Total Ice-Covered Area and Extent
NSIDC distributes several products for ice-covered area and extent. Along with one historical data set from the Electrically Scanning Microwave Radiometer (ESMR), these products are derived from the Scanning Multichannel Microwave Radiometer (SMMR), the Special Sensor Microwave/Imager (SSM/I), and the Special Sensor Microwave Imager/Sounder (SSMIS). The data sets are computed using either the NASA Team or Bootstrap algorithm, and the products are provided by the NASA Goddard Space Flight Center (GSFC). See the Sea Ice Products Frequently Asked Questions page for more information about the differences between the algorithms. See the descriptions on this page for information about each product and to access the data. Click one of the links below to jump to a product description.
- Regional Graphs: Time-Series Plots
- Daily and Monthly Total Sea Ice Extent and Total Ice-Covered Area
- Merged ESMR, SMMR, and SSM/I-SSMIS Sea Ice Extent
- References
Regional Graphs: Time-Series Plots
The regional graphs show monthly time-series plots of ice-covered area, area anomalies, ice extent, and extent anomalies for both the NASA Team and Bootstrap algorithms. Table 1 lists the Arctic regions covered by this data set along with their abbreviations found in the data files. Table 2 lists the Antarctic regions covered by this data set along with their abbreviations found in the data files.
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To see the time-series plots of area,
area anomalies, extent, and extent anomalies for each region, click on the region names in the maps in Figures 1 and 2.
To see graphs
for an entire area, click on the Total Arctic or Total Antarctic links below.
Total Arctic |
Total Antarctic |
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| Figure 1. Northern Regions | Figure 2. Southern Regions | ||
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Daily and Monthly Total Ice-Covered Area and Total Sea Ice Extent
In computing the total ice-covered area and ice extent with both the NASA Team and Bootstrap Algorithms, pixels must have an ice concentration of 15 percent or greater to be included. Total ice-covered area is defined as the area of each pixel with at least 15 percent ice concentration multiplied by the ice fraction in the pixel (0.15 to 1.00). Total ice extent is computed by summing the number of pixels with at least 15 percent ice concentration multiplied by the area per pixel, thus the entire area of any pixel with at least 15 percent ice concentration is considered to contribute to the total ice extent.
There is a circular section over the Northern Hemisphere pole (known as the "pole hole") which is never measured due to orbit inclination. For the purposes of ice extent, pixels under the pole hole are always considered to be at least 15 percent. For total ice-covered area, the pixels under the pole hole are not used. The Southern Hemisphere also has a pole hole, but it does not affect this sea ice data set because there is only land under this hole. For SMMR, the hole is 611 km in radius and is located poleward of 84.5 degrees North. For SSM/I and SSMIS, the hole is 311 km in radius and is located poleward of 87.2 degrees North. Note: The difference in pole hole areas between SMMR and SSM/I-SSMIS results in a discontinuity in the Northern Hemisphere ice-covered area time series across the instrument transitions.
NASA Team Algorithm Data Sets
Daily and monthly total ice-covered area and total sea ice extent data spanning the SSM/I-SSMIS record from October 1978 through the most recent processing date are provided by Don Cavalieri of the NASA Goddard Space Flight Center (GSFC) Oceans and Ice Branch, and are produced from the NASA Team Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data. Preliminary data produced by NSIDC are used in this data set while the GSFC processes and quality checks the most recent SSM/I and SSMIS data. The data are flagged as preliminary in the version column of the data files with the letters pre. Once the GSFC data become available, the files are updated with the final version of the data.
Bootstrap Algorithm Data Sets
Daily and monthly total ice-covered area data and total sea ice extent spanning the SMMR and SSM/I-SSMIS record from October 1978 through the most recent processing date are provided by Joey Comiso of the NASA GSFC Oceans and Ice Branch, and are produced from the Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS data set.
File Descriptions
The following file descriptions pertain to both the NASA Team and Bootstrap algorithm data sets. Data are provided in two different temporal resolutions: daily and monthly.
