About 15,000 years ago the world began to warm out of the last ice age. The huge ice sheets that covered North America and Northwest Europe began to melt, and sea level began to rise. But 12,900 years before present, the climate of much of the northern hemisphere made a rapid return to full ice age conditions — cooling by as much as 10ÂºC. The Big Freeze, as it’s sometimes called, could have started in as little as one season. The cold snap lasted 1,300 years before warming resumed and the current interglacial began. This cooling episode is known as the Younger Dryas, because it was associated with an increase in pollen of the Arctic plant Dryas octopetala in Norwegian lake sediments.