Arctic sea ice extent is declining at a rapid rate; the extent in September, 2019 was about 30% lower than the average September extent over 1980-2010. Sea ice in both hemispheres can be easily monitored now, with data from a series of satellites that have been operating since the late 1970s. Every year, NOAA publishes the Arctic Report Card. In it, scientists summarize how sea ice, along with snow cover, tundra greenness, marine algae, caribou, and other indicators of change are responding to warming that is taking place about two times faster in the Arctic than elsewhere on the planet.
But how did Arctic sea ice vary in the months, years, and decades before satellites? The history of Arctic exploration goes back at least a thousand years, to when the northern reaches of Russia we being mapped, and Norse sagas recorded ice conditions around Iceland even further back in time. There aren’t many of these early observations. The economic incentive of whaling changed that, beginning in the 1800s. Whaling ship log entries remarked on ice conditions, and archives have preserved many of those records. In the 1890s, the Danish Meteorological Institute began to compile annual yearbooks of ice observations from ships engaged in Arctic trade and exploration. Militaries began to chart ice conditions by air beginning in the mid 20th century.