Cryosphere
- 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.
- There has been an abrupt decrease in Antarctic sea-ice extent since 2015, after a small increase over the period from 1979–2014.
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 1993–2018, melt from Greenland and Antarctica combined contributed around 1.7 cm of global sea level rise.
Changes in sea ice have little direct impact on sea level because sea ice is frozen seawater 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 changes in climate 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 1979–2014, but with substantial regional variations. The largest daily recorded wintertime extent of approximately 20.2 million km2 was in September 2014. Since 2014, there has been a marked, abrupt and relatively persistent decrease in sea-ice extent, which in early 2022 dropped below 2.0 million km2 for the first time since observations began.
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 since 2015 has been attributed to a combination of atmospheric and oceanic anomalies.
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.
The rate of mass loss from the Antarctic ice sheet has increased over recent decades, with a total loss of 2670 ± 870 Gt of ice over the period 1992–2020.
