Resolving the paradox of the Antarctic Sea Ice

While the Arctic sea ice has been diminishing in recent decades, the Antarctic sea ice extent has been increasing. Here, we present an explanation for the seeming paradox of increasing Antarctic sea ice in a warming climate. For the latter half of the 20th Century, as the Southern Ocean warmed, the hydrological cycle accelerated and there was more precipitation in the Southern Ocean. Increased precipitation stabilized the upper ocean, and insulated it from the ocean heat below, decreasing the heat available from the ocean to melt the ice from below. The increase in precipitation was mostly in the form of snow, which slowed the melting of sea ice from above as the snow is highly reflective. This created conditions that have allowed the Antarctic sea ice to grow.

Climate models generally reproduce the observed behavior of the Antarctic sea ice in recent decades, with the Southern Ocean sea surface temperature variability being dominated by natural variability associated with the Antarctic Oscillation. With increased loading of greenhouse gases in the 21st century, climate model simulations show an accelerated warming that exceeds the natural variability. This increased warming heats the upper ocean and results in decreased snowfall (but more rain), enhancing the melting of the Antarctic sea ice both from above and below. As a consequence, we may see, on a time scale of decades, a switch in the Antarctic, where the sea ice extent begins to decrease.