Image copyright Reuters Image caption There's a "goldilocks" point for ice and emperor penguins, say researchers

Climate change is likely to cut Antarctica's 600,000-strong emperor penguin population by at least a fifth by 2100, a study suggests.

The main threat to the penguins comes from changes to sea-ice cover in the Antarctic, which will affect their breeding and feeding.

Dynamics will differ between penguin colonies, but all are expected to be in decline by the end of the century.

Details are published in Nature Climate Change journal.

The US, British and Dutch researchers urge governments to list the birds as endangered. Such a listing could impose restrictions on tourism and fishing.

The team, led by Stephanie Jenouvrier of the Woods Hole Oceanographic Institution, said the global population of emperor penguins would probably decline by between 19 and 33% from current levels.

Dr Jenouvrier said the penguins "face possible extinction throughout a significant portion of their range in the foreseeable future".

She added that emperor colonies in Antarctica's Ross Sea may experience population declines later than others because sea ice conditions are still suitable for them.

"Implementing a marine protected area in the Ross Sea could help buy time to avoid extinction and to put in place needed conservation and greenhouse gas mitigation strategies," she said.

To feed their young, emperor penguins leave the colony for months at a time and travel long distances across Antarctic ice to reach open water to find nourishment, such as krill.

They are dependent upon an optimal amount of sea ice cover for a variety of reasons, including refuge from predators while foraging.

Changes to the sea-ice cover can also significantly affect the abundance of krill, the emperor's primary food source and a critical species in the Antarctic food web.

More sea-ice is good for krill, but means that parents might have to waddle further to the sea.

"There is a goldilocks point for ice and emperor penguins," Phil Trathan, an expert at the British Antarctic Survey (Bas), told Reuters.

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

Satellite measurements of Antarctic sea-ice extent show winter coverage to be at record levels. However, climate computer modelling expects this trend to be reversed in the future, as conditions in the Antarctic warm.