Understanding climate: Antarctic sea ice extent

Author: 
March 26, 2021

As it does in the Arctic, the surface of the ocean around Antarctica freezes over in the winter and melts back each summer. Antarctic sea ice usually reaches its annual maximum extent in mid- to late September, and reaches its annual minimum in late February or early March. The 2020 maximum extent (on September 28, 2020) was 7.32 million square miles, which was above the 1981–2010 climatological average but not a record high. The 2021 minimum extent, on or near February 21, 2021, was 1.01 million square miles, below the 1981–2010 climatological average but well above the record low recorded in 2017.

Map of sea ice concentration around Antarctica on Sept 28, 2020

Antarctic sea ice concentration on September 28, 2020, the day that the ice reached its maximum winter extent. Compared to median conditions from 1981-2010 (yellow line), the most recent maximum was slightly below average. Map by NOAA Climate.gov, based on satellite data provided by National Snow and Ice Data Center.

The timing of the seasonal cycles isn’t the only way that Antarctic sea ice differs from the Arctic. One key difference is the larger range between austral winter maximum extent and summer minimum extent. Antarctic sea ice extends to about 7.2 million square miles in winter, versus 6 million square miles in the Arctic; the Antarctic summer minimum is about 1.1 million square miles versus 2.5 million square miles for the Arctic.

Map of sea ice concentration around Antarctica on February 18, 2021, near the summer minimum

Antarctic sea ice concentration on February 18, 2021, the day that the ice reached its summer minimum extent, compared to the 1981-2010 median extent (yellow line). Map by NOAA Climate.gov, based on satellite data provided by National Snow and Ice Data Center.

The differences in seasonal extremes are due to basic geography. The Arctic is an ocean basin largely surrounded by land. Sea ice forms over the North Pole itself—the hemisphere’s coldest latitudes—but its expansion is checked by Eurasia, North America, and Greenland. The Antarctic is a continent surrounded by a vast ocean. Sea ice can expand freely across the Southern Ocean in winter, but it can get no closer to the South Pole than the Antarctic coastline will allow.

Because it forms at lower, warmer latitudes, less Antarctic sea ice survives the summer. On average, about 40 percent of the Arctic Ocean’s winter ice cover remains at the summer minimum, whereas in the Southern Ocean only about 15 percent does. Because so little Antarctic ice persists through the summer, the majority of Antarctica’s sea ice is only one winter old at most. As a result, Antarctic sea ice is relatively thin, often 1 meter (about 3 feet) or less.

(In the Arctic, multiyear ice that survives at least one summer is generally 3 to 4 meters thick, and even seasonal ice that formed since the previous summer can often reach about 2 meters in thickness.) So overall, average Antarctic ice thickness is much lower than Arctic sea ice. However, snowfall often thickens Antarctic sea ice. The heavy snow burden can depress ice floes, and seawater can subsequently flood those floes.

Aircraft camera photo of a mixture of thin and heavily compacted sea ice in Antarctica's Amundsen Sea on October 19, 2009

Young, thin ice floats in the Amundsen Sea on October 16, 2009. The brighter the ice, the more it has been compacted by wind and waves. Photo captured by the DMS camera on the first flight of NASA’s Operation Ice Bridge Campaign. Photo courtesy NASA Earth Observatory. 

Variability and long-term change

Sea ice waxes and wanes with the seasons, but minimum and maximum extents rarely match from year to year; over years and decades, summer and winter extents vary. Compared to the Arctic, Antarctic sea ice shows less variability in summer, and more variability in winter. These changes largely result from the geographic differences mentioned above, namely Antarctic sea ice’s distance from the pole (sea ice can melt back all the way to the coast in summer, making for less summer-to-summer variability) and unconstrained growth potential in winter. Weather events often drive variability, but have different effects in the Northern and Southern Hemispheres. Weather exerts a greater influence on the Arctic minimum, and the Antarctic maximum.

The sea ice satellite record dates back to October 25, 1978. Unlike the Arctic, where sea ice extent is declining in all areas in all seasons, Antarctic trends are less apparent. Over 1979–2017, Antarctic-wide sea ice extent—for the annual average, winter maximum, and summer minimum extents—showed a slightly positive trend overall, although some regions experienced declines. Those exceptions have occurred around the Antarctic Peninsula. The region south and west of the Antarctic Peninsula has shown a persistent decline in the Southern Hemisphere summer and autumn (primarily January through May), but this downward trend is small compared to the high variability of Antarctic sea ice overall. Another region near the northern tip of the Peninsula, in the Weddell Sea, showed strong sea ice declines in the autumn and winter (primarily April through September) until 2006, but the ice in that region has rebounded in recent years. The eastern Ross Sea region has shown a modest increase in ice extent during the summer and autumn (December through June).

Line graph showing September average ice extents in Antarctica from 1979-2020

Antarctic sea ice extent each September from 1979 through 2021. Based on satellite data, extent is the total area where the ice concentration is 15 percent or higher. In the past decade, the September winter maximum has been extremely variable, hitting record and near-record highs as well as near-record lows. NOAA Climate.gov image, based on data from the National Snow and Ice Data Center.

Overall, the long-term trend in Antarctic sea ice is nearly flat, for the annual average and for every month. Spanning over four decades, the satellite record shows periods of increasing and decreasing sea ice, but few of those trends have been statistically significant. In 2013, 2014, and 2015, Antarctic sea ice yearly minimum extents (occurring in February or March) not only exceeded the 1981–2010 average, they also exceeded almost all the values in the satellite record for that time of year. In 2012, 2013, and 2014, yearly maximum extents (occurring in September) were successively the highest on record.

