Summary: A catastrophic die-off of emperor penguin chicks has been observed in the Antarctic, with up to 10,000 young birds estimated to have been killed.
The sea-ice underneath the chicks melted and broke apart before they could develop the waterproof feathers needed to swim in the ocean.
The birds most likely drowned or froze to death.
The event, in late 2022, occurred in the west of the continent in an area fronting on to the Bellingshausen Sea.
It was recorded by satellites.
Dr. Peter Fretwell, from the British Antarctic Survey (BAS), said the wipeout was a harbinger of things to come.
More than 90% of emperor penguin colonies are predicted to be all but extinct by the end of the century, as the continent's seasonal sea-ice withers in an ever-warming world.
"Emperors depend on sea-ice for their breeding cycle; it's the stable platform they use to bring up their young. But if that ice is not as extensive as it should be or breaks up faster, these birds are in trouble," he told BBC News.
"There is hope: we can cut our carbon emissions that are causing the warming. But if we don't we will drive these iconic, beautiful birds to the verge of extinction."
There is hope: we can cut our carbon emissions that are causing the warming.
Maybe 25 years ago. It's too late now. There is no hope. We could stop 100% of emissions tomorrow and it would change nothing. We've entered the runaway train scenario.
This is what the oil companies want you to think. The less hope you have the longer they'll keep making profit. They still receive trillions in public subsidies, it's criminal.
We may never go back, but if we did cut 100% of emissions tomorrow, it would have a huge impact. These penguins would have a much better chance for the future, for one, and so would plenty of other species.
That's not entirely true. If we cut off carbon emissions right now, you'll still get the runaway train that is global warming, yes, but it will end sooner or will have a lesser peak temperature increase.
More than 90% of emperor penguin colonies are predicted to be all but extinct by the end of the century, as the continent's seasonal sea-ice withers in an ever-warming world.
But the research team watched as sea-ice under emperor rookeries fragmented in November, before thousands of chicks had had time to fledge the slick feathers needed for swimming.
Antarctic summer sea-ice has been on a sharp downturn since 2016, with the total area of frozen water around the continent diminishing to new record lows.
Between 2018 and 2022, roughly a third of the more than 60 known emperor penguin colonies were affected in some way by diminished sea-ice extent - whether that's ice forming later in the season or breaking up earlier.
She links the causes for the current decline to anomalously warm ocean water around the continent and a particular pattern of winds, which in the case of the Bellingshausen, has pushed ice back towards the coast, making it difficult to spread.
Currently, emperors are classified as "Near Threatened" by the International Union for Conservation of Nature (IUCN), the organisation that keeps the lists of Earth's most endangered animals.
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