Polar ice caps, continental ice sheets, glaciers, permafrost and all other frozen aspects of the planet are collectively known as the cryosphere.

Polar ice holds at least 80 percent of the planet’s fresh water. The biggest share is in the Antarctic, whose ice sheets constitute more than 7 million cubic miles of ice, one-third of the ice that existed during Earth’s regular glacial periods, or ice ages, which are currently occurring every 100,000 years or so. These glacial cycles are paced by the cycles in Earth’s orbital mechanics, regular changes in its tilt, wobble and eccentricity, but are amplified by ice and snow.

Changes to the cryosphere can occur in many ways, however, with these changes affecting overall global climate in three major ways. One involves changes in the reflectivity of the surface as snow and ice melt and vegetation cover changes; the second involves changes to ocean circulation as Arctic ice melts, adding fresh water to the oceans; and the third involves changes in the amounts of greenhouse gases emitted to the atmosphere from the land as warming progresses.