Overview

From 1893 to 1956, map makers at the Danish Meteorological Institute (DMI) created hand-drawn charts of observed and inferred sea ice extent for each summer month, generally April to August but occasionally March and September as well. Scanned versions of these charts were released as the Arctic Sea Ice Charts from Danish Meteorological Institute, 1893 - 1956 data set and are the input data for the sea ice concentration and extent estimates described in this document.

The original DMI charts are based on compiled observations of ice conditions reported by a variable network of national organizations, shore-based observers, scientific expeditions, and ships that are detailed by month in a report that accompanies each year of charts. In cases where no observations were available, the lead map makers may have drawn further ice cover using their knowledge of ice climatology and movement. They show observed or inferred ice extent as a light-colored field and ship observation locations as red symbols, with the symbol representing a type of ice cover that can be used to infer concentration (for example, tight pack-ice, for which we infer a concentration of 7/10 to 9/10, or 80% ± 10%). Figure 1 shows an example of these original charts. The original charts are particularly valuable for research about Arctic sea ice because of the length of the record and the early time period they cover.

The data set described in this document was created to convert the original DMI charts to a more usable format for research. To accomplish this, ice extent and areas of the same concentration were traced by hand on top of each digitized chart using a graphical image manipulation program. Areas of ice where observations had been made were color-coded red, with a saturation value that indicated percent concentration. Areas where the presence of ice was inferred by map makers rather than observed by ships were assigned a light-blue color. Each resulting image of concentration and extent was then projected into a common map framework using a Graphical Information System (GIS). Shapefiles were created and concentration and other information were included in attribute tables. Because the original hand-drawn DMI charts are in a polar stereographic projection, the derived sea ice concentration and extent estimates were kept in this projection as well.

This derived sea ice concentration and extent data set is comprised of both JPEG and GIS shapefile formats. There are 253 processed charts, one for each of the original DMI charts from 1901 to 1956. For most of the record, there is one chart per month covering April through August; however, for the years 1901 and 1907, there are charts for September; and 1902 has a chart for March. There is a gap in the record for the years between 1940 and 1945. Spatial coverage varies from year to year, but observations are much more highly concentrated in the North Atlantic region. Although the original hand-drawn DMI charts used to create these estimates begins in 1893, this data set begins in 1901 because charts earlier than 1901 do not cover the entire Arctic.

These data are contributing to larger data collections, including the Historical Sea Ice Atlas of Alaska sea ice, mid-1800s to the present, that are part of a joint project funded by the Alaska Ocean Observing System (AOOS), the Alaska Center for Climate Assessment and Policy (ACCAP), and the Scenarios Network for Alaska and Arctic Planning (SNAP)

Background

The early history of this collection is described in Thomsen (1947). In 1885, the DMI began systematically collecting information on sea ice in the Davis Strait. Ship captains sailing to settlements in west Greenland and residents of Greenland were asked to record all ice they observed. This sea ice observation program quickly grew to include observers on ships from other nations and to cover a wider geographic area. Beginning in 1900, these observations were collated and published annually in reports entitled "The State of the Ice in the Arctic Seas." In 1901, the charts' coverage expanded from a North Atlantic view to pan-Arctic coverage. Each chart shows observed ice (using standard phrases like unbroken polar ice or open ice) as well as the inferred ice edge in non-observed areas. The charts were published yearly along with a detailed written summary of pertinent ship logbook observations and ship traffic patterns.

Kelly (1978, 1979) recognized the value of these charts for research and digitized the ice edge shown in them as a graduate student at the University of East Anglia. He described the DMI charts as "the only major chart series containing circumpolar ice limits for the first half of this century" (Kelly 1978). Kelly digitized only the inferred ice edge and only to a spatial resolution of about 100 km, depending on the distance of the ice boundary to the pole (1978). He chose not to improve the resolution because of the low accuracy of the inferred ice edge itself. The eventual plan was for this digitized data to be incorporated on a one-degree grid into larger sea ice data products as an ice/no ice indicator.

Kelly also worked on assessing the reliability of DMI chart data. He notes that coverage differs spatially and temporally and planned to use both auxiliary historical sources and statistical methods to find and flag gross errors in ice extent data. He also suggested giving the data, as a whole, a series of reliability ratings. To the best of our knowledge, this work was never completed.

Kelly's work was cited by Sear (1988) as part of a larger group of Arctic sea ice data sets from which Sear created a simple index of sea ice extent. Kelly's digitized edge was also used with other historical data in a gridded sea ice data set first published in 1991 (Chapman and Walsh 1991). For complete details on the original charts, see the Arctic Sea Ice Charts from Danish Meteorological Institute, 1893 - 1956 documentation.