About the data | National Snow and Ice Data Center

Sea Ice Today

Analyses and daily images of sea ice conditions
Sea Ice Today
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About the data
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About the data

The Daily Images at the top of Sea Ice Today include two maps and a graph that update daily, with a one-day lag. Visitors may notice that the date on the image is occasionally more than one day behind. This is likely due to short-term delays and data outages that are usually resolved in a few days.

These near-real-time images are derived from the NOAA@NSIDC Sea Ice Index data product. The default display shows figures for the Arctic.You can toggle between Arctic and Antarctic by clicking the tabs at the top of the image.

arctic sea ice daily images example

Maps

Sea ice extent map: This map shows the area of ocean with at least 15 percent sea ice concentration. All ocean areas meeting the 15-percent threshold are filled with white and are considered ice-covered in NSIDC’s calculations. 

Sea ice concentration map: This map shows the concentration, expressed as a percentage, of sea ice cover. The color bar on the right side of the map shows the concentration, from 0 percent (dark blue) to 100 percent (white). The data shown in this map are the basis for calculating sea ice extent. Concentrations below 15 percent are considered ice-free. The dark gray circle at the pole is where satellites have historically been unable to collect data due to their orbits. The size of this data gap varies by time of day. See NSIDC’s  Sea Ice Index documentation for more information.

Median extent line: In each map, the orange line shows the median ice edge for the current date over the years 1981 to 2010. Half of the years had smaller extents, and half had larger extents on this date from 1981 to 2010. The median line is based on ice extent, and the same line is used for both the extent and concentration maps.

Read the following Ask a Scientist articles for more information:

Graph

What it shows: For both the Arctic and Antarctic, the graph compares sea ice extent (area of ocean with at least 15 percent sea ice concentration) for the current year to the most recent record-low extent, and to the long-term climatology. Each graph includes a color key for the lines and shading shown. 

Year versus extent: Months of the year appear across the horizontal axis—five months roughly centered around the most recent observation date. Extent—measured in millions of square kilometers—appears along the vertical axis. Every year or climatological range shown gets its own line or shading.

Climatological data: The median extent line (charcoal gray) is the middle value for all the sea ice extents for the given hemisphere and the given date from 1981 to 2010. The darker gray band around the median line is the interquartile extent: the innermost 50 percent of all values for that date. The lighter gray band is the interdecile extent: the innermost 80 percent of all values for that date.

Notes: 

  • NSIDC does not include a previous year’s extent line as the record low until that entire year has passed. For example, Antarctic sea ice experienced a new record-low extent in 2023, making it the new record holder, well before that year was over. For the duration of 2023, however, 2022 was still displayed as the record-low year in the graph. 
  • Read Why use the 1981 to 2010 average for comparisons? for more information.

Where do the daily images come from?

Sea Ice Today images are are derived from the NOAA@NSIDC Sea Ice Index data product which relies on NASA-developed methods and NASA data products using passive microwave data from the Defense Meteorological Satellite Program (DMSP) F-18 Special Sensor Microwave Imager/Sounder (SSMIS). The basis for the Sea Ice Index is the data set, “Near-Real-Time DMSP SSM/I Daily Polar Gridded Sea Ice Concentrations,” and the NASA-produced “Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I Passive Microwave Data.” For more details and data visualizations, visit the Sea Ice Index Daily and Monthly Image Viewer.

The passive-microwave observations used for the Sea Ice Index are especially helpful because the sensors can “see” through clouds and deliver data even during the six months of polar darkness and frequently cloudy conditions. The sensors can also observe the entire region of the globe where sea ice exists every day. The passive microwave sea ice record dates back to 1979, one of the longest continuous environmental data sets available for sea ice.

The underlying data used to produce these images and graphs are freely available from NSIDC. Please see the Sea Ice Index documentation for more information about the daily values, and links to the data. 

The sea ice extent and concentration maps in the Daily Image Update show the values for the specific day indicated on the maps, not values averaged over five-day periods. The time series graph showing daily extent, however, is derived from a five-day running average. It is the average of the given day and the four previous days. 

NSIDC also produces the 4km Multisensor Analyzed Sea Ice Extent (MASIE) product which is an alternate sea ice extent data visualization tool that uses a different combination of data sources. The MASIE site includes an FAQ page that explains the differences between it and the Sea Ice Index daily product.

Updated August 14, 2024

 

How accurate are the data images?

