Climate Change: Arctic sea ice

September 28, 2021

Sea ice is ocean surface water that has frozen. Sea ice grows throughout the fall and winter, and melts throughout the spring and summer. In the Arctic Ocean, sea ice extent—the area of ocean with at least 15 percent sea ice concentration—typically reaches its maximum in March and its minimum in September.

The 2021 summer minimum extent was 1.82 million square miles (4.72 million square kilometers), reached on September 16, 2021. This was the 12th-lowest extent in the 43-year satellite record, but the largest extent since 2014.

Map of Arctic sea ice concentration on Sept. 16, 2021 compared to the a981-2020 median extent

Sea ice concentration across the Arctic on September 16, 2021—the date of the smallest ice cover of the year—based on satellite observations. (The satellites' orbits pass just shy of the North Pole, leaving a "pole hole" where there are no data.) The gold line shows the median ice extent from 1981-2020. Median means "in the middle;" half of the years in the record had larger extents, and half had smaller extents. NOAA map, based on data from the National Snow and Ice Data Center.

Current trends in Arctic sea ice 

From the start of the satellite record in November 1978 through August 2021, Arctic sea ice showed a declining trend in all months, with the smallest declines in February through May, and the largest declines in August through October. The biggest declines are occurring in September.

According to the National Snow and Ice Data Center, the downward trend for the summer minimum in September was 13 percent per decade relative to the 1981–2010 average. According to NOAA's 2020 Arctic Report Card, if we split the satellite record into thirds, the average minimum extent for each third has successively declined: 2.64 million square miles (6.85 million square kilometers) for 1979–1992, 2.37 million square miles (6.13 million square kilometers) for 1993–-2006, and 1.71 million square miles (4.44 million square kilometers) for 2007–2020. (The 2021 Arctic Report Card will come out in December 2021.)

Although wintertime losses in Arctic sea ice extent are smaller than summertime losses, the experts reported that declines were still significant: March declines are 2.6 percent per decade. Besides declines in sea ice extent, Arctic Report Card: Update for 2020 described significant declines in sea ice age. In March 1985, sea ice more than four years old comprised 33 percent of the Arctic Ocean ice pack. In March 2020, equally old ice comprised just 4.4 percent of the ice pack.

Read more in the Arctic Report Card: Update for 2020.

Explore this interactive graph: Click and drag to display different parts of the graph. To squeeze or stretch the graph in either direction, hold your Shift key down, then click and drag. This graph shows the average area covered by sea ice during September each year. Minimum sea ice extent has decreased 13.0 % per decade since 1979. Data provided by the National Snow and Ice Data Center.

How sea ice freezes, melts, and moves

Just as ponds and lakes in northern states develop a layer of ice on their surfaces during cold winters, the surface of the Arctic Ocean also freezes, forming sea ice. Seawater has a lower freezing point than fresh water, but once it is chilled to around -2°C (about 28°F), the salty liquid begins to solidify. Ice crystals appear on the sea surface, and if the air is cold enough, the crystals expand to form a slushy mix, then a solid covering of ice that can thicken over time.

In the Arctic Ocean, the area covered by sea ice grows and shrinks over the course of the year. Each fall, as less sunlight reaches the Arctic and air temperatures begin to drop, additional sea ice forms. The total area covered by ice increases through the winter, usually reaching its maximum extent in early March. Once spring arrives, bringing more sunlight and higher temperatures, the ice begins to melt back, shrinking to its minimum extent each September. Sea ice minimum and maximum extents occur toward the end of summer and the end of winter in part because the ocean lags behind the atmosphere in warming up and cooling down.

Sea ice that hasn't yet survived a summer melt season is first-year ice. This thin, new ice is vulnerable to melt and disintegration in stormy conditions. Ice that survives a summer melt season can grow thicker and less salty—two things that make it more resistant to melt. Multiyear ice is more likely to survive temperatures that would melt first-year ice, and to survive waves and winds that would break up first-year ice.

Although sea ice moves more slowly than ocean water, ice still moves. Carried by ocean currents, Arctic sea ice regularly flows out toward the warmer waters in the Atlantic via the Fram Strait east of Greenland.

