Emperor penguins require stable fast ice, sea ice anchored to land or ice shelves, on which to lay eggs and raise chicks. As the climate warms, changes in sea ice are expected to lead to substantial declines at many emperor penguin colonies. The most southerly colonies have been predicted to remain buffered from the direct impacts of warming for much longer. Here, we report on the unusually early breakup of fast ice at one of the two southernmost emperor penguin colonies, Cape Crozier (77.5°S), in 2018, an event that may have resulted in a substantial loss of chicks from the colony. Fast ice dynamics can be highly variable and dependent on local conditions, but earlier fast ice breakup, influenced by increasing wind speed, as well as higher surface air temperatures, is a likely outcome of climate change. What we observed at Cape Crozier in 2018 highlights the vulnerability of this species to untimely storm events and could be an early sign that even this high-latitude colony is not immune to the effects of warming. Long-term monitoring will be key to understanding this species' response to climate change and altered sea ice dynamics.

The arrival in January 2001 in the south-west Ross Sea of two giant icebergs, C16 and B15A, subsequently had dramatic affects on two emperor penguin colonies. B15A collided with the north-west tongue of the Ross Ice Shelf at Cape Crozier, Ross Island, in the following months and destroyed the penguins' nesting habitat. The colony totally failed in 2001, and years after, with the icebergs still in place, exhibited reduced production that ranged from 0 to 40% of the 1201 chicks produced in 2000. At Beaufort Island, 70 km NW of Crozier, chick production declined to 6% of the 2000 count by 2004. Collisions with the Ross Ice Shelf at Cape Crozier caused incubating adults to be crushed, trapped in ravines, or to abandon the colony and, since 2001, to occupy poorer habitat. The icebergs separated Beaufort Island from the Ross Sea Polynya, formerly an easy route to feeding and wintering areas. This episode has provided a glimpse of events which have probably occurred infrequently since the West Antarctic Ice Sheet began to retreat 12 000 years ago. The results allow assessment of recovery rates for one colony decimated by both adult and chick mortality, and the other colony by adult abandonment and chick mortality.