Quality control for near-real-time-data

Satellite-derived images in our Daily Image Update are near-real-time and have not yet undergone rigorous quality control. Issues are especially likely along coastlines. Areas near land may show some ice coverage where there is none because the sensor’s resolution is not fine enough to distinguish ice from land. Data relayed to NSIDC may have geolocation errors caused by instrument issues. Near-real-time data may have areas of missing data (displayed on the daily map as wedges, speckles, or spider web patterns). Satellite sensors occasionally experience outages. NSIDC corrects technical issues in the final sea ice products, which replace the near-real-time data in about six months to a year.

Despite areas of inaccuracy, near-real-time data are still useful for assessing changes in sea ice coverage, particularly when averaged over an entire month. The monthly average image is more accurate than the daily images because weather anomalies and other errors have less impact.

Limitations of near-real-time data mean they should be used with caution when seeking to extend a sea ice time series, and should not be used for operational purposes such as navigation. To look at monthly images that have been through quality control, click on “Archived Data and Images” on the Sea Ice Index Daily and Monthly Visualizer.

Resolution of the data

Data are averaged over an area of approximately 25 kilometers by 25 kilometers (16 by 16 miles). This means that the ice edge could be off by as much as 25 to 50 kilometers (16 to 31 miles) in passive-microwave data, compared to higher-resolution satellite systems. In addition, we define ice extent as any 25-by-25-kilometer grid cell with an average of at least 15 percent ice. Ice-free areas may nevertheless exist within an area that is defined by our algorithms as ice covered. See What is the difference between sea ice area and extent? for more information.

Passive microwave data characteristics

The daily image is derived from remotely sensed passive microwave data, which can be collected even during cloudy or dark conditions. Passive microwave data may show ice where none actually exists due to signal variation between land and water along coastlines, or because of atmospheric interference from rain or high winds over the ice-free ocean.

Passive microwave data may not detect ice due to the presence of thin, newly formed ice; the presence of liquid water on the surface of actively melting ice; and atmospheric interference. Thin, newly formed ice is consistently underestimated by these data. Centers such as the US National Ice Center and the Canadian Ice Service that publish sea ice data for navigation employ higher spatial resolution data that is better able to detect such thin ice.

Despite limitations, passive microwave data still yield good large-scale estimates for the overall extent pattern and values of the ice. Plus, the limitations are consistent from year to year. So, when comparing from year to year, these types of errors do not affect the comparison.

While passive microwave data products may not show as much detail or be as accurate “on the ground” as other satellite data, they provide a consistent time series to track sea ice extent going back to 1979. Higher resolution sensors only go back to 2002. This type of long-term, consistent data record is important to scientists who study whether or not change is occurring. To learn more about how scientists study sea ice, see our Learn about Sea Ice: Science page.

Updated August 14, 2024

How NSIDC compares observations

[Walt should definitely take a fresh look at this section, to ensure I haven’t messed up.]

To make sense of sea ice extent over time, NSIDC compares observations from any day to the long-term average for that time of year. NSIDC shows both the average and the expected range of variability around the average. NSIDC computes the average and the expected range of variability for each day of the year, drawing from the 30 years of the baseline period. NSIDC has used more than one convention to convey this information in sea ice extent graphs.

Mean and standard deviation

The mean is a measurement of the average calculated by summing all values and dividing that sum by the number of values added together. For example, the mean of 6,7,6,5,4,3,6, and 7 equals 5.5. 

When using the mean, NSIDC also displays a measure of variation around it, the standard deviation. One standard deviation is defined as encompassing 68 percent of the variation, and two standard deviations encompass 95 percent of the variation. Standard deviations serve as an estimate of the expected range of natural variability.

NSIDC has changed our default convention from showing mean and two standard deviations to showing median, interquartile, and interdecile ranges, but both conventions of showing the average and range of variability are available through our Charctic tool.

Median, interquartile, and interdecile

The median is the middle value of a range of values. For example, the median of a range of integers from 3 to 9 is 6. In NSIDC’s sea ice extent data, the median is the middle value of the data for all years; half the years from 1981 to 2010 had smaller values, and half the years had larger values. 

The interquartile range shows the 25 percent of values above and 25 percent of values below the median, so the interquartile range is the central 50 percent of values. The interdecile range shows the 40 percent of values above and 40 percent of values below the median, so the interquartile range is the central 80 percent of values.

Updated August 14, 2024

What is the error range for your images?

NSIDC does not have error bars on the time series plot shown in the Daily Image Update and the daily time series plot because we strive to keep the images concise and easy to read. Plus, the error bars would be quite small compared to the total extent values in the images.

