Sea Ice Concentration
Is there more or less sea ice in the Arctic Ocean?
About this map
How much does the area of the ocean covered by sea ice change throughout the year and over time? This interactive map shows monthly sea ice concentration anomalies in the Arctic Ocean. A different map display is available for every month of every year in the time series.
Sea ice concentration is a measurement of the amount of sea ice in a given area, usually described as a percentage. Each monthly map shows how sea ice concentration compares to long-term average concentrations for that month over the period of 1979 to 2015. The corresponding bar graph shows how much the overall sea ice concentration for that month departs from the long-term average.
Color key and bar graph
When you select a month on the dropdown selector and a year on the lower-left slider, the map will show sea ice concentrations.
- Anomalies of higher-than-average sea ice concentrations will appear in shades of blue, and anomalies of lower-than-average concentrations will appear in shades of orange and red.
- Areas with sea ice concentration near the long-term monthly average are very light blue or orange.
- Areas with significant departures from average sea ice concentration appear in darker colors. The greater the departure from average, the darker the color.
- Areas with no sea ice are white. The area of the ocean covered by sea ice changes over the course of each year.
- The area over the North Pole is gray, indicating there is no data, because satellites have historically missed flying directly over the pole, leaving that area unobserved.
The bar graph indicates the sea ice extent anomaly (departure from the long-term average) for the entire Arctic. Red bars indicate negative anomalies (years in which sea ice extent is less than the long-term average) and blue bars show positive anomalies (years in which sea ice extent is greater than the long-term average). The bars show every year in the time series for the selected month, and the bar that correlates with the map on display is highlighted in light gray.
Sea ice extent is the measurement NSIDC uses to determine whether an area of ocean is ice-covered. NSIDC derives measurements of sea ice extent from estimates of sea ice concentration, by setting a threshold of 15 percent. Where sea ice concentration is 15 percent or higher, the area is considered ice-covered; where sea ice concentration is below 15 percent, the area is considered ice-free. The no-data area over the North Pole is considered ice-filled.
Although each month has areas with above-average and below-average sea ice extent, the bar graph shows the overall departure from the long-term average for that month, for the whole region from 60°N of the equator to the North Pole.
How to change the display
- To change the month displayed on both the map and the graph, use the month dropdown selector in the bar graph box (lower right).
- To change the year displayed, move the slider in the year box (lower left).
- To animate the time series for the selected month, click the play arrow. The animation will display maps for the selected month for all years in the time series.
Why sea ice matters
Sea ice is ocean water that freezes and floats on the ocean surface. It grows throughout the fall and winter, and melts throughout the spring and summer. In the Northern Hemisphere, sea ice typically reaches its maximum extent in March or early April, and melts to its minimum extent around mid-September.
Sea ice plays an important role in Earth's climate system, regulating the exchange of heat between the polar seas and the atmosphere. The ice’s bright surface reflects most of the sunlight that reaches it back into space, keeping temperatures low. In contrast, sea ice melt exposes dark ocean water that absorbs nearly all of the sunlight that reaches it, causing temperatures to rise. As a result, a declining trend in sea ice cover can result in a self-reinforcing cycle. Less ice leads to more heat absorption by the ocean, which leads to even less ice.
Compared to an ice-covered sea surface, an ice-free sea surface increases the temperature of the overlying atmosphere. As a result, decline in sea ice cover is a significant contributor to Arctic amplification—the more rapid warming of the Arctic compared to the rest of the globe.
The presence or absence of sea ice also plays an important role in human activity in the Arctic. Sea ice retreat eases passage of ships through areas that were previously impassable, but also raises the risk of accidents requiring rescue operations. Sea ice loss affects fisheries and subsistence hunters in the Arctic as well, since it prompts some fish species to move to different waters, and can deprive marine mammals of resting platforms. l. Sea ice loss also increases wave action along coastlines and this action, combined with permafrost thaw, can lead to coastal erosion and infrastructure damage.
What the data show
Since 1978, polar-orbiting satellites have produced a continuous record of Arctic sea ice behavior, allowing scientists to study trends over time. Besides the annual pattern of sea ice melt to its minimum in September, and growth to its maximum in March or April, the data also show a long-term declining trend in overall sea ice extent. The Arctic Report Card: Update for 2020 reports that the sharpest declines in sea ice extent have occurred around the time of the sea ice minimum. But over the course of the satellite record, sea ice extent has also declined in all months.
By viewing different months and years, you can use this map to see how sea ice concentration has changed over time and where changes are most pronounced. You may be able to detect patterns in these changes. Try using these maps to answer questions such as:
- Which months show the biggest declines in sea ice concentration and extent over time? Which months show the smallest declines?
- What years show the greatest anomalies in Arctic sea ice extent, either positive (more ice than the long-term average) or negative (less ice than the long-term average)?
- Are the greatest anomalies in sea ice concentration likely to occur in the middle of the ice pack or along the margins?
By comparing sea ice maps with other maps in Satellite Observations of Arctic Change, you can see how sea ice relates to other changes in the Arctic, such as near-surface air temperatures and sea ice age.
These sea ice maps are generated by data products archived and distributed by NSIDC. The data products are derived from satellite sensors that can “see” through clouds and in the dark. This observation capability is crucial to continuous monitoring of sea ice conditions despite storms and months of polar darkness.
Data source(s)
The bar graphs of sea ice extent and the maps of ice concentration anomalies are based on the same data used in the NSIDC Sea Ice Index. Sea Ice Index products are derived from two data sets: the Near-Real-Time DMSP SSM/I-SSMIS Daily Polar Gridded Sea Ice Concentrations (NRTSI product, NSIDC-0081) and the Sea Ice Concentrations from Nimbus-7 SMMR and DMSP SSM/I-SSMIS Passive Microwave Data (GSFC product, NSIDC-0051). These satellite passive microwave-derived data sets are used to generate the daily and monthly images and numbers that comprise the Sea Ice Index record of sea ice extent and concentration from November 1978 to present. Information on the accuracy and precision of passive microwave-derived sea ice concentration products can be found in documentation for the Sea Ice Index. Daily concentrations are averaged to generate monthly mean concentrations for each grid cell.
Data processing steps
To create this map, NSIDC took the following steps:
- Acquire the monthly gridded seaice concentration data which is computed as part of the Sea Ice Index, from NSIDC internal servers. The data is in polar stereographic projection on a Hughes ellipsoid (PROJ4 string ‘+proj=stere +lat_ts=70 +lat_0=90 +lon_0=-45 +a=6378273 +b=6356889.449 +no_defs’).
- Acquire the monthly seaice extent CSV data from the Sea Ice Index.
- Compute the monthly anomalies CSV from the extents by averaging extents over the 1979-2015 time period and subtracting the climatological mean from each month’s extent.
- Generate monthly climatological gridded seaice concentration, i.e., for each month:
- Calculate the mean concentration across the years 1979-2015.
- Mask the larger pole hole found in earlier years of the record (SMMR)
- Generate monthly anomaly images for every year/month in the full timeseries by subtracting the climatological mean for that month from each month’s concentration grid.
- Reproject monthly anomaly image to EPSG: 3413.