In Antarctica, a huge iceberg blocked emperor penguins' routes, leading to a tragedy in which about 7 out of every 10 chicks starved to death. Mothers that went out to forage failed to return to the nest in time. The incident is interpreted as evidence that emperor penguins can face a survival threat if an iceberg grounded off land blocks their path, even when the sea ice in their habitat remains intact.
Kim Jeong-hun, principal researcher at the Korea Polar Research Institute (KOPRI), said on the 24th that "satellite imagery and field surveys confirmed a 69% decline in the number of chicks at the emperor penguin habitat on Coulman Island in the Ross Sea, Antarctica, in 2025 compared with 2024," in Communications Earth & Environment, a sister journal of Nature.
Kim Jeong-hun's team surveyed the emperor penguin habitat on Coulman Island by aircraft in November and December 2025. At that time, 6,658 live chicks were counted, 69% fewer than 21,242 in 2024. The 2017–2024 annual average was 21,202.
◇ A sheer iceberg face blocked mothers' return
In Antarctica, accumulated inland snowfall becomes a glacier. A slowly flowing glacier that spreads over the ocean forms an ice shelf. An iceberg is a block of freshwater ice that breaks off from a glacier or ice shelf and drifts at sea. The researchers said it is highly likely that mothers that left to find food months before hatching were blocked by the iceberg and failed to return on time, causing a mass die-off of chicks.
Last year, scientists from the Korea Polar Research Institute (KOPRI) observed a large iceberg grounded 3–5 km north of an emperor penguin colony during field observations. The iceberg measured 13.9 km long, 4 km wide, and 42.7 km² in area, about 90% the size of Seocho District in Seoul. The iceberg separated from the Nansen Ice Shelf in Mar. 2025 and ultimately grounded near the penguin colony on Jul. 28, four months later.
That period is the penguin hatching season. Females mate in May and lay eggs around June. They then leave the eggs with the males and go hunting, returning in late July to early August when the chicks hatch to feed them fish and krill stored in the stomach and esophagus. But a large iceberg that did not exist when the mothers left to hunt drifted in just before their return, blocking the route home.
The researchers analyzed that, due to the iceberg's steep southern face and grounding position, the distance penguins traveled to forage likely increased from 9.3 km in 2024 to 14.9 km in 2025. The southern face of the iceberg rose more than 20 meters, like a vertical cliff. Penguins may have climbed the seaward side of the iceberg, which had a gentler slope, only to be unable to proceed farther on the southern side facing the breeding site. Adult penguins were in fact observed gathering and lingering for long periods below the southern ice wall.
◇ Even intact sea ice becomes risky when an iceberg runs aground
Emperor penguins, the largest penguins on Earth, are perfectly adapted to extreme cold. They spend their entire lives on sea ice, handling mating, egg-laying, and chick-rearing without setting foot on land. But their numbers have recently continued to decline. According to satellite observations by the World Wide Fund for Nature (WWF), emperor penguin numbers in West Antarctica fell by about 22% between 2018 and 2023.
Scientists have long pointed to the rapid decline of sea ice, the emperor penguins' home, due to climate change as the cause. According to WWF, Antarctic sea ice used to expand and contract seasonally, but since 2016 both its extent and duration have been trending downward. Emperor penguin chicks cannot enter the water until their waterproof feathers fully develop. If sea ice collapses early, chicks with underdeveloped waterproof plumage are forced into the ocean and inevitably die.
Scientists at the Korea Polar Research Institute (KOPRI) now show that icebergs can also threaten emperor penguins. The team explained that sea-ice conditions at the breeding site were favorable in 2025, but breeding success collapsed because the passage connecting the foraging grounds and the breeding site was blocked by an iceberg. Kim Jeong-hun said, "There have been previous cases in the Ross Sea where giant icebergs such as B15A and C16 had major impacts on access to emperor penguin breeding habitats and feeding grounds."
Of course, it is not yet known whether incidents like this iceberg event are increasing. That is why long-term monitoring is needed. Kim said, "The formation and movement of icebergs are natural phenomena in Antarctica," and added, "There are not enough long-term data to show that events in which icebergs hinder access to emperor penguins' feeding grounds are increasing."
References
Communications Earth & Environment (2026), DOI: https://doi.org/10.1038/s43247-026-03764-w
Communications Earth & Environment (2025), DOI: https://doi.org/10.1038/s43247-025-02345-7
Antarctic Science (2007), DOI: https://doi.org/10.1017/S0954102007000065