We estimate error based on accepted knowledge of the sensor capabilities and analysis of the amount of “noise,” or daily variations not explained by changes in weather variables. For average relative error, or error relative to other years, the error is approximately 20,000 to 30,000 square kilometers (7,700 to 11,600 square miles), a small fraction of the total existing sea ice. For average absolute error, or the amount of ice that the sensor measures compared to actual ice on the surface, the error is approximately 50 thousand to 1 million square kilometers (19,300 to 386,100 square miles), varying over the year. During summer melt and autumn freeze-up, the extent may be underestimated by 1 million square miles; during mid- and late winter before melt starts, the error will be on the low end of the estimates. It is important to note that while the magnitude of the error varies through the year, it is consistent from year to year. This gives scientists high confidence in interannual trends at a given time of year.

The absolute error values may seem high, but it is important to note that each year has roughly the same absolute error value, so the decline over the long term remains clear. NSIDC has high confidence in sea ice trend statistics and the comparison of sea ice extent between years.

Updated August 14, 2024

Why do different years appear on the graph?

Each year at the beginning of January, the reference year on the Daily Extent Graph changes. The graph of daily sea ice extent for the Northern Hemisphere shows ice extent in the current year, the 1981 to 2010 average, and the year with record low ice extent. The graph has a five-month window. This means that in December, the graph would show the Northern Hemisphere record-low year for reference, and extend into the following year although that extent line is not yet populated. Starting in January, to show five months beginning in October, the graph would show the end of the previous year into the current year, plus the record-low year extending into the next year.

Updated August 14, 2024

 

Why is the daily change in sea ice extent in the Northern Hemisphere larger at the beginning of each month?

[Walt, I just want to confirm: You’re okay with the term “mask” here?]

If you plot the average daily change in sea ice extent in the Northern Hemisphere, based on the data from Sea_Ice_Index_Daily_Extent_G02135_v3.0.xlsx file, you may notice that at the beginning of each month, particularly in the summer, the daily change is larger.

This is related to the valid ice masks that are used in the processing of the Sea Ice Index. It is really a land spillover effect: that is, even when there is not ice in a coastal sea, ice can appear to fringe the coast, and fill fjords. This happens because there are mixed land-ocean areas within the sensor’s field of view. That mixture of land and ice looks like sea ice to the algorithms interpreting the sensor data.

A correction for land spillover is applied, but it is not perfect. Monthly valid ice masks are also used and these mask out areas where sea ice is not realistic in a given month, including along the coast due to land spillover. When you switch to the next month there is a change in the ice mask. Going from May to June to July, the valid ice mask moves north in the Arctic and crops out more potential ice areas south of the valid ice line. Ice may have receded in a coastal sea by the end of May, for instance, but may still appear to be along the coastline. On the first day of June the new mask removes more of the invalid ice, which is why you see a sudden change in sea ice.

Updated August 14, 2024

Using the five-day trailing average, not the daily extent, to announce the minimum/maximum

We use a five-day trailing average to smooth out the day-to-day variability from the influence of weather (such as storms causing false retrievals) and coastal or surface effects on the data. Five days is a typical synoptic timescale that helps create a continuous curve that is easier to follow and interpret. Note: Before April 2012, NSIDC used a five-day centered average instead of the current five-day trailing average, which shifts the date of the extents by two days.

For more information on calculating daily sea ice extent values, see the Sea Ice Index documentation.

Updated August 14, 2024

 

Related Ask a Scientist articles

Other sources of sea ice data and how they differ

Other researchers and organizations monitor sea ice independently, using a variety of sensors and algorithms. While these sources agree broadly with NSIDC data, extent measurements differ because of variation in the formulas (algorithms) used for the calculation, the sensor used, the threshold method to determine whether a region is “ice-covered,” and processing methods. NSIDC’s methods are designed to be as internally consistent as possible to allow for tracking of trends and variability throughout our data record. Links to other sources of sea ice data are listed below:

Other sources of sea ice data include operational centers that provide support to ships navigating in the Arctic. There are often discrepancies between information from these centers and our data because they employ additional data sources to capture as much detail on sea ice conditions as possible. However, unlike our data, because the quality and availability of their data sources vary, their products do not provide a long-term, consistent time series suitable for tracking climate trends and variability. Several Arctic nations have operational sea ice centers. The two North American centers are:

Updated May 23, 2024

Other NSIDC sea ice data visualization tools

  • The Data Tools page of Sea Ice Today lists many more useful tools including
    • The Sea Ice Today Charctic tool which gives you the option of switching between the average with standard deviation, or the median with interquartile and interdecile ranges (the default display).
    • The Sea Ice Today sea ice analysis tool gives you the option to use the 1981 to 2010 climatology or  choose any baseline climatology period you like.

       
  • NSIDC also hosts