And finally, Milankovitch discovered, every 41,000 years there is a change in the tilt of the planet's axis, moving either the Northern or Southern Hemisphere farther from the Sun. These cycles mean that at certain times there is less sunshine hitting Earth, so there is less melting of snow and ice. Instead of melting, these cold expanses of frozen water grow. The snow and ice last longer and, over many seasons, begin to accumulate. Snow reflects some sunlight back into space, which also contributes to cooling.
Temperatures drop, and glaciers begin to advance.
Ice Age Not all ice ages occur on a large, 100,000-year scale. Still, for the people who watched glaciers advance year to year, it was certainly dramatic. In England, the Thames froze and heavy snow fell. In France, bishops were ushered to the edge of glaciers to halt their advance with prayer. During the last 10 million years, Earth has had 10 major ice ages and numerous smaller glaciations such as the recent Little Ice Age.