Sea-ice cover is the most significant feature of the polar ocean environment. Annual cycles of sea-ice cover and their interannual variations contribute significantly to changes in surface reflectivity and turbulent heat fluxes from the Earth’s surface that affect high-latitude climate sensitivity. Polar sea ice also influences the deepwater formation in the world ocean. We know little about sea-ice conditions before the late nineteenth century. Consistent weekly mapping of ice extent in the Arctic seas dates from the early 1950s and for the Antarctic from the 1970s. Moreover, the fractional coverage (or concentration) of sea ice in the interiors of the polar packs was largely unknown until the advent of polar-orbiting satellites.