Why Antarctic sea ice peaks in September

Antarctica is surrounded by a vast ocean, giving sea ice plenty of room to spread outward during the Southern Hemisphere winter. This differs from the Arctic, where ocean basins are largely surrounded by land.

Antarctica’s open-ocean geography allows winter sea ice to expand over long distances. However, that expansion also pushes the ice into warmer latitudes, making it more vulnerable to melting during summer. Antarctic sea ice typically reaches its maximum extent in September before shrinking to its annual minimum in February.

According to NASA and the National Snow and Ice Data Center (NSIDC), Antarctic sea ice may have reached its 2026 maximum extent on September 14. At its peak, the ice covered approximately 17.59 million square kilometers (6.79 million square miles). That made 2026 the year with the third-lowest winter maximum sea ice extent in the satellite record.

The map below compares seasonal changes in Antarctic sea ice. It shows average ice concentration for September on the left and February of the following year on the right, using data from 1990 through 2026. The solid white areas show the highest ice concentrations, while dark blue represents open ocean.

Only locations where ice covers at least 15 percent of the water are included. Scientists use this threshold to determine which areas count as ice-covered when calculating total sea ice extent.

Antarctic sea ice went from record highs to historic lows

For decades, Antarctic sea ice appeared to be moving in the opposite direction of the Arctic’s rapidly shrinking ice cover. Between 1979 and 2014, Antarctic sea ice extent increased by about 1 percent per decade.

However, Antarctic sea ice naturally fluctuates widely from year to year. Researchers therefore remain uncertain whether the gradual increase during that period represented a meaningful long-term trend.

The expansion was especially pronounced from 2012 to 2014, when Antarctic winter sea ice reached a record high for three consecutive years. Those records contrasted sharply with conditions in the Arctic, where sea ice was declining rapidly.

That upward trend ended in 2015. As 2016 began, Antarctica experienced a significant drop in sea ice extent, marking a notable reversal from the previous decades. Although some years have brought modest recovery, Antarctic sea ice has generally remained below historical averages.

The most extreme conditions occurred in 2023, when Antarctica recorded both its lowest summer minimum and lowest winter maximum since satellite observations began. The unusually low 2026 maximum adds another important data point to the region’s changing ice conditions.

Scientists continue to monitor Antarctic sea ice each year to determine whether these declines signal a permanent shift in conditions or remain within the range of natural statistical fluctuations.

How satellites measure Antarctic sea ice through clouds

Scientists measure Antarctic sea ice with satellites equipped with instruments called passive microwave sensors. Instead of relying on visible light, these instruments detect microwave energy naturally emitted from Earth’s surface.

Sea ice produces stronger microwave signals than open ocean water, allowing researchers to distinguish between frozen and ice-free areas. Microwaves can also pass through clouds, enabling scientists to observe Antarctic sea ice even when thick cloud cover blocks traditional satellite cameras.

The long-term satellite record began in late 1978 with observations from NASA’s Nimbus 7 spacecraft. Beginning in 1987, satellites operated by the Defense Meteorological Satellite Program (DMSP) supplied additional measurements. NASA’s Aqua satellite also contributed data from 2002 to 2011.

Monitoring efforts continue to evolve as satellite technology changes. In 2025, researchers began incorporating observations from the Japan Aerospace Exploration Agency’s GCOM-W satellite, helping maintain the decades-long record used to study changes in Antarctic sea ice.

Image by Michala Garrison using data from NASA Earth Observatory and the National Snow and Ice Data Center.