The document discusses the key concepts and evidence related to ice ages. It defines an ice age as a period where temperatures greatly decrease, causing expansion of ice sheets in both hemispheres. Geologists classify the current time as an ice age based on the existence of ice sheets in Greenland and Antarctica. Evidence of ice ages comes from geological features left by glaciers, isotopes in fossils and sediments, and changes in fossil distributions. There have been five major ice ages in Earth's history. During glacial periods, climates are cooler and dryer and large ice masses extend from poles. Theories for the causes of ice ages include changes in Earth's orbit, atmospheric composition, meteorite impacts, and shifts in tectonic plates