Scientists discovered that colonies of chinstrap penguins -- also known as ringed or bearded penguins -- have dropped dramatically since they were last surveyed almost 50 years ago.
Every colony surveyed on Elephant Island, an important penguin habitat northeast of the Antarctic Peninsula, experienced a population fall, according to independent researchers who joined a Greenpeace expedition to the region.
At the last survey in 1971, there were 122,550 pairs of penguins across all colonies on Elephant Island. But the recent count revealed just 52,786 pairs -- a drop of almost 60%.
The size of the population change varied from colony to colony on Elephant Island. The biggest decline -- 77% -- was recorded at a colony known as Chinstrap Camp.
Climate change has led to reduced sea ice and warmer oceans, which has meant less krill, the main component of the penguins' diet.
"Climate change is probably the underlying factor and the effects are rippling through the food chain," Noah Strycker, an ornithologist and penguin researcher at Stony Brook University, told CNN from Greenpeace's Esperanza ship in the Antarctic.
"Penguins, seals and whales all depend on krill, which depends on ice. So if climate change affects the ice, that impacts on everything else."
The latest study is published just days after temperatures in the Antarctic hit an all-time high, with 18.3 Celsius (64.94 Fahrenheit) recorded on February 6. The previous high -- 17.5 C (63.5 F) -- was recorded in March 2015.
The temperature was taken by scientists at Argentina's Esperanza research station, according to the country's meteorological agency.
Heather J. Lynch, associate professor of ecology and evolution at New York's Stony Brook University and one of the expedition's research leads, said: "Such significant declines in penguin numbers suggest that the Southern Ocean's ecosystem has fundamentally changed in the last 50 years, and that the impacts of this are rippling up the food web to species like chinstrap penguins."
She added that "while several factors may have a role to play, all the evidence we have points to climate change as being responsible for the changes we are seeing."
The chinstrap penguin is characterized by a cap of black plumage, a white face and a continuous band of black feathers that extends from one side of the head to the other -- the "chinstrap."
The species inhabits the northern part of the Antarctic Peninsula, several Antarctic and subantarctic islands and the uninhabited Balleny Islands between Antarctica and New Zealand.
Until now, the chinstraps have been deemed of "least concern" by the International Union for Conservation of Nature (IUCN), which made the findings all the more surprising.
"We really didn't know what to expect, and then we found this huge decline on Elephant Island," Strycker said. "It's a little bit worrying as it means that something is shifting in the ecosystem and the fall in penguin numbers is reflecting that shift."
There was some good news, however, as the researchers reported an increase in gentoo penguins in neighboring colonies, beyond Elephant Island.
"It's interesting, like a tale of two penguins on the Antarctic Peninsula," said Strycker. "Gentoo are a species from further north and they appear to be colonizing the area and are actually increasing in numbers."
The scientists have been traveling aboard the Greenpeace ship Esperanza, which has been documenting the threat to the world's oceans.
The researchers, from Stony Brook and Northeastern University in Boston, also used manual and drone techniques to survey a series of large but relatively unknown chinstrap penguin colonies on Low Island in the South Shetland Islands, north of the Antarctic Peninsula. This is the first time the island has been properly surveyed, though results are not yet available.
Greenpeace has been campaigning for the establishment of three Antarctic sanctuaries, which it says would offer protection to many of the colonies surveyed. These would be totally off-limits to humans.