March through August Ice Edge Positions in the Nordic Seas, 1750-2002


This product contains ice edge coordinates for the Nordic Seas from 1750 through 2002 for months March through August. Ice edge in this product is defined as the outer boundary of ice having at least 30% ice concentration. Data are derived from publicly available sea ice products and use observational data from ship logbooks, diaries and other sources in addition to more recent satellite data products from the Scanning Multichannel Microwave Radiometer (SMMR) and the Special Sensor Microwave Imager (SSM/I). The data are distributed via FTP in ASCII, JPEG, and shapefile format.

Access to the March through August Ice Edge Positions in the Nordic Seas, 1750-2002, product is unrestricted, but users are encouraged to register for the data. Registered users will receive e-mail notification about any product changes.

Citing These Data

As a condition of using these data, you must cite the use of this data set using the following citation. For more information, see our Use and Copyright Web page.

Divine, D. V., and C. Dick. 2007. March through August ice edge positions in the Nordic Seas, 1750-2002. Boulder, Colorado USA: National Snow and Ice Data Center.

Overview Table

Category Description
Data format ASCII text (data); JPEG (browse images); shapefile (GIS format)
Spatial coverage and resolution Northernmost Latitude: 86° N
Southernmost Latitude: 57.7° N
Easternmost Longitude: 64.2° E
Westernmost Longitude: 36.2° W

See Figure 5 for spatial coverage of the data.

Spacing between ice edge points varies throughout the data set. Post 1978 the ice edge is defined by latitude / longitude points approximately every 25 km corresponding to the cell size of satellite passive microwave data. Prior to this, the separation of points is typically between 5 and 10 km, which is determined by the resolution at which the original analog charts were digitized and does not necessarily imply anything about the resolution of the original observations.
Temporal coverage and resolution March through August; 1750 through 2002
File naming convention

ice edge positions (iep) and browse images

iepYYYYMMDD.dat and iepYYYYMMDD.jpg

monthly means (mm) and browse images

mmiepYYYYMM.dat and mmiepYYYYMM.jpg

multidecadal means (mdm) and browse images

mdmiepYYYY-YYYYMMM.dat and mdmiepYYYY-YYYYMMM.jpg

shapefiles (for monthly means and multidecadal means):

Example for monthly mean:

File size The data range from 27 bytes to 61.1 KB per file.
Parameter Sea ice edge positions are defined by longitude (0-360 in decimal degree) and latitude (0-90 in decimal degree) coordinates
Metadata access View metadata
Data access FTP

Table of Contents

  1. Contacts
  2. Overview
  3. Detailed Data Description
  4. Data Access and Related Collections
  5. References and Related Publications
  6. Acknowledgements
  7. Document Information
  8. Appendix

1. Contacts


Dmitry Divine
Norwegian Polar Institute
Tromsø, Norway

Chad Dick
CliC International Project Office, Norwegian Polar Institute
Tromsø, Norway

Technical Contact

NSIDC User Services
National Snow and Ice Data Center
University of Colorado
Boulder, CO 80309-0449  USA
phone: +1 303.492.6199
fax: +1 303.492.2468
form: Contact NSIDC User Services

2. Overview

Ice edge is defined by a consecutive set of geographical coordinates. These positions have at least a 30% ice concentration. This means that nominally the 30% contour line defines the ice edge.

This product is derived from four sea ice input products. In this document, the phrase "input products" refers to the four products in Table 1. The input products span a longer temporal period than does this product. For example, the Arctic Climate System Study (ACSYS) Historical Ice Chart Archive spans from 1553 through 2002, but only data from 1750 through 1978 are used as input to this product. In this document, the ACSYS archive is described as two subsets, constituting the files from 1750-1966 and the files from 1967-1978. These subsets differ in the way they characterize the sea ice conditions.

