Sea Ice Extent
Sea ice is frozen seawater that floats on the ocean surface. Sea-ice cover expands in Earth’s polar regions each autumn and winter, as ocean water freezes in response to cooling of the atmosphere and ocean. Summer warming melts much of this seasonal ice, with annual sea-ice minima in each hemisphere typically occurring in late summer or early autumn. Changes in sea-ice cover affect ocean circulation, atmospheric dynamics and surface heating. Sea ice extent is the area of the ocean where the sea ice concentration is greater than 15%. A concentration of 15% is used to define the edge of the ice pack. Sea ice has been measured by satellites since 1978.
In 2026, annual maximum Arctic sea-ice extent was the lowest or second lowest in the satellite record
Explanation
Large variations in sea ice extent from day to day can be driven by changes in weather as winds and ocean currents spread or compact the ice. In the Arctic, the annual maximum of sea ice extent typically occurs in March at the end of winter. The minimum occurs in September at the end of summer.
Methodology
Sea-ice concentration, the fraction of each ocean grid cell covered by sea ice, is estimated from satellite passive microwave measurements of brightness temperature, using the different microwave emission properties of sea ice and open water. Extent is the area of ocean grid cells where the sea-ice concentration exceeds 15%. There are modest differences in the absolute extent between datasets, but they agree well on the year-to-year changes and trends.
Continuous satellite-based sea-ice records extend back to 1978, providing more than four decades of consistent observations.
-0.91 million km2
in 2025, lowest or second lowest on record.
Explanation
Annual sea ice extent in the Arctic has declined since satellite observations began in the late 1970s. The annual average is shown as a difference from the 1991-2020 average.
Methodology
Sea-ice concentration, the fraction of each ocean grid cell covered by sea ice, is estimated from satellite passive microwave measurements of brightness temperature, using the different microwave emission properties of sea ice and open water. Extent is the area of ocean grid cells where the sea-ice concentration exceeds 15%. There are modest differences in the absolute extent between datasets, but they agree well on the year-to-year changes and trends.
Continuous satellite-based sea-ice records extend back to 1978, providing more than four decades of consistent observations.
To produce the plot, the following processing steps were performed:
- Calculated monthly average from values using arithmetic mean of all dates that fall within each month
- Selected years within the range 1979 to 2025.
- Rebaselined to 1991-2020 for each month separately by calculating the arithmetic mean of the data over the baseline period and subtracting the mean from all data values. This is done for each month separately (Januarys, Februarys etc).
- Calculated annual average from monthly averages using arithmetic mean
In 2026, the annual maximum Antarctic sea-ice extent was the third lowest in the satellite record
Explanation
Large variations in sea ice extent from day to day can be driven by changes in weather as winds and ocean currents spread or compact the ice. In the Antarctic, the annual maximum of sea ice extent typically occurs in September or October at the end of winter. The minimum occurs in February at the end of summer.
Methodology
Sea-ice concentration, the fraction of each ocean grid cell covered by sea ice, is estimated from satellite passive microwave measurements of brightness temperature, using the different microwave emission properties of sea ice and open water. Extent is the area of ocean grid cells where the sea-ice concentration exceeds 15%. There are modest differences in the absolute extent between datasets, but they agree well on the year-to-year changes and trends.
Continuous satellite-based sea-ice records extend back to 1978, providing more than four decades of consistent observations.
-1.01 million km2
in 2025, third lowest on record
Explanation
Annual sea ice extent in the Antarctic has declined since satellite observations began in the late 1970s with most of the decline occurring since 2014 after a period of little change or even a slight increase. The annual average is shown as a difference from the 1991-2020 average.
Methodology
Sea-ice concentration, the fraction of each ocean grid cell covered by sea ice, is estimated from satellite passive microwave measurements of brightness temperature, using the different microwave emission properties of sea ice and open water. Extent is the area of ocean grid cells where the sea-ice concentration exceeds 15%. There are modest differences in the absolute extent between datasets, but they agree well on the year-to-year changes and trends.
Continuous satellite-based sea-ice records extend back to 1978, providing more than four decades of consistent observations.
To produce the plot, the following processing steps were performed:
- Calculated monthly average from values using arithmetic mean of all dates that fall within each month
- Selected years within the range 1979 to 2025.
- Rebaselined to 1991-2020 for each month separately by calculating the arithmetic mean of the data over the baseline period and subtracting the mean from all data values. This is done for each month separately (Januarys, Februarys etc).
- Calculated annual average from monthly averages using arithmetic mean