Sometimes, data points deemed to be “outliers” are met with suspicion—possibly the result of an error in the measuring process, for example. But outliers can also represent a puzzling thing that really happened. This year’s floating sea ice cover around Antarctica falls into that latter category.

On September 25, the US National Snow and Ice Data Center (NSIDC) published preliminary dates and numbers for the annual maximum sea ice coverage in the Antarctic and minimum coverage in the Arctic. With the last few days of September in the books, NSIDC noted Wednesday that those determinations have held.

Arctic sea ice

Arctic sea ice coverage hit its end-of-summer low point on September 19, the sixth lowest in the satellite record that started in 1979. The average across all of September was fifth lowest. The record is still held by an unusual 2012 season, but sea ice is steadily declining over time.

Cooler weather prevailed over the Arctic Ocean, even as the planet as a whole set monthly records for warmth. But very little ice survives through multiple years anymore, so the ice is thinner now and disappears more easily in the warmer waters.

Antarctic sea ice

The Arctic usually gets the bulk of public attention, but the status of Antarctic sea ice has been shocking all year. Antarctic sea ice is a different beast, ringing a polar continent rather than growing from the center of a polar sea, and a number of factors cause its behavior to be complex. After smashing the satellite-era record for minimum extent in February, Antarctic sea ice coverage continued to track well below the range of previous years through the Southern Hemisphere winter months. It maxed out just shy of 17 million square kilometers on September 10 at the end of winter—a full 1 million square kilometers below the previous record set in 1986.

There has not been a consistent trend in Antarctic sea ice coverage. The largest extents of the last 40 years occurred between 2006 and 2015. The numbers have swung in the opposite direction since, with new summer minimum records set in 2017, 2022, and now 2023. But the amount by which 2023 separated from the pack is remarkable.

So what do we make of this erratic behavior? It’s important to understand that this is not as simple as an ice cube melting in a glass of warm water. Sea ice is more of a loose jumble than a continuous slab, and storms and wind patterns can push it around. Ocean currents move warmer and cooler water around over time. Air temperatures vary with weather patterns.

Atmospheric warming isn’t even the only longer-term trend interacting with these things. The hole in the ozone layer over Antarctica has trickle-down effects on winds. And meltwater from shrinking glacial ice on land affects circulation in the ocean.

Focusing just on 2023, it’s not easy to pin down the combination of factors responsible. But some research is pointing to water temperatures, which have been notably warm recently. But this, too, depends on more than global warming pushing heat into the ocean, as the variable circulation of seawater is important to local temperatures.

While climate models have long projected that Arctic sea ice would waste away over time, the last IPCC report determined that “there remains low confidence in existing future projections of Antarctic sea ice.” Most projections have shown significant declines this century, though less precipitous than for the Arctic, but there could be a lot of ups and downs along the way.

Clear answers have been elusive for Antarctica, and not just because it’s hard to get there. So, while more data is collected, and research yields hard-won insights, it won’t be a surprise for the frozen continent to continue surprising us. Certainly, researchers will be eager to see how Antarctic sea ice evolves over the next few years after 2023’s extraordinary curveball.