The amount of water frozen in Earth's ice has changed throughout the planet's history. Earth's ice budget (total ice volume) grows when the planet's average temperature falls and shrinks when it rises. During colder periods called ice ages, ice caps (large dome-shaped glaciers) extend far from the North and South Poles, and mountain glaciers (large masses of moving ice) advance into lowlands. During warmer periods, ice retreats back toward the poles and up mountain valleys. Earth has even been completely ice-free several times during its long history.

Earth's water budget, the total amount of water on the planet, does not change over time. When more water freezes into ice on land, there is less water in the oceans, so global sea level falls during ice ages. Shorelines move seaward, and the edges of the continents are exposed above sea level...