Data Access
| NASA Team | Data are available via FTP. |
|---|---|
| Bootstrap | Data are available via FTP. |
Daily Data
Data Format
Data are in ASCII text format arranged in columns as described in Table 3.
| Column | Description |
|---|---|
| Year | 4-digit year |
| Mon | 1-digit and 2-digit month |
| Day | 1-digit and 2-digit day of month |
| DOY | 3-digit day of year |
| Ver | Version number (pre: preliminary, v01: version 01, v02: version 02) |
| Region | The last columns of the Arctic and Antarctic daily area and daily extent files list the total ice-covered area (km2) and total sea ice extent (km2), respectively. The columns are labeled by region; see Table 1 and Table 2 for a list of abbreviations for the column names for the Arctic and Antarctic regions, respectively. |
File Naming Convention
There are two different types of daily files: ice covered area files and sea ice extent files. The files are named according to the following convention, and as described in Table 4:
gsfc.algorithm.daily.area.1978-yyyy.r
gsfc.algorithm.daily.extent.1978-yyyy.r
where:
| Variable | Description |
|---|---|
| gsfc | Indicates that this is GSFC data |
| algorithm | Algorithm used to process the data (nasateam or bootstrap) |
| daily | Indicates that this file contains daily data |
| area | Indicates that this file contains ice covered area measurements |
| extent | Indicates that this file contains sea ice extent measurements |
| yyyy | 4-digit year (most recent year of processing) |
| r | Region (n: Arctic, s: Antarctic) |
Monthly Data
Data Format
Data are in ASCII text format arranged in columns as described in Table 5 and Table 6.
Monthly mean sea ice concentrations were used to derive
monthly extents, rather than averaging daily data. Using monthly mean sea ice concentrations may provide a
more accurate monthly ice extent since the effects of storms on
the ice edge are less pronounced in the monthly mean sea ice data
than in the daily sea ice data. However, this has not been verified by NSIDC.
| Column | Description |
|---|---|
| Year | 4-digit year |
| Mon | 1-digit and 2-digit month |
| Ver | Version number (pre: preliminary, v01: version 01) |
| Region | The last columns of the Arctic and Antarctic monthly area, extent, and anomaly files list the total ice-covered area (km2), total sea ice extent (km2), and the difference from the mean, respectively. The columns are labeled by region; see Tables 2 and 3 for a list of abbreviations for the column names for the Arctic and Antarctic regions, respectively. |
| Column | Description |
|---|---|
| Mon | 1-digit and 2-digit month |
| Region | The last columns of the Arctic and Antarctic monthly mean files list the data (area or extent) averaged over the entire temporal coverage for each month. The columns are labeled by region; see Tables 2 and 3 for a list of abbreviations for the column names for the Arctic and Antarctic regions, respectively. See the Note below for more information on monthly mean files. |
Note: Sea ice data are missing for most days in October 1978 and December 1987, so monthly values are not calculated for these months. January 1988 is missing data from the first half of the month. A value is given for this month, but the value may not represent true monthly mean since the sea ice data are incomplete.
Monthly anomalies of ice-covered area and ice extent are calculated from monthly mean ice concentrations throughout the period of study. These anomalies show the deviations from the mean monthly values averaged over the entire time series.
File Naming Convention
There are six different types of monthly files: ice covered area files, sea ice extent files, ice covered area anomaly files, sea ice extent anomaly files, monthly mean ice covered area files, and monthly mean sea ice extent files. The files are named according to the following convention, and as described in Table 7:
gsfc.algorithm.month.area.1978-yyyy.r
gsfc.algorithm.month.extent.1978-yyyy.r
gsfc.algorithm.month.anomaly.area.1978-yyyy.r
gsfc.algorithm.month.anomaly.extent.1978-yyyy.r
gsfc.algorithm.month.mean.area.1978-yyyy.r
gsfc.algorithm.month.mean.extent.1978-yyyy.r
where:
| Variable | Description |
|---|---|
| gsfc | Indicates that this is GSFC data |
| algorithm | Algorithm used to process the data (nasateam or bootstrap) |
| month | Indicates that this file contains monthly averaged data |
| area | Indicates that this file contains ice covered area measurements |
| extent | Indicates that this file contains sea ice extent measurements |
| anomaly | Indicates that this file contains the difference between the monthly data (area or extent) and the total monthly mean for the entire temporal coverage |
| mean | Indicates that this file contains monthly data averaged over the entire temporal coverage for each month |
| yyyy | 4-digit year (most recent year of processing) |
| r | Region (n: Arctic, s: Antarctic) |
Merged ESMR, SMMR, and SSM/I-SSMIS Sea Ice Extent
Daily and monthly sea ice extent summaries from 01 January 1972 to 31 December 2002 are provided for the Northern Hemisphere and 01 January 1973 to 31 December 2002 for the Southern Hemisphere.
