Antarctica’s west coast is missing an area of winter sea ice the size of France, sparking concerns for threatened penguins other marine life and global sea levels.

One expert said the loss of ice in the Bellingshausen Sea was “depressing” and the failure of ice to form could have intensified a heatwave over the continent’s peninsula last week that saw daytime temperatures peak at 15.4C which is more than 20C above average.

It’s winter in Antarctica, when sea ice expands rapidly around the continent peaking in September.

But satellite observations showed the Bellingshausen Sea – on the west side of the Antarctic peninsula and which by June would usually be covered by ice – was almost completely ice free.

Scientists said the region was missing about 650,000 sq kilometres (250,000 sq miles) of sea ice, compared with the average between 1991 and 2020. That is an area about the size of France and almost 10 times the size of Tasmania.

“I’m concerned. It’s depressing,” said Dr Will Hobbs, an Antarctic sea ice expert at the University of Tasmania with the Australian Antarctic Program Partnership.

“It is remarkable that we are in June and there is no sea ice there.”

He said this was the third time in four years that sea ice had been very low in the region. “I don’t think we will see sea ice there any more. It’s done,” he said.

He said the loss of sea ice was likely linked to changes in the ocean and scientists were trying to understand if global heating was a factor.

He said the region was important for krill – a critical part of the food web for species in the region. Krill would usually be hiding from predators under the ice in winter, where they graze on algae.

On 10 June there was about 11.4m square kilometres of sea ice around the entire continent compared to a long-term average for that date of 12.6m sq km.

Dr Phil Reid, who monitors Antarctic conditions at Australia’s Bureau of Meteorology, said the Bellingshausen Sea had seen “incredible coastal exposure” in winter and summer in recent years.

He said just to the area’s west were the Pine Island and Thwaites glaciers – the continent’s major contributors to ice loss and sea level rise.

Floating ice shelves in front of the glaciers could break up faster if protective sea ice is absent for longer periods, he said, and this could then speed up the loss of ice from the glaciers, pushing up global sea levels in the future.

The Bellingshausen Sea’s coastline was the site of tragedy in late 2022 when thousands of emperor penguin chicks died during a “catastrophic breeding failure” in four colonies.

That event contributed to UN advisers pushing the species up two categories to “endangered” on its international threatened species list earlier this year.

Dr Peter Fretwell, a scientist at the British Antarctic Survey who has been documenting the penguin’s decline, said the current loss of sea ice in the region was “a serious problem for penguins, especially emperors”.

“Sea ice is forming too late and breaking up too early. It leads to reduced breeding success and longer trips to moulting grounds.”

Adelie penguin numbers were also falling and crabeater seals were being forced to migrate in summer to find stable ice, he said.

This month the Antarctic peninsula witnessed an extreme temperature spike over several days. Hobbs said while “nobody has done the numbers” it was reasonable to suggest the heatwave was “made worse by the lack of sea ice”.

Sea ice would usually help to cool any warmer airflow entering the region from the north, he said.

Officials at Argentina’s national weather service, Servicio Meteorológico Nacional, said the country’s Esperanza base at the peninsula’s northeastern tip had experienced an “extreme temperature event” that peaked on 5 and 6 June.

Maximum temperatures of 15.4C and 13.4C, respectively, were recorded at a period when average daily maximums were -6.2C. The previous June temperature record at the base of 13.3C was set on 12 June 1998.