What causes ice ages?

Denis Robertson, Lima, Peru

Scott Elias, a paleoecologist at the University of London, answers:

Illustration by Christoph Niemann

The most likely causes of ice ages are changes in Earth’s orbit and orientation. The tilt of Earth’s axis increases and decreases over a 41,000-year cycle. A relatively large tilt generally leads to hotter summers and colder winters. Meanwhile, the shape of Earth’s orbit around the sun varies on a 96,000-year cycle. When the orbit is at its most elliptical, the amount of sunlight hitting Earth increases and decreases more intensely over the year. Finally, Earth’s axis wobbles on a 26,000-year cycle. Its changing direction alters the season when Earth is closest to the sun. Major ice ages over the last 1.6 million years have occurred when the variables line up to give the Northern Hemisphere the least amount of summer warmth. At those times, snows from previous winters do not melt completely, eventually accumulating into miles-thick ice sheets. The ice advances, then retreats when the Northern Hemisphere begins to experience particularly warm summers again. The last ice age ended 11,000 years ago. Most interglacial periods have persisted for 10,000 to 15,000 years, so it seems likely that a new ice age will begin, but perhaps not for thousands of years. Human-caused global warming may prevent or stall the next ice age, although no one knows how much of a factor this will be.