Antarctica's population of emperor penguins could fall by at least a fifth by 2100 as the sea ice on which the birds breed becomes less secure, a new study predicts.

The study urges governments to list the birds as endangered, even though populations in 45 known colonies were likely to rise slightly by 2050 before declining by the end of the century. Such a listing could impose restrictions on tourism and fishing companies.

"The emperor penguin is fully deserving of endangered status due to climate change, and can act as an iconic example of a new global conservation paradigm for species threatened by future climate change," the study's authors write in the journal Nature Climate Change.

The study is the first to project the long-term outlook for Antarctica's largest penguins, which can grow 1.2 metres tall, seeking to fill a gap in understanding climate change and wildlife in one of the remotest parts of the planet.

Overall, numbers were set to fall by at least 19 per cent from current levels by 2100 as sea ice melts. And two-thirds of colonies of the birds, which have distinctive golden head patches, would decline by more than half.

"It's not happy news for the emperor penguin," study co-author Hal Caswell, of the U.S. Woods Hole Oceanographic Institution, said.

Most penguins in decline

Populations of most of 18 types of penguins are decreasing, according to a list run by conservation experts. Emperors are one of three species rated stable, with around 600,000 birds.

Colonies of emperor penguins are dotted right around Antarctica.

West Antarctic colonies, located from the Eastern Weddell Sea to the Western Indian Ocean, are expected to have the most rapid decline as this area of Antarctica experiences the greatest variability in sea ice cover.

Colonies in the Ross Sea, which will experience the smallest sea ice loss and variability in cover, are still projected to decline by 2100, the scientists report.

"At the global level, large population declines at most colonies are buffered by increases in the Ross Sea until 2080, but by the end of the 21st century there will no longer be any viable refuges," the study's authors write.

Frozen huddle

Emperor penguins breed on sea ice, with the males huddling together to keep eggs warm amid winter darkness and temperatures down to minus 50 Celsius, and are vulnerable to shifting sea ice.

"There is a Goldilocks point for ice and emperor penguins," said Phil Trathan, an expert at the British Antarctic Survey.

Too much ice means the females, which can travel 100 kilometres to the sea to catch fish, must waddle ever further. Too little ice means waves could break up colonies in spring.

Changes in sea ice also affect the availability of krill, the penguin's major food source.

Trathan says it was unclear whether the ungainly birds could adapt by climbing onto land or higher ice. Four emperor penguin colonies had recently been found on ice shelves, above sea level where glaciers spill off the land.

Environmentalists have urged governments to agree to proposed marine reserves in the Ross Sea and off East Antarctica — so far opposed by Russia — to safeguard wildlife.

"Marine reserves are one of the best ways to protect penguins," said Andrea Kavanagh of the Pew Charitable Trusts.

Satellite images have made it easier to estimate emperor populations, especially since a discovery in 2009 that droppings from their colonies leave stains visible from space.

Other penguins, which breed on land, are far harder to spot by satellite amid rocks and shifting snows.

With files from Reuters