In Antarctica, a massive iceberg blocked emperor penguins' migration route, leading to a tragedy in which about 7 out of 10 chicks starved to death. Mothers that went to find food failed to return to the nest in time. The incident is seen as evidence that even when sea ice, the emperor penguin's habitat, remains intact, a grounded iceberg separated from land can block the route and threaten survival.
Kim Jeong-hun, a principal researcher at the Korea Polar Research Institute (KOPRI), said on the 24th that "satellite imagery and field surveys confirmed a 69% decrease in the number of emperor penguin chicks in 2025 compared with 2024 at the colony on Coulman Island in the Ross Sea, Antarctica," in Communications Earth & Environment, a sister journal of Nature.
Kim Jeong-hun's team surveyed the emperor penguin colony on Coulman Island by aircraft in November and December 2025. At that time, 6,658 living chicks were counted, 69% fewer than the 21,242 recorded in 2024. The 2017–2024 annual average was 21,202.
◇A cliff-like iceberg blocked mothers' return
When snowfall accumulates inland in Antarctica, it becomes a glacier. A glacier that flows slowly and spreads over the sea forms an ice shelf. An iceberg is a mass of freshwater ice that has broken off from a glacier or ice shelf and floats in the ocean. The researchers said it is highly likely that mothers who went in search of food for several months before hatching were blocked by the iceberg and failed to return in time, triggering mass chick deaths.
Last year, scientists at the Korea Polar Research Institute (KOPRI) observed during fieldwork that a large iceberg had grounded 3–5 km north of the emperor penguin colony. The iceberg measured 13.9 km long, 4 km wide, and 42.7 square km in area, equivalent to 90% of the area of Seocho District in Seoul. The iceberg separated from the Nansen Ice Shelf in March 2025 and ultimately grounded near the penguin colony on July 28, four months later.
That period is 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 crop and esophagus. But a large iceberg, which was not there when the mothers departed to hunt, drifted in just before their return and blocked the way home.
The team analyzed that, due to the iceberg's steep southern face and its grounding position, the distance penguins traveled to find food 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 sheer cliff. The penguins may have climbed onto the iceberg from the gently sloping seaward side but been unable to proceed on the southern side toward the breeding site. In fact, adults were observed gathering and lingering for a long time beneath the southern ice wall.
◇Even with intact sea ice, a grounded iceberg spells danger
Emperor penguins, the largest penguins on Earth, are perfectly adapted to extreme cold. They spend their entire lives on sea ice—mating, laying eggs, and rearing chicks—without setting foot on land. Yet their numbers have declined in recent years. According to satellite observations by the World Wide Fund for Nature (WWF), the emperor penguin population in West Antarctica decreased by about 22% between 2018 and 2023.
Scientists have pointed to the rapid decline of sea ice, the emperor penguin's home, due to climate change as the cause. According to WWF, Antarctic sea ice used to grow and shrink seasonally, but since 2016 both its extent and duration have trended downward. Emperor penguin chicks cannot enter the water until their waterproof feathers fully develop. If sea ice collapses early, chicks whose waterproof feathers are not yet developed are forced into the ocean and inevitably die.
Scientists at the Korea Polar Research Institute (KOPRI) have now shown that icebergs can also threaten emperor penguins. The team explained that sea ice conditions at the breeding site were good in 2025, but breeding success collapsed because an iceberg blocked the corridor connecting the foraging grounds and the breeding site. 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 sites and foraging areas."
Of course, it is not yet known whether incidents like this involving icebergs are increasing. That is why long-term monitoring is needed. Kim said, "The formation and movement of icebergs are natural phenomena in Antarctica," and noted, "There is not enough long-term survey data showing an increase in incidents where icebergs hinder access to emperor penguin foraging areas."
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