Viewpoint: Yes, ice age cycles of the Northern Hemisphere are driven by complex forces in the Southern Hemisphere, and possibly even the tropics.

Viewpoint: No, the ice age cycles of the Northern Hemisphere are not driven by processes in the Southern Hemisphere; the Milankovitch cycle and disruption of the ocean's thermohaline circulation are the primary initiators.

Ice-age conditions—with a global temperature 9°F (5°C) lower than today's climate and glaciers that cover most of Europe, Asia, and North and South America in a deep blanket of ice—have predominated for 80% of the past 2.5 million years.

Ice ages come in 100,000-year cycles controlled by the shape of Earth's orbit around the Sun. The shape varies, becoming more circular or elliptical every...