 So, why do we go in and out of ice ages? One of the major factors there is the amount of CO2 that's in the atmosphere. If we want to put a lot of CO2 in the atmosphere, the best place to do it is via lots of CO2 coming out of volcanoes into the atmosphere. That sets us off where we start to go into warming trend. As we do that, the ice starts to melt from the poles and the reflectivity of the planet goes down and we absorb more heat and we go into a heating trend. Now how do we get out of that heating trend? One good way is to put a lot of volcanic ash in the upper atmosphere and block the surface for some tens of years. That sends us into a cold spell. The other part of this whole sequence is the weathering of rocks where we have CO2, and it's a lot of CO2 in the atmosphere. We have carbonic acid in the rain, which then interacts with the rocks in the mountains, goes down into the rivers and into the ocean, and we start precipitating limestone. As we precipitate that limestone, we pull the CO2 down and we go into a cold period. So there's this amazing interaction between the organisms in the sea, the weathering of the rocks in the mountains, the positions of the continents relative to the poles, and all of this combines to put us in and out of ice ages.