carbon. We live in a warm pause between them, the Holocene, whose mild stability made farming and civilization possible; the glacial record also shows how sharply the planet responds to a shift in its carbon balance.
Twenty thousand years ago, a wall of ice more than a kilometer thick sat where Chicago and Stockholm now stand. So much of the world's water was locked into that ice that the seas fell hundreds of feet, and people walked from Siberia into Alaska on dry land. This was the last great cold of the Ice Age. Our ancestors lived through it. They hunted mammoth, sheltered in caves, and painted the walls by firelight. Then, about 11,700 years ago, the ice pulled back and the world warmed into the mild climate we still live in. In that gentler world, farming spread, cities eventually followed, and written history much later began. We are not after the Ice Age so much as inside a warm pause within it.