On June 12th the continent’s sea ice covered just 10.7m square km. That is 1.15m square km below the previous minimum record for that date, from 2019, and 2.3m square km below the average for 1981-2010. The first difference represents an area a little larger than Colombia; the second an area the size of Mongolia—with three Britains tacked on.

Though it fluctuates, sea ice at the opposite end of the globe, in the Arctic, has generally been declining steadily as greenhouse gases build up in the atmosphere and the planet heats up. Since 1979 its summer surface area has fallen by roughly 12.6% per decade; global average temperatures are 1.0-1.3°C above pre-industrial levels, with most of that warming occurring since 1975. Despite average temperatures rising there too, the Antarctic fared rather differently. Between 1979 and 2014 its annual sea-ice extent (the area of ocean covered by ice) increased slightly overall. It then declined rapidly, falling from an all-time high in 2014 to an all-time low in 2017. That record was then broken in 2022, and again in February 2023. There have been two further record lows this year.

It is not yet clear why there has recently been such a precipitous drop in Antarctica’s sea ice, nor why it rose somewhat before that. (Climate models mostly projected that it would behave more like the Arctic: the fact it has not is sometimes called “the Antarctic paradox”.) Various theories have been floated for the sudden decrease, including shifts in the strength of winds circling the continent and a warmer ocean. Scientists, disturbed by the steep downward trend in recent years, are calling for more research into exactly how climate change is affecting Antarctica. They are also increasingly worried about the consequences. Sea ice helps protect the ice shelves that fringe the continent’s land. The shelves are already splintering apart more frequently as temperatures rise, exposing the vast ice sheets behind them. Their deterioration could prove catastrophic: the western Thwaites glacier alone contains enough water to increase global sea levels by around 65cm. Whatever is making the Antarctic more volatile—and thus vulnerable—is cause for concern.

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