Charctic-derived graph, 2012-2021

Over the past decade, Antarctic sea ice extents have shown great variability, with both record-high winter maximum extents and record-low ones. In 2014, the maximum extent (purple dashed line) was record high. Just a few years later, the 2017 and 2018 summer minimum were record low. Climate.gov graph, adapted from NSIDC’s Charctic.

Midway through 2015, Antarctic sea ice was exhibiting values closer to the 1981–2010 average. Then Antarctic sea ice began to fall below the long-term range of variability (encompassing 80 percent of the range of values around the 1981–2010 average). Starting in September 2016, Antarctic sea ice extents mostly fell well below the 1981–2010 average. Extents for 2017 and 2018 were the lowest on record for both winter maximum and summer minimum. In 2019, both the minimum and maximum extents fell below the 1981–2010 average, but neither was a record low for that time of year. Starting in mid-2020, Antarctic sea ice was mostly near or above normal. The September 2020 maximum was above the 1981–2010 average, and the February 2021 minimum was below the 1981–2010 average.

Line graph showing February average ice extents in Antarctica from 1979-2021

Antarctic sea ice extent each February from 1979 through 2021. Based on satellite data, extent is the total area where the ice concentration is 15 percent or higher. In the past decade, the February summer minimum has been extremely variable, hitting both near-record highs and record lows. NOAA Climate.gov image, based on data from the National Snow and Ice Data Center.

According to the National Snow and Ice Data Center’s Sea Ice Index, from the start of the satellite record in November 1978 through early March 2021, Antarctic sea ice showed a slightly positive long-term trend in all months but November, which showed a very slight negative trend. But in most months, the error bar exceeded the trend: year-to-year variability dwarfed long-term trends.

Impacts of change

Land-sea configurations affect sea ice extents not only by limiting where ice can form, but also by introducing their own effects. In the Arctic, landmasses surround and influence the sea ice in the Arctic Ocean. Ice and (especially) snow are highly reflective, bouncing much of the Sun’s energy back into space. As Northern Hemisphere spring and summer snow cover declines, the underlying land surface absorbs more energy and warms. Warmer conditions on land affect the nearby ocean, and more sea ice melts as a result. The melt-warmth-melt feedback cycle means that the Arctic is warming faster than the rest of the globe.

No such polar amplification effect has occurred on a large scale in the Southern Hemisphere, however. Antarctica is surrounded by ocean, not a land surface that is losing its reflective snow and ice cover in the spring and summer. It was already normal, historically, for summertime sea ice to melt back nearly to the Antarctic coastline, leaving large expanses of the Southern Ocean exposed to heating from the summer sun. By contrast, the loss of reflective snow and ice in high northern latitudes surrounding the Arctic Basin represents a profound change from what was historically normal.

A pair of satellite images of the sea ice off Oates Coast of Antarctica in October 2018 and January 2019

Satellite images of sea ice off the Oates Coast of Antarctica on October 7, 2018, (left) and January 12, 2019 (right). Unlike the Arctic, the Antarctic typically holds on to very little sea ice in the summer. Suomi NPP satellite images from Worldview. 

The Southern Ocean is vast, a fact often underplayed in map projections focused on the Northern Hemisphere. Natural cycles in the Southern Ocean can have pronounced effects on Antarctic sea ice. Atmospheric patterns, partly influenced by greenhouse gas emissions, are also at work.

The Southern Annular Mode (SAM) is a pattern westerly winds circling Antarctica. SAM is influenced by El Niño-Southern Oscillation conditions, so it is partly driven by natural oscillations. At the same time, anthropogenic global warning tips SAM into more frequent positive mode, and the resulting wind effects generally increase Antarctic sea ice extent. SAM also has a relationship with the Amundsen Sea Low, which exerts a complex influence on sea ice transport on the western side of the Antarctic Peninsula.

Long story short: Climate change has a discernible influence on Arctic sea ice, but it has a complicated, messy influence on Antarctic sea ice. (Meanwhile, the Antarctic ice sheets are losing mass.)

Where sea ice does melt away completely in the Antarctic summer, the ice’s absence can have cascading effects. For example, sea ice retreat in the Weddell Sea along the northern tip of the Antarctic Peninsula probably contributed to Larsen Ice Shelf losses. Ice shelves—thick slabs of floating ice attached to coastlines and usually fed by glaciers—fringe the frozen continent. Intact sea ice in front of an ice shelf buffers the shelf from ocean swells. When the ice is gone, ocean waves can flex the shelf and make it more vulnerable to disintegration. Depending on how much an ice shelf disintegrates, the glacier feeding it may accelerate into the ocean. But sea ice retreat alone rarely, if ever, initiates the disintegration process; other factors such as warm ocean water and surface melt on the ice shelf are usually at work, too.

References

Antarctica is colder than the Arctic, but it’s still losing ice. (2019, March 12). Climate.gov.

Arctic Sea Ice News and Analysis, National Snow and Ice Data Center. Accessed March 9, 2021.

Charctic, National Snow and Ice Data Center. Accessed March 9, 2021.

Sea Ice Index, National Snow and Ice Data Center. Accessed March 9, 2021.

State of the Cryosphere: Ice Shelves, National Snow and Ice Data Center. Accessed March 27, 2019.

State of the Cryosphere: Sea Ice, National Snow and Ice Data Center. Accessed April 10, 2019.