There are several theories for the cause of glacial periods–Cycles of glaciers followed by interglacial warm periods and then repeating that cycle. Of those theories, the Milankovitch Cycles theory seems to have a broad base of adherents who believe it to have the best answer that question. The Milankovitch theory has some weaknesses. So this posting remains skeptical, however, it appears that there are good reasons why it is probably the most accepted theory. That Richard Lindzen is a supporter of the theory is one of the good reasons.
The Milankovitch theory says that moving in and out of glacial periods is a result of variation in the Earth’s orbit and orientation. Three parameters—Earth’s eccentric orbit around the Sun, the planet’s axial tilt and the procession of its axis are the basis of the theory. These parameters are pretty well defined. The coincidence with certain combinations of the three parameters and the paleohistory of glacial periods is reasonably close. A posting by Doug Hoffman on his blog, the Resilient Earth “Confirmed! Orbital Cycles Control Ice Ages” is very good. I could not say it as well, so this posting will lift much from his.
From Hoffman’s posting:
Earth’s orbit goes from measurably elliptical to nearly circular in a cycle that takes around 100,000 years. Presently, Earth is in a period of low eccentricity, about 3%. This causes a seasonal change in solar energy of 7%. The difference between summer and winter is a 7% difference in the energy a hemisphere receives from the Sun. When Earth’s orbital eccentricity is at its peak (~9%), seasonal variation reaches 20-30%. Additionally, a more eccentric orbit will change the length of seasons in each hemisphere by changing the length of time between the vernal and autumnal equinoxes. (Click on the Chart to enlarge.)
Variation in Axial Obliquity, Orbital Eccentricity, and Polar Precession.NOAA.