Long-term changes in Arctic sea ice

Early observations of sea ice coverage in the Arctic come to us both from the oral histories of native populations and from the records of early European mariners who were seeking a "Northwest Passage" to the lucrative markets in Asia.

Records of sea ice near the coast of Iceland data back to the 9th century, and records became more routine in the 17th century. British and Russian records of sea ice conditions along sailing routes became routine in the 18th and 19th centuries. Detailed shipping charts, available from the United Kingdom Met Office Hadley Centre, show that Arctic sea ice has declined at least since the mid-1950s. Satellite data have documented continued declines; since continuous satellite-based measurements began in November 1978, data show a trend of more ice melting away during summers and less new ice forming during winters.

Overall, less sea ice survives in the Arctic. The Beaufort Gyre, a looping current north of Alaska, historically acted as a nursery for young ice, enabling it to thicken and grow. Ice growth in the Beaufort Gyre roughly offset the flow of ice out of the Arctic via the Fram Strait. Since the start of the 20th century, however, summers in the southern portion of the gyre have been too warm for sea ice to survive.

In the early 20th century, explorer Roald Amundsen took three years (1903–1906) to traverse the Northwest Passage. Since the turn of the 21st century, that passage has experienced relatively ice-free conditions multiple times, though it's not yet a dependable pathway for commercial ships. The Northern Sea Route along the coast of Siberia has begun experiencing summertime sea ice declines that may transform it into a reliable shipping route. The opening of shipping lanes across the Arctic may provide shippers with viable alternatives to traveling through the Panama Canal or around the southern tip of South America. This new reality will have impacts not only on the environment, but also on the world economy and national security, as nations compete to gain rights to shipping lanes and newly accessible resources in the Arctic.

Arctic amplification: a not-so-positive feedback

Arctic sea ice declines are related to the phenomenon known as Arctic amplification: more intense warming in the Arctic than over the rest of the globe. Arctic amplification fits with current scientific understanding of Earth's climate system, and with model projections of global warming caused by greenhouse gas emissions. Multiple factors contribute to Arctic amplification, and sea ice loss is one of them.

Sea ice's white or light gray surface reflects up to 80 percent of incoming sunlight, deflecting additional energy away from the planet. With less ice present, the dark surface of ocean water absorbs considerably more sunlight energy, leading to further warming of the atmosphere and more melting of ice, which leads to further warming... Scientists are actively studying the effects of this positive feedback loop to help them understand and predict how the observed decrease in Arctic sea ice will affect the global climate system.

A gray horizontal bar centered in the body text column

Popular Arctic sea ice visualizations

A map in polar projection of Arctic sea ice concentration on September 18 2018, and a graph of daily ice concentration throughout the satellite record

(top) Sea ice concentration (light blue to white) on September 18, 2019, the day of the summer minimum extent. The gold line is the median extent for 1981-2010: half of years had smaller extents, half had larger. (bottom) A graph of daily ice extent each year of the satellite record. Earlier years are in shades of light blue; recent years are in dark blue. The 2019 daily extent line is in dark pink. NOAA image, based on data from the National Snow and Ice Data Center.

Polar map showing Arctic sea ice concentration at the winter maximum in March 2018

Sea ice concentration on March 17, 2019—the maximum extent of the year—compared to the median winter maximums between 1981-2010 (gold line). Full story

A gray horizontal bar centered in the body text column

Pair of maps showing Arctic sea ice age at the winter maximum in 1985 (left) versus 2018 (right)

These maps show the age of sea ice in the Arctic winter maximum ice pack in March 1985 (left) and March 2018 (right). Less than 1% of ice is 4 or more years old. Full story

A gray horizontal bar centered in the body text column

1 minute, 20 seconds

 This animation tracks the relative amount of ice of different ages from 1990 through early November 2016. Seasonal ice is darkest blue. Ice that is 9 or more years old is white. Video produced by the team, based on NOAA and NASA data provided by Mark Tschudi, University of Colorado-Boulder. Data processing by Hunter Allen. Editing by Bruce Sales. Narration by Deke Arndt, NCEI. 

A gray horizontal bar centered in the body text column

Previous years' minimums

2018 Arctic sea ice minimum

Arctic sea ice concentration on September 19, 2018, one of the two dates that sea ice experienced its minimum extent in 2018. Higher concentration appears white, and lower concentration appears blue. Extent is the area covered by at least 15% sea ice, and the yellow line indicates the 1981–2010 median extent for this date. Image adapted from NASA Earth Observatory using data from the National Snow and Ice Data Center.

