Understanding climate: Antarctic sea ice extent

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.

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.

In September 2021, Antarctic sea ice reached its annual peak early, on the first of the month. At 18.75 million square kilometers (7.24 million square miles), the maximum extent was well above the long-term average for that day of the year. However, ice extents dropped off rapidly throughout the month, and September as a whole was near average. On February 25, 2022, Antarctic sea ice reached its annual minimum extent. At 1.92 million square kilometers (741,000 square miles), it was the lowest on record, but the low value was likely due to natural variability rather than a long-term declining trend.

Antarctic sea ice extent each February from 1979 through 2022. 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.

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 million square miles in winter, versus 6 million square miles in the Arctic; the Antarctic summer minimum is about 1 million square miles versus 2.5 million square miles for the Arctic.

Antarctic sea ice concentration in September 2021, the month of the maximum winter extent in the Southern Hemisphere. Conditions in 2021 were close 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. This barrier means that sea ice can't form over the very coldest parts of the Southern Hemisphere.

Average Antarctic sea ice concentration in February 2022, the month when sea ice reaches its summer minimum extent. Compared to median conditions from 1981-2010 (yellow line), the most recent minimum was below average. Map by NOAA Climate.gov, based on satellite data provided by National Snow and Ice Data Center.

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.)

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.

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. This saturated snow can freeze, forming “snow ice,” an important component of southern sea ice. Although this can occur in the north as well, it is less common.

Variability and long-term change

Sea ice waxes and wanes with the seasons, but minimum and maximum summer and winter extents vary. Compared to the Arctic, Antarctic sea ice shows lower variability in summer, and higher 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 year-to-year 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. From 1979–2017, Antarctic-wide sea ice extent 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, 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 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 over the same period.

Overall, the long-term trend in Antarctic sea ice is nearly flat. The satellite record spans over four decades, and although the ice has shown increasing and decreasing trends over portions of that record, few of those trends have been statistically significant. Year-to-year variability has dominated, especially over the last decade. Since the year 2013, Antarctic sea ice has exhibited its highest and lowest extents on record—highest-ever winter maximum in September 2014, and lowest-ever summer minimum in February 2022. But the overall trend, as of early 2022, is nearly zero.

Daily ice extent in the Southern Hemisphere from 1979 through February 2022. The dark gray dashed line shows the average, while the light gray area shows the range of variability (95% of values). February 2022 (dark blue) replaced 2017 (pink) as the smallest summer minimum on record. The summer minimum has been below-average for the past 7 years, but the long-term trend remains statistically insignificant thanks to the region's history of big swings in ice extent from year to year. NOAA Climate.gov graph, based on Charctic 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 February 2022, Antarctic sea ice showed a slightly positive long-term trend in most months, with the exceptions of January, February, and November, which showed slightly negative trends. None of the trends are statistically significant.

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.

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 its positive mode more frequently, and the resulting wind effects generally tend to increase Antarctic sea ice extent. SAM also affects the Amundsen Sea Low, which in turn affects sea ice transport and weather conditions over a broad area from the western Antarctic Peninsula to the eastern Ross Sea.

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 Sheet is 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 8, 2022.

Charctic, National Snow and Ice Data Center. Accessed March 8, 2022.

Sea Ice Index, National Snow and Ice Data Center. Accessed March 8, 2022.