Sea ice is frozen seawater that floats on the ocean surface. Sea-ice cover expands in Earth’s polar regions each autumn and winter, as ocean water freezes in response to cooling of the atmosphere and ocean. Summer warming melts much of this seasonal ice, with annual sea-ice minima in each hemisphere typically occurring in late summer or early autumn. Changes in sea-ice cover affect ocean circulation, atmospheric dynamics and surface heating. Sea ice extent is the area of the ocean where the sea ice concentration is greater than 15%.  A concentration of 15% is used to define the edge of the ice pack. Sea ice has been measured by satellites since 1978.

Glaciers are formed from snow that has compacted to form ice, which then deforms and flows downhill. An ice sheet is an expanse of ice originating on land that covers an area of more than 50 000 km². There are two principal ice sheets, one in Greenland and one in Antarctica. Loss of ice from glaciers and ice sheets contributes to global mean sea-level rise. Changes in land ice also have implications for freshwater resources, ecosystems, and communities.