There is also the Snowball Earth hypothesis which maintains that the severe freezing in the late Proterozoic was ended by an increase in CO2 levels in the atmosphere. Conversely the drop in Greenhouse gasses in the atmosphere was what began the Ice age. This theory proposes that changes in the earths Atmosphere cause the ice age to occur. It can be seen in current times, rising CO2 and other greenhouse gas levels are in fact, causing the Earths temperature to rise. However it is hard to determine what caused the greenhouse gas levels to drop in the first place.
Position of the Earths orbit This theory is perhaps the most plausible. The Earth is not orbiting the Sun in a perfectly orderly circle. The orbit can fluctuate. Given how sensitive the Earths biosphere is even just a small change in the earths orbit an cause global temperatures to drop dramatically. This theory is supported by evidence that the orbits of other planets are not consistent. The main con is that based on these orbital fluctuation cycles, the Earth should be in ice age right now.