What Caused the Ice Ages?

There are many factors that together can be used to try and explain what caused the ice ages the Earth has experienced. In order for an event to occur many things need to happen. There need to be certain boundary conditions, a trigger (which without those specific conditions would be irrelevant), feedbacks and a tipping point. 
To explain the ice ages we have to set our boundary conditions: the Earth rotating on it’s axis. This is how the Earth is and how it rotates around the Sun, plain and simple. The trigger would then be the Sun’s output of radiation as well as the Earth’s position on the axis as well as in relation to the Sun. These things alone do not cause an ice age, but when factored all together the Earth experiences a feedback. The Earth may be farther away from the Sun at a point and temperature drops because of the lessened amount of radiation reaching the surface. The temperature drops, which causes ice to form. The more ice that forms, the more radiation is reflected from the surface, which causes the temperature to drop even more. The tipping point is the resulting glaciers and ice sheets that cover large masses of land. 

Figure 1: Earth's Axis and Rotation
Image Source: www.eoearth.org
In order to help illustrate the effects that the Earth's position in relation to the Sun has, here is a figure to illustrate how the Earth's axis changes. You can see how the position of the Earth on it's axis has changed as well as it's relative position to the Sun.

Tipping Points

Tipping points sorted by their likelihood of occurring within the next few decades.

Very Likely:

Increased production and emission of carbon dioxide will continue to be a major cause in the increase of the Earth’s temperature and climate changes. If we don’t start cutting emissions now, the effects could become more devastating.

Somewhat Likely:

Having ice-free Arctic summers could start a waterfall effect as higher temperatures could melt permafrost, releasing carbon dioxide and causing further warming and melting.

Not Very Likely:

The runaway greenhouse effect.

One version of the feedback is described as water vapor accumulating in the atmosphere as it warms. The warmer the atmosphere, the more water vapor. The more water vapor that accumulates, the more heat gets trapped, and the temperature continues to rise. This feedback would continue uncontrollably until the oceans have evaporated.

Another version would be that as snow and ice accumulate over the Earth, more and more sunlight is being reflected back into space. This causes a cooling, resulting in more snow and ice, until the entire Earth is covered and becomes frozen.

To read more check out these articles:

• Tipping Points Within Our Lifetime
• Do Global Climate Tipping Points Exist?