ESMR sea ice extent values were derived from single channel brightness temperatures using a simple linear algorithm. The SMMR and SSM/I-SSMIS sea ice concentrations were derived using the NASA Team algorithm.
The temporal data gap in the satellite-derived sets were filled with supplemental data sources. In the Northern Hemisphere, the U.S. National Ice Center sea ice climatologies (Dedrick et al. 2001) were used, which were based on weekly operational sea ice charts. In the Southern Hemisphere, a weekly sea ice extent time series (Ropelewski 1983) from the U.S. National Ice Center was used. These supplemental data sets were also used to match or calibrate the three satellite-based products and eliminate biases between the products from the three sensors. See Cavalieri, Parkinson, and Vinnikov (2003) or visit the National Ice Center Web site for more information.
Note: Since supplemental data are used to create this data set, it should not be combined with the SMMR and SSM/I-SSMIS data sets described above.
Data Access
Data are available via FTP.
Data Format
Data are provided in two different temporal resolutions: daily and monthly. Data are in ASCII text format arranged in columns as described in Table 8 and Table 9 for the daily and monthly data, respectively.
| Column No. | Description |
|---|---|
| 1 | 4-digit year |
| 2 | 1-digit and 2-digit month |
| 3 | 1-digit and 2-digit day of month |
| 4 | 1-digit to 3-digit day of year |
| 5 | Observed/interpolated sea ice extent (106 km2) |
| Column | Description |
|---|---|
| Year | 4-digit year |
| Months (last 12 columns) | 3-character month abbreviation |
File Naming Convention
Files are named according to the following convention and as described in Table 10:
Daily Files: gsfc.nasateam.extent.1972-yyyy.r
Monthly Files: gsfc.nasateam.month.extent.1972-yyyy.r
where:
| Variable | Description |
|---|---|
| gsfc.nasateam | Indicates this is GSFC data processed with the NASA Team algorithm |
| extent | Indicates that this file contains sea ice extent measurements |
| month | Indicates that this file contains monthly averaged data (otherwise daily) |
| yyyy | 4-digit year (most recent year of processing) |
| r | Region (n: Arctic, s: Antarctic) |
References
Comiso J. C. and F. Nishio. 2008. Trends in the Sea Ice Cover Using Enhanced and Compatible AMSR-E, SSM/I, and SMMR Data. Journal of Geophysical Research 113(C02S07), doi:10.1029/2007JC004257.
Cavalieri, D. J., C. L. Parkinson, and K. Y. Vinnikov. 2003. 30-Year Satellite Record Reveals Contrasting Arctic and Antarctic Decadal Sea Ice Variability. Geophysical Research Letters 30(18), doi: 10.1029/2003GL018031.
Comiso, J. C. et al. 1997. Passive Microwave Algorithms for Sea Ice Concentration: A Comparison of Two Techniques. Remote Sensing of the Environment 60(3).
Dedrick, K. R., K. Partington, M. Van Woert, C. A. Bertoia, and D. Benner. 2001. U.S. National/Navy Ice Center Digital Sea Ice Data and Climatology. Canadian Journal of Remote Sensing 27: 457-475.
Parkinson C. L. and J. C. Comiso. 2008. Antarctic Sea Ice Parameters from AMSR-E Data Using Two Techniques and Comparisons with Sea Ice from SSM/I. Journal of Geophysical Research 113(C02S06), doi:10.1029/2007JC004253.
Ropelewski, C. F. 1983. Spatial and Temporal Variations in Antarctic Sea Ice (1973-82). Journal of Climate and Applied Meteorology 22: 470-473.
