Question

How do scientists think earth's orbit relates to an ice age?

Answer 1

The amount of solar energy received by the earth is directly related to its proximity to the sun. Thus, changes in the orbit can significantly affect the global temperatures.

Explanation:

Deviations in either the orbit or inclination of the Earth towards the Sun will cause significant changes in the solar radiation received by the earth. This is the source of essentially ALL of the sustaining energy of planetary life.

Relatively small changes in solar radiation can have significant changes in the overall global temperature, which will affect the amount of ice that will exist.

Scientific version here:
http://www.roperld.com/science/SolarInsolationVSLatitude.htm

The cycle of these trends are called Milannkovitch Cycles. Details are here:

https://en.wikipedia.org/wiki/Milankovitch_cycles