Question: why do ice ages occur

Susan Cartwright answered on 16 Jun 2015:

We believe that ice ages occur because the orbit and tilt of the Earth are not completely stable – owing to the small gravitational effects of other planets, the shape of the orbit flexes slightly, and the tilt of our rotation axis (currently 23.5 degrees) also changes slightly.

These changes affect the intensity of the different seasons: for example, a larger axial tilt will produce colder winters and hotter summers.

If more ice forms in the polar regions during the winter, it is harder to melt it, because ice is white – that is, it efficiently reflects most incoming sunlight back into space, instead of absorbing it and heating up. If the conditions are such that not all the snow and ice that forms in the winter melts in the summer, then a vicious cycle is set up in which the extent of ice steadily increases. This is an ice age. The amount of ice in an ice age varies: early in the Earth’s history, there seems to have been at least one “Snowball Earth” episode in which the ice covered the entire planet. This is unlikely to ever happen again because the Sun is brighter now than it was then.

Ice ages stop when the orbital parameters no longer favour the growth of the ice sheet: if more ice melts in summer than is produced in winter, the ice will retreat.

The actual response of the Earth’s temperature to these orbital changes is very complicated, and depends on where the continents are at the time (continents drift, and sea and land respond very differently to temperature changes). It’s really not very well understood.