That sea ice is disappearing in the Arctic regions is often the headline of science news articles today. One read “The least sea ice in 800 years” and the press release explains the evidence—drawn from ships’ logbooks, ice cores, tree rings, and satellite images. “Piece all of the information together and you get a picture of how much sea ice there has been throughout time,” according to Aslak Grinsted, geophysicist with the Centre for Ice and Climate at the Niels Bohr Institute at the University of Copenhagen.

Grinsted’s team concentrated on the ice in the sea between Greenland and Svalbard archipelago from the 13th century to the present. Ice cores taken from an ice cap on Svalbard and information from annual growth rings of trees in Finland gave the researchers a picture of the curve of past climate. 

To determine how much sea ice was there, the researchers turned to the logbooks in which whalers and fishermen kept records of their voyages. The logbooks go back to the 16th century. Harbor records in Iceland go back to the end of the 18th century.

The 13th century was a warm period in geologic history, yet the extent of sea ice was greater than today’s. A sharp decline was noted in the middle of the 17th century, but it lasted for only a brief period. The greatest cover of sea ice occurred around 1700-1800. A period of sharp shrinkage occurred from 1910 to 1920.

 In an article in Climate Dynamics, the research team reports that the present low sea ice extent between Svalbard and Greenland is unique over the last 800 years, and results from a decline started in late-nineteenth century after the Little Ice Age.