Sea ice around Antarctica has shrunk to the smallest annual extent on record after years of resisting a trend of man-made global warming, preliminary US satellite data shows.
Ice floating around the frozen continent usually melts to its smallest for the year around the end of February, the southern hemisphere summer, before expanding again as the autumn chill sets in.
This year, sea ice extent contracted to 2.287 million square kilometres on February 13, according to daily data from the US National Snow and Ice Data Centre (NSIDC).
That extent is a fraction smaller than a previous low of 2.29 million square kilometres recorded on February 27, 1997, in satellite records dating back to 1979.
Mark Serreze, director of the NSIDC, said he would wait for a few days' more measurements to confirm the record low.
"But unless something funny happens, we're looking at a record minimum in Antarctica. Some people say it's already happened," he told Reuters. "We tend to be conservative by looking at five-day running averages."
In many recent years, the average extent of sea ice around Antarctica has tended to expand despite the overall trend of global warming, blamed on a build-up of greenhouse gases in the atmosphere, mainly from burning fossil fuels.
People sceptical of mainstream findings by climate scientists have often pointed to Antarctic sea ice as evidence against global warming. Some climate scientists have linked the paradoxical expansion to shifts in winds and ocean currents.
"We've always thought of the Antarctic as the sleeping elephant starting to stir. Well, maybe it's starting to stir now," Serreze said.
World average temperatures climbed to a record high in 2016 for the third year in a row. Climate scientists say warming is causing more extreme days of heat, downpours and is nudging up global sea levels.
At the other end of the planet, ice covering the Arctic Ocean has set repeated lows in recent years.
In the northern winter, sea ice is growing in winter and is at the smallest extent for mid-February, at 13.927 million square kilometres.
Combined, the extent of sea ice at both ends of the planet is about two million square kilometres less than the 1981-2010 averages for mid-February, roughly the size of Mexico or Saudi Arabia.