Researchers maintain a catalogue of satellite images of the frozen Antarctic seas that dates back to the late 1970s. While the northern Arctic has lost an average of 20,800 square miles a year in that time, NASA says, the Antarctic has gained an average of 7,300. On an average year, Antarctica's sea ice maxes out at about 7.23 million square miles. On September 20, it peaked at 7.78 million--the most ever. Satellite images show how much of the ocean's surface freezes, but ignore other factors like ice thickness and volume. Massive, ancient chunks of ice have disappeared into the ocean even as this temporary, seasonal, and relatively thin cover has grown.

NASA researchers believe the new record was a product of changes in climate patterns of air and water circulation near Earth's southern pole. Among the possible culprits: shifting winds and ocean currents shoving colder air and water toward the area, and increased snow seeding the ocean with slush that could form a more resilient ice cover. Antarctica, far from other continents, also has few obstacles preventing its ice from expanding in the right conditions.