The rapid loss of Antarctic sea ice brings grim scenarios into view
W hen the area near Antarctica covered by sea ice fell to 1.8m km2 (800,000 square miles) on February 19ththe smallest extent since satellite-based data became available in 1978few climatologists were surprised. Such records fall frequently in a warming world, as they did at the same time of year in 2017 and 2022. In March ice cover in the Antarctic began bouncing back at a rate of 70,000 km2 per day, close to the historical average. However, between April and June the ice did not recover as expected, with a growth rate around 15% lower than normal. And at the start of July its edge turned south, receding at a time when it should have been advancing rapidly. By July 6th Antarctic sea ice covered just 12m km2, roughly as large as Brazil and India put together. Compared with the average for that date in 1981-2010, its area was 2.84m km2 smallera gap as large as Argentina or Kazakhstan. This abrupt decline looks even more extreme set against long-run patterns. Unlike the Arcticwhere, in recent decades, the maximum sea-ice extent has been gradually shrinking, and the minimum has plummetedthe range of ice cover in the Antarctic has been relatively stable during the past half-century. Between 1976 and 2014, the average area covered by mid-winter sea ice in the region actually grew slightly. A wide range of factors account for the historical divergence in ice-cover trends between Earths poles, one a continent surrounded by water and the other an ocean surrounded by land. But after years in which Antarctic waters appeared to be a rare ocean of climatological calm, the regions sea-ice charts now look just as stormy as the notoriously turbulent Southern Ocean. It is too early to tell whether the Antarctic seas exceptional failure to freeze over represents an unusual confluence of temporary factors; a new normal driven by climate change; or something in between. One of the leading short-term explanations is unusual patterns of wind and waves. Throughout June and July, gusts travelling from the Bellingshausen Sea towards the South Pole prevented ice from forming near the Antarctic Peninsula. Weather systems emanating from storms in the Indian Oceanbrought about by shifts in two regular atmospheric fluctuations, the El Nino Southern Oscillation and the Southern Annular Modemay also have broken up sea ice as it began to form in East Antarctica. Warmer waters could also have played a role. Recent research suggests that since 2016, an increase in the Southern Oceans average temperature has caused Antarctic sea ice to shrink. This pattern may stem from overall atmospheric warming, but could also result from variation in normal cycles of heat transfer between deep and shallow parts of the ocean. Even if ice cover eventually returns to recent norms, however, its vanishing act this year aligns with scientists long-run expectations. Based on current forecasts for global air temperatures, climate models predict that the extent of Antarctic sea ice will shrink during the second half of the century. And although the disappearance of floating sea ice does not affect sea levels directly, it could accelerate the loss of glaciers on land. Sea ice helps keep the edges of Antarctica cool. White ice reflects sunlight, reducing the amount of energy absorbed by the ocean and thus lowering both air and water temperatures. Moreover, the ring of sea ice around Antarctica holds in place the continents coastal ice shelves, which in turn do the same for its glaciers and ice sheets. If those ice shelves were to collapseas the Conger shelf in east Antarctica did in 2022the gates would open for continental ice to flow rapidly into the oceans. The west Antarctic ice sheet alone contains enough water to increase global sea level by 3.3 metres (11 feet). Chart sources: NSIDC; Copernicus; Polar Geospatial Centre