Date range used in this Nordic Seas data set
Input data source
Name referred to in this document
ACSYS Historical Ice Chart Archive (ACSYS, 2003) ACSYS archive (1750-1966); Norwegian Meteorological Institute (NMI) files, a subset of the ACSYS archive, (1967-1978)
Data based on Soviet aircraft reconnaissance files for the Barents Sea (Fetterer and Troisi, 1997) Soviet reconnaissance files
SMMR passive microwave data (Gloerson et al., 1990) SMMR data
SSM/I passive microwave data (Cavalieri et al., 1990) SSM/I data

Table 1. The four input products. The first column indicates the date range selected for use in this product, the second column lists the input product name, and the third column lists the product short name that is referenced throughout this document. See Related Data Collections for the data set citations.

3. Detailed Data Description


This section summarizes the input products and describes how the ice edge is derived. See Divine and Dick (2006) for additional information, or refer to the original data sources.

Arctic Climate System Study (ACSYS) archive (1750-1966)

The original ACSYS archive is a compilation of historic sea ice data from various sources, including newspapers, ship observations, aircraft observations, diaries and more (ACSYS, 2003). The sea ice data from the original ACSYS archive from 1750-1966 are distributed as shapefiles (ACSYS, 2003), with polylines depicting the sea ice extent. A polyline is defined as straight line segments merged together to form a continuous line. The polylines characterize the ice conditions that are bounded by the line. For example, the red line in Figure 1 depicts the edge of ice concentrations that range from 70-80% ice concentration.


Figure 1. Ice file from the ACSYS archive from May 1906. Ice concentrations are shown with different colors (see legend). The X's in the figure indicate which polylines were selected to construct the ice edge from the original file for this Nordic Seas data set.

Figure 2 shows the attribute table that corresponds to Figure 1. Notice the "ICE_COND" column that is highlighted. These numbers represent different ice conditions. For example, the number "3" corresponds to the red line in Figure 1 that represents 70-80% ice concentration. Another column is "SOURCE": through the data in this column, each polyline can be traced back to its original source (for example, the newspaper the sea ice data originated from).

Figure 2. Sample attribute table from the ACSYS archive from May of 1906. The ICE_COND column contains numbers that reference different ice concentration ranges.

There are usually two files per month in the ACSYS archive from 1750 through 1966. However, only data from one file per month is used in this data set.

Once the shapefile was chosen, the data providers decided which polylines to use to construct the ice edge. Preference was given to lines with 30% or more ice concentration. An ASCII file with geographical coordinates of the ice edge (at least 30% ice concentration) was produced.

Hunters, primarily consisting of sealers and whalers, collected most of the data prior to 1950 and consistently reported ice concentrations ranging from 30% to 60%. Therefore, users should understand that the ice edge is not defined exactly at the 30% ice concentration contour. If sea ice edge data were available as a set of segments (likely from varying sources), they were lined up in sequence to form a continuous set of coordinates in the ice edge file. See Divine and Dick (2006) for further details.

NMI files (subset of ACSYS archive from 1967-1978)

The ACSYS ice chart archive from 1967-1978 contains ice charts collected by the Norwegian Meteorological Institute (NMI) and then digitized at the Norwegian Polar Institute (NPI). These data are based primarily on satellite imagery. Whereas the earlier ACSYS subset from 1750-1966 are distributed as polylines, the data from 1967-1978 use polygons to denote the different classifications of ice concentration (see Figure 3). The ice conditions are categorized as follows: open water (ice concentration below 10%), very open drift ice (10-40%), open drift ice (40-70%), close drift ice (70-90%), very close drift ice (90-100%) and fast ice (ACSYS, 2003). The boundary between very open drift ice and open drift ice was used by the data providers to determine the ice edge. Occasionally, the shape of the marginal ice zone made this inapproprite, and a different boundary (eg. between close drift ice and open drift ice) was used. The file selected contained the best spatial coverage, but if both charts were of comparable coverage and quality, the later file was used (Divine and Dick, 2006). The polygons were converted to a gridded (raster) format directly in the GIS and then the "grdcontour" program of the Generic Mapping Tools (GMT) was used (Wessel and Smith, 1998) to draw the boundary.

