Each year during the Southern Hemisphere's winter, the ocean around Antarctica freezes hundreds of miles beyond the continent, with the maximum reach usually observed in September or October, before the thawing cycle begins.

This year, the ice appeared to peak on September 17 at 17.81 million square kilometers (6.88 million square miles), according to preliminary figures by the US National Snow and Ice Data Center at the University of Colorado Boulder.

The 2025 maximum ranks as the third lowest in the 47-year record, behind the all-time low in 2023 and the second-lowest in 2024 -- but still well below the historic normal.

Senior research scientist Ted Scambos at CU Boulder told AFP that until 2016, measurements of Antarctic sea ice showed "an erratic but slight increase over time."

But "what seems to be happening is that warmth from the global ocean is now mixing into the water that's closest to Antarctica" -- meaning that climate change finally caught up with the southern continent's frozen seas.

Floating sea ice does not add to sea level when it melts.

But its retreat does replace white surfaces that reflect almost all of the Sun's energy back into space with deep blue water, which absorbs the same amount instead.

The sea ice also acts as a stabilizing buffer -- protecting the Antarctic Ice Sheet from entering the ocean and amplifying sea level rise by reducing the impact of waves before they reach the coast and lessening the effect of winds over the ocean.

On the other hand, it also triggers a competing effect.

"We may see more snowfall in Antarctica, because the humid air over the ocean would be closer to the coast... storms that arrive over the ice sheet would carry more moisture and therefore produce more snowfall over the continent, and that offsets sea level rise," said Scambos.

He added, however, that while increased snowfall could offset destabilization effects for decades, over longer timescales past records show that when the climate stays warmer, the ice sheet shrinks.

The Antarctic Ice Sheet holds enough land ice to raise seas high enough to inundate low-lying coastlines around the world, though such a catastrophic impact would likely unfold over centuries.

Ninety percent of the heat generated by human-caused global warming is soaked up by oceans.