Time Magazine: Glaciers are thought to change at, well, a glacial pace. Certainly that has been true throughout the planet’s history. The current ice age — known as the Pilocene-Quatenary glaciation, which began 2.6 million years ago — has witnessed some 20 cycles of glacial (freezing) and interglacial (thawing) periods, with ice sheets advancing and retreating completely on roughly 100,000-year time scales. But scientists are unsure exactly what prompts the shifts in cycles. In glacial periods, …