Final answer:
Two possible causes of large continental ice sheets are global cooling driven by lowering CO2 and changes in ocean circulation, and changes in the tilt of Earth's rotational axis caused by the gravitational effects of other planets.
Step-by-step explanation:
Two possible causes of large continental ice sheets are global cooling driven by lowering CO2 and changes in ocean circulation, and changes in the tilt of Earth's rotational axis caused by the gravitational effects of other planets. In the case of global cooling, when the CO2 levels lower past a critical threshold, ice sheets form in places like Antarctica. Changes in the rotational axis of the Earth can also cause ice sheets to form, as seen in the evidence of past ice ages.