Ice Ages and Warm Spells

Human history sits inside these swings. At the peak of the last Ice Age, ending about 11,700 years ago, ice roughly 600 meters (about 2,000 feet) thick covered the site of New York City.

The Medieval Warm Period (about 900–1300 AD) was mild enough that Norse settlers farmed parts of Greenland. Then the Little Ice Age (about 1300–1850 AD) flipped the script — the Thames in London froze hard enough for whole 'Frost Fairs' on the ice. A swing of just one or two degrees did all of this.