Most people view our planet’s vanishing ice as a symptom of climate change. And if they pay a bit more attention, some people might even be aware of some of its effects, including sea level rise and the opening up of the Arctic to shipping. But ice is also an active player in the Earth’s climate—it doesn’t only respond to warming by melting. Changes in our planet’s ice are capable of feeding back on the climate system, creating further consequences for the globe.

The regions of our planet where temperatures fall below the freezing point are characterized by ice and snow, lots of ice and snow. Across land masses, seas, and oceans, roughly 70 percent of the fresh water exists as ice. But now, in response to the warming of our planet, that entire system is changing.

This part of the Earth, where water exists in its frozen state, is called the cryosphere. On land, this includes the giant ice sheets on Greenland and Antarctica, the ice in mountain glaciers, snow on mountain tops, and frozen soil in boreal and tundra regions of the Northern Hemisphere—including large parts of Canada and Russia. The system is dynamic. In the world’s polar regions, sea ice grows in winter and recedes in summer. On land there are also frozen rivers and lakes, which can also experience seasonal variation. The cryosphere is the global story of ice, and it’s a highly active and important component in our Earth’s climate system.