A gray horizontal bar centered in the body text column

Monthly sea ice concentration in the Arctic during the 2017 summer melt season from April–August, ending with the daily concentration on September 13, the preliminary date of the annual minimum extent. Animation by NOAA Environmental Visualization Lab, based on data from the National Snow and Ice Data Center. Full story

A gray horizontal bar centered in the body text column

Arctic sea ice minimum as of September 10, 2016

Arctic sea ice concentration on the date of the 2016 minimum extent, September 10, 2016. NOAA image based on NOAA and NASA satellite data from NSIDC. Full story

A gray horizontal bar centered in the body text column

Arctic sea ice map

Map of Arctic sea ice on September 11, 2015, the date it reached its fourth-lowest extent in the satellite record. Full story

A gray horizontal bar centered in the body text column

Map of Arctic sea ice concentration in September 2014 in shades from light blue to white

Sea ice concentration on September 17, 2014. Areas where the surface was less than 15% ice covered are deep blue; places that were 100% ice covered are solid white. The orange line shows the 1981-2010 median extent for September 17. Full story 

A gray horizontal bar centered in the body text column

Sea ice concentration in September 2013 compared to the median extent from 1981-2010 (gold line) and the 2012 record low (gray line).  Map by NOAA, based on data provided by the National Snow and Ice Data Center. Adapted from Figure 5.12(b) in State of the Climate in 2013. Full story

A gray horizontal bar centered in the body text column

Map shows ice concentration on September 16, 2012—the smallest summer minimum on record—along with the extent of the previous record low, in 2007 (yellow line), and the mid-September median extent from 1981-2010 (black line). Full story

A gray horizontal bar centered in the body text column

A graph of Arctic sea ice extent from 500 AD to the present, overlaid on a photo of sea ice

Arctic sea ice extent in millions of square kilometers over the past roughly 1,500 years. These records show that while there have been several periods over the past 1,450 years when sea ice extents expanded and contracted, the decrease during the modern era is unrivaled. Full story

A gray horizontal bar centered in the body text column

Map of Bering Strait and Chukchi Sea showing percent ice cover

Sea ice extent (percent ice cover) on November 19, 2017, mapped by a combination of satellites (synthetic aperture radar and passive microwave sensors) and ground-based radar. Except for a few areas of nearshore ice, the entire Bering Sea is ice-free as are thousands of square miles of the Chukchi Sea to the north. Full story

A gray horizontal bar centered in the body text column


All About Sea Ice, National Snow and Ice Data Center. Accessed March 17, 2009.

Bobylev L.P., Miles, M.W. (2020) Sea Ice in the Arctic Paleoenvironments. In: Johannessen O. et al. (eds) Sea Ice in the Arctic. Springer Polar Sciences. Springer, Cham.

State of the Cryosphere: Sea Ice, National Snow and Ice Data Center. Accessed January 11, 2017.

Arctic Sea Ice News and Analysis, National Snow and Ice Data Center. Accessed September 26, 2019.

Intergovernmental Panel on Climate Change. (2013). Climate Change 2013: The Physical Science Basis Summary for Policymakers. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.

Perovich, D., Meier, W., Tschudi, M., Hendricks, S., Petty, A. A., Divine, D., Farrell, S., Gerland, S., Haas, C., Kaleschke, L., Pavlova, O., Ricker, R., Tian-Kunze, X., Wood, K., & Webster, M. (2020). Arctic Report Card 2020: Sea Ice. “United States. National Oceanic and Atmospheric Administration. Office of Oceanic and Atmospheric Research. Pacific Marine Environmental Laboratory (U.S.) Thayer School of Engineering National Snow and Ice Data Center (U.S.) University of Colorado (Boulder Campus) Alfred-Wegener-Institut Für Polar- Und Meeresforschung / Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research Goddard Space Flight Center Norsk Polarinstitutt / Norwegian Polar Institute University of Alaska Fairbanks. Geophysical Institute.”


September Sea Ice Extent, National Snow and Ice Data Center.

Do you have feedback to offer on this or another article?
Let us know what you think.