Earlier in the text this question is provided for, you should be able to find the passage, "The currently accepted theory attributes ice ages to small changes in Earth's orbit, known as Milankovitch cycles. These cycles describe how Earth's orbit gradually changes shape from more circular to more elliptical and back again. This happens roughly every 100,000 years. The orbital changes take Earth nearer and further from the Sun, which would affect the amount of energy reaching Earth."
Answer: Small changes in Earth's orbit
