Data Sources

These data are derived from the following sources: 

• Bootstrap Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS, Version 3 and/or
• Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data, Version 1 (also referred to as NASA Team)
• Polar Stereographic Valid Ice Masks Derived from National Ice Center Monthly Sea Ice Climatologies (Used in processing to remove spurious ice caused by residual weather effects and land spillover in passive microwave data)

Processing Steps

Total Ice-Covered Areas and Sea Ice Extent

In computing the total ice-covered area and ice extent data 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. Anomalies are the difference between the current time period's average value and the long-term average.  A positive anomaly indicates that the current period has greater extent or area than the average.

Ice Persistence

Individual ocean pixels containing a minimum of 15 percent ice cover are summed throughout the time series. The maximum ice persistence value is 100 percent. Each pixel represents the percentage of years for which ice was present during that month. Only the NASA Team algorithm is used to compute these data.

Monthly Climatology of Sea Ice Concentration

A threshold of 15 percent concentration is applied to the monthly climatology fields. Only the NASA Team algorithm is used to compute these data.

Missing Data and Monthly Anomalies

Periods for which there are no available data are described in Table 12.

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.

Gap in Coverage over the North Pole

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.

Table 13 provides a summary of the version history of this product. 

Version 3 Update Notice

01 March 2018

The climatology presents a summary of the long-term record of satellite-derived sea ice concentrations calculated using the NASA Team (NT) and Bootstrap (BT) algorithms. Prior to this update, the climatology included NT and BT data from 1978 through 2015. Because no changes were made to the NT data that were used in the previous version of this product, only small differences due to averaging data from an additional year are expected.

Findings

Because the ice persistence and monthly climatology fields are based on the NT data set, there are only small differences. A few grid cells have small changes, but overall there are minimal changes in the ice persistence and monthly climatology fields in both hemispheres for all months.

The NT-derived fields of regional ice extent and areas similarly consist of just the additional year of data; the values through the end of 2015 are unchanged.

The BT-derived fields do show widespread changes because the underlying Bootstrap data have been updated to a new version. While monthly hemisphere-wide areas show average area increases of less than 2%, some smaller regions—for which even small changes in ice area can be a large percentage of the region’s total—can show much larger percentage changes.

While monthly hemisphere-wide areas show average area increases of less than 2%, some smaller regions–for which even small changes in ice area can be a large percentage of the region’s total–can show much larger percentage changes.

Recommendations

The updated Sea Ice Trends and Climatologies data are consistent in format and value with the previous version and can be used for the same purposes. Because the Bootstrap data have been updated for the entire time series, users of the Bootstrap data should update those applications with the entire Bootstrap data set. 

Version 2 Update Notice

01 January 2017

In the Northern Hemisphere, very small differences in sea ice extent were found: less than 0.2 percent for the NASA Team algorithm and less than 0.05 percent for the Bootstrap algorithm.

Differences in Northern Hemisphere sea ice area were also small. Most years had less than 0.05 percent. Differences in the Bootstrap sea ice area were generally less than 0.01 percent.

For both the Bootstrap and NASA Team algorithms, the differences in the Southern Hemisphere were negligible.

These small differences are found in the persistence and monthly climatology data also. The Northern Hemisphere monthly fields showed no difference in 95-98 percent of the sea ice values, and fewer than 0.02 percent of pixels had differences in ice concentration of greater than 2 percent.

Version 1 Update Notice

28 July 2014

Data are now available through 31 December 2013 for all Sea Ice Trends and Sea Ice Trends and Climatologies from SMMR and SSM/I-SSMIS data sets.

Version 1 Update Notice

04 September 2013

Reprocessing is complete and updated data are now available through 31 December 2012 for all Sea Ice Trends and Sea Ice Trends and Climatologies from SMMR and SSM/I-SSMIS data sets. With this update, the following changes have been implemented:

Total Ice-Covered Area and Extent

The cutoff value which defines a region as ice-covered or not ice-covered has been adjusted from 14.8 percent to 15 percent to more closely match the methodology used to calculate other sea ice extent and area products at NSIDC.

Ice Persistence and Monthly Climatology of Sea Ice Concentration (Monthly Means)

• Includes the new 15 percent cutoff value described above.
• To prevent inconsistencies between different algorithms, these parameters are now calculated exclusively using the NASA Team algorithm.

Monthly Ocean Masks and Maximum Extent Masks

The ocean mask files and maximum extent mask files were removed as they are not the masks used in the production of these data sets.

This most recent reprocessing also rectifies two previous releases of these data sets that contained erroneous data. In January/February of 2012, and May of 2013, the products were to have contained data from 2007 to 2010 and from 2010 to 2011, respectively. However, the climatology products contained data only through 2007. We recommend replacing previously downloaded 2010 and 2011 climatology data files with the climatology files that now extend through 2012. The Total Ice-Covered Area and Extent data set did contain post-2007 data, but reprocessed data utilizing the new methodology are available in the 2012 release.