What causes ice ages?
Denis Robertson, Lima, Peru
Scott Elias, a paleoecologist at the University of London, answers:
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 ...
