No one knows why ice ages occur, but there are several theories. One possibility is that the Sun’s energy varies in intensity over time. Each time there is a decrease in activity, an ice age may occur as the Earth cools. Another possibility is an increase in dust in the atmosphere, either from volcanoes or a large meteorite impact. The debris from either event would reflect more of the Sun’s light into space (albedo), cooling down the atmosphere and causing more snow and ice to form. This would also further increase the world’s albedo, as even more sunlight would reflect off the ice and snow. However, this theory has a problem, as many of the other theories do, in that it doesn’t explain what causes the ice sheets to retreat.