Figure 3 shows an example of the original ACSYS file "ice19750403.shp" containing polygons with the different "ICE_TYPE" categories.

Figure 3. This image shows the "ice19750403.shp" file from the ACSYS archive (ACSYS, 2003). The different polygons contain different ice type ("ICE_TYPE") categories as displayed in the legend.

Soviet reconnaissance files (1953-1965)

Data from Fetterer and Troisi (1997) were used to fill in gaps in the Barents Sea that exist in the ACSYS archive. These data are available as gridded ice concentrations with 12.5 km cell spacing. The data set is a 10-day product and usually only one 10-day file per month falls within the spatial extent of this data set. Therefore, the Soviet reconnaissance files provide ice edge data for approximately one file per month from 1953-1965. To produce contour lines corresponding to a given ice concentration, the "grdcontour" program was used (Wessel and Smith, 1998) with the 30% ice concentration denoting the ice edge.

In some instances, the Soviet reconnaissance data overlapped temporally with the ACSYS files. When this overlap occurred, data from both sources were merged, meaning that a file in the current data set can contain data from more than one input product. For example, there may be segments in an ice edge file with one segment from the ACSYS archive and another segment from the Soviet reconnaissance files. For a complete list of merged files, see the Appendix.

The segments of the ice edges obtained for the Barents Sea were subsequently merged with the respective data for the eastern part of the study area. Figure 4 shows an example of an ice edge file.


Figure 4. Merged ice edge file for July 1956 based from the ACSYS archive (second 10-day period of July 1956, dashed line) and Soviet reconnaissance data (from Fetterer and Troisi, 1997; third 10-day period of July 1956, solid line).

SMMR and SSM/I data (1978-2002)

Passive microwave ice concentration data were used from SMMR (1978-1987) and SSM/I (1987-2002). The input data are in an approximate 25 km grid in a polar stereographic projection. The region over the Nordic Seas was extracted and ice concentrations were passed through the "grdcontour" program of the Generic Mapping Tools (GMT) (Wessel and Smith, 1998). The 30% contour line determined the ice edge.

Monthly means

Monthly means were derived using a technique described by Shapiro et al. (2003). With this technique, each ice edge is expressed as a sequence of rectangular coordinates, parameterized by their distance along the ice edge from a fixed location. Points can then be grouped into bins according to their distance along the edge. The monthly mean ice edge is then computed by calculating the mean location of points for each bin for each month of each year.

Multidecadal means

The multidecadal means are calculated from the monthly means described above. They are calculated for each calendar month for these subperiods: 1870-1920, 1921-1961, 1962-1988 and 1989-2002. These subperiods were chosen to correlate with the warming and cooling in the Arctic region described by Polyakov et al (2003).

Spatial and Temporal Coverage and Resolution

Temporal and spatial coverage vary over the period of record. The data density is irregular throughout the data set, with approximately one data file per month from 1850-1966. The ice edge data gradually increase from one file per month to weekly files per month to daily files per month. Spatial coverage varies as well, as seen in Figures 4 and 8.

The geographic domain of the product is shown in Figure 5.

Northernmost Latitude: 86° N
Southernmost Latitude: 57.7° N
Easternmost Longitude: 64.2° E
Westernmost Longitude: 36.2° W


Figure 5. Map of the Nordic Seas with surrounding areas. The ice edge coordinates in this data set fall within the bounding box.

An example of data coverage from one 50-year period from 1850-1899 is shown in Figure 6.


Figure 6. Images illustrate the data density in April (a) and August (b) of sea ice edge positions from the ACSYS archive.

