State of the Climate 2020


  • The ice sheets and ice shelves of Antarctica and Greenland are losing ice and contributing to global sea level rise due to a warmer climate.
  • The extent of sea ice in the Arctic has steadily reduced, while in the Antarctic there has been regional and seasonal variability in sea-ice cover.

The Earth’s ice sheets—glacial ice that has accumulated from precipitation over land—and ice shelves, which are floating sheets of ice formed from glacial ice sheets, play crucial roles in our global climate system. Ice shelves help stabilise the Antarctic ice sheet by restricting the flow of glacial ice from the continent to the ocean. Warm ocean water penetrating below the ice shelves of the West Antarctic ice sheet is destabilising several glaciers, increasing the Antarctic contribution to sea level rise. Atmospheric warming is also causing surface melting of ice sheets and ice shelves, particularly in Greenland and on the Antarctic Peninsula. From 2003 to 2019, melt from Greenland and Antarctica combined contributed around 1.4 cm of global sea level rise.

Changes in sea ice have little direct impact on sea level because sea ice is frozen sea water that floats. When it melts it returns the original volume of water to the sea. However, the presence or absence of sea ice influences the climate, including the rate of regional climate warming.

Antarctic sea ice also acts as a protective buffer for ice shelves against destructive ocean swells. Changes in Antarctic sea-ice cover can also be an indicator of wider climate changes because it is an integrator of ocean, atmosphere and cryosphere components, from local change to the tropics.

Satellite monitoring of sea ice began in the late 1970s. Since then, Arctic sea-ice cover has consistently decreased, whereas net sea-ice cover changes within the Antarctic have been mixed. Overall, Antarctic sea-ice extent showed a small increase from 1979 to 2014, but with substantial regional variations. The largest daily recorded wintertime extent of approximately 20.2 million km2 was in September 2014. However, since then the sea-ice cover has decreased to below the long-term average, with the lowest recorded daily summertime extent of approximately 2.1 million km2 occurring in 2017.

The overall increase in Antarctic sea-ice extent during 1979–2014 has mostly been attributed

to changes in westerly wind strength, whereas the marked decrease from 2015 to 2017 was predominantly related to warming in the uppermost layers of the ocean.

Observed changes in Antarctic sea-ice cover are also regionally variable, as depicted in the trends of yearly sea-ice duration. Statistically significant increases of up to two days per year in sea-ice duration have occurred in the Ross Sea, between 160° E and 150° W. Decreases in sea-ice duration of as much as four days per year are seen west of the Antarctic Peninsula and the Bellingshausen Sea offshore of West Antarctica. Recent sea-ice seasons (since 2015) have shown opposite regional responses to the long-term trend.

Changes in the Arctic sea-ice cover, since 1979, have been more seasonally and regionally uniform than those in the Antarctic, with decreasing trends in all months and virtually all regions. Sea-ice thickness in the Arctic has also decreased steadily over the last few decades, making it easier to break and melt in springtime.


Antarctic and Arctic sea-ice extent (shown as the net anomaly from the 1981–2010 monthly averages in order to remove the seasonal variability) for the period January 1979 to December 2019 (106 km2). Thin lines are monthly averages and indicate the variability at shorter time-scales, and thick lines are eleven-month running averages.


Trends in the length of the sea-ice season each year (in days per year) around Antarctica, from 1979–1980 to 2018–2019. Each year sea ice around Antarctica starts expanding in February and retreats from October. Duration is a measure of the number of days that a particular location is covered by sea ice.