Record low Antarctic sea ice in the spring of 2022 caused four emperor penguin colonies to suffer “catastrophic breeding failure”. Since 2009, Peter Fretwell and colleagues from the British Antarctic Survey have used satellite imagery to monitor penguin colonies at five remote sites in the Bellingshausen Sea, east of the Antarctic Peninsula. These birds have never seen a human, but we can detect the brown staining of their poo from space.

The scientists estimate about 9000 chicks typically hatch on the ice here each spring, with 6000-8000 surviving to fledge in December. But in 2022, Fretwell watched anxiously as the waves edged closer to the animals. Well before the chicks had developed their waterproof feathers, the ice melted underneath three of the five colonies. The thousands of young penguins likely drowned, or froze to death due to their lack of insulation. If they did manage to scramble onto a drifting iceberg, there was only a slim chance of their parents finding them again.

A fourth colony was perched high on the ice shelf. Penguins there were not pitched into the frigid ocean—but the melting sea ice dissolved a snow ramp the adult birds used to climb to the colony after they’d been fishing. Unable to reach their chicks, the adults were forced to abandon them. Only the Rothschild colony, containing perhaps 800 chicks, avoided disaster.

This spring, Fretwell says, is looking “worse still”. As this issue went to press in mid-October, the scientists had already watched the ice melt beneath one colony, and several others looked vulnerable.

Multiple years of breeding failure spells trouble for emperor penguins long-term—and they’re just the canary in the coalmine. “Emperor penguins are the things we can see,” says Fretwell. “They’re our window on this world.”