Projection (GIS files)

The GIS shapefiles were created at NSIDC and are distributed in a geographic projection with WGS84 datum. Here is an example projection (.prj) file:


The ice edge positions, monthly means and multidecadal means are stored in the form of ASCII files. Each row in a data file contains coordinates in sequence specifying points along the ice edge. The coordinates are space delimited by longitude (0-360) and latitude (0-90) in decimal degrees.

If the ice edge positions were available as a set of segments (a situation typical for historical ice charts) the points constituting the segments were sequenced as if they stemmed from a continuous line. In this case the ">" symbol is used to separate the ice edge segments in the data file. For example, the iep19660715.dat file from the ACSYS archive contains three">" symbols to separate the ice edge segments in this file. Figure 7 is an example from iep19660715.dat showing a portion of this file with the ">" symbols:

Figure 7. An example ice edge file ("iep19660715.dat") with ">" symbols, which indicate different segments.

Browse images are also available. They are JPEG images that depict the ice line shown in the corresponding data file. Figure 8 (below) shows the iep19660715.jpg browse image that corresponds to the iep19660715.dat file in Figure 7 (above). Although the ice edge appears as line segments in the browse images, the data are actually distributed as points (coordinates). If the ice edge points were connected, lines similar to the ones in Figure 8 (below) would appear.

Figure 8. The iep19660715.jpg file that corresponds to the iep19660715.dat file from Figure 7.

GIS shapefiles are also available. These files can be imported into a GIS software application.

File and Directory Structure







File Naming Convention

ice edge position data: iepYYYYMMDD.dat

Example: iep17650422.dat

monthly means (1967-2002): mmiepYYYYMM.dat

Example: mmiep198404.dat

multidecadal means (1870-2002, divided into ranges: 1870-1920, 1921-1961, 1962-1988, and 1989-2002): mdmiepYYYY-YYYYMMM.dat

Example: mdmiep1870-1920apr.dat

browse images: iepYYYYDDYY.jpg; mmiepYYYYDD.jpg; mdmiepYYYY-YYYYMMM.jpg

Example: iep17650422.jpg

shapefiles: mdmiepYYYY-YYYYMMM.shp;


where iep=ice edge position, mm=monthly mean, mdm = multidecadal mean, GIS=geographic information system, YYYY=year, MM=month and MMM=month (eg. 04=april and 'apr'=april), and DD=day (when the data were collected)

Sample Data Record

A sample browse image from iep17510715.jpg:

The first portion of iep17510715.dat:


Quality Assessment

There are some known limitations in the input data collections referenced in Divine and Dick (2006). March through August months were selected since these months contain a high data density. Therefore, the annual cycle of sea ice advance and retreat is not captured. Some other known limitations are listed below, by data collection:

ACSYS archive (1750-1966):

The NMI data (subset of ACSYS archive, 1967-1978):

The frequency of files increased from four (at the beginning of this period) to eight (by the end of this period) files per month.

Soviet reconnaissance files (1953-1965):

Each input data file falls in one of three 10-day periods of the month. Typically, only one file was available over the study region per month.

SMMR and SSM/I data (1978-2002):

Ice concentration fields from passive microwave data were available every other day from 1979-1987 (SMMR data) and on a daily basis after 1987 (SSM/I data).

Monthly mean:

If only one file is available for a given month, that one file is used as the monthly mean.

Merged data:

Between 1953 and 1965, data from more than one source (Soviet reconnaissance data and ACSYS data) are included in a file. See the Appendix for files that contain more than one source of data.

Users should consider the uncertainties related to the ice edge definition and the difficulty of navigating in the Arctic and sub-Arctic areas prior to the satellite era. An additional source of errors is uncertainty in ice concentrations derived from passive microwave data; this is of particular importance during the melting season, when the emissivity of the ice surface undergoes substantial changes.

Note that the historical ice files from the ACSYS archive were used “as they are”, without applying any reconstruction or interpolation procedures.

NSIDC did not perform a quality check on the sea ice edge data.

