There are many theories as to why ice ages occur. the currently accepted theory is that ice ages are caused by
increases in the number of living organisms on earth
giant bursts of carbon dioxide into the atmosphere
small changes in earth's orbit
large amounts of oxygen released to the atmosphere from the oceans

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."

Small changes in Earth's orbit

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