4. Data Access and Related Collections

Data Access

Data are available via FTP.

Related NSIDC Data Collections

Arctic Climatology Project. 2000. Environmental Working Group joint U.S.-Russian sea ice atlas. Edited by F. Tanis and V. Smolyanitsky. Ann Arbor, MI: Environmental Research Institute of Michigan in association with the National Snow and Ice Data Center. CD-ROM.

Fetterer, F., and V. Troisi. 1997. AARI 10-Day Arctic Ocean EASE-Grid Sea Ice Observations. Boulder, CO, USA: National Snow and Ice Data Center. Digital media.

Gloerson, P., D. Cavalieri, W.J. Campbell, and J. Zwally. 1990. Nimbus-7 SMMR polar radiances and Arctic and Antarctic sea ice concentrations. Boulder, CO: National Snow and Ice Data Center. CD-ROM.

National Ice Center. 2006. National Ice Center Arctic sea ice charts and climatologies in gridded format. Edited and compiled by F. Fetterer and C. Fowler. Boulder, Colorado USA: National Snow and Ice Data Center. Digital media.

National Snow and Ice Data Center. 2004. Morphometric characteristics of ice and snow in the Arctic Basin: aircraft landing observations from the Former Soviet Union, 1928-1989. Compiled by I.P. Romanov. Boulder, CO: National Snow and Ice Data Center. Digital media.

Other Related Data Collections

5. References and Related Publications

Ackley, S., P. Wadhams, J.C. Comiso and A.P. Worby,. 2003. Decadal decrease of Antarctic sea ice extent inferred from whaling records revisited on the basis of historical and modern sea ice records, Polar Research, Vol.22(1), 19-25.

Divine, D.V. and C. Dick, 2006. Historical variability of sea ice edge position in the Nordic Seas, J. Geophys. Res., Vol. 111, No. C1, C01001, 10.1029/2004JC002851.

Polyakov, I., R.V. Bekryaev, G.V. Alekseev, U. Bhatt, R. Colony, M.A. Johnson, A.P. Makshtas and D. Walsh, 2003. Variability and trends of air temperature and pressure in the maritime Arctic, 1875-2000, J. Climate, 16, 2067-2077.

Shapiro, I., R. Colony, and T. Vinje, 2003. April sea ice extent in the Barents Sea, 1850-2001, Polar Research, Vol.22(1), 5-10.

Vinje, T., 2001. Anomalies and trends of sea ice extent and atmospheric circulation in the Nordic Seas during the period 1864-1998, J. Climate, 14, 255-267.

Wessel, P., and W. H. F. Smith, 1998. New, improved version of the Generic Mapping Tools Released, EOS Trans. AGU, 79, 579.

6. Acknowledgements

This data set is maintained at NSIDC with support from the NOAA National Geophysical Data Center. We thank A. Mahoney (NSIDC) for reviewing this document and for assisting with an explanation for the spacing between ice edge points.

7. Document Information


The following acronyms are used in this document:

ACSYS: World Climate Research Programme's (WCRP) Arctic Climate System Study Mission
DIF: Directory Interface Format
GCMD: Global Change Master Directory
NMI: Norwegian Meteorological Institute
NPI: Norwegian Polar Institute
SMMR: Scanning Multichannel Microwave Radiometer
SSM/I: Special Sensor Microwave/Imager

Document Authors

L. Ballagh prepared this document based on Divine and Dick (2006), ACSYS (2003) and on correspondence with D. Divine and F. Fetterer.

Document Creation Date

June 4, 2007

Document URL

8. Appendix

Merged ACSYS and Soviet reconnaissance data

Some files in this data set use ice edge data from two input sources. The left column below is the name of the file in this current data set (originating from the ACSYS archive) while the right hand column lists the file name from the Soviet reconnaissance files (Fetterer and Troisi (1997)). There are 43 combined data files.