The emperor has no ice

Antarctica is best described in superlatives: It is the coldest, windiest, driest and highest continent on Earth - along with being the largest and most unforgiving of the world's deserts.

The sun sets only once every year on Antarctica, and in the twilight between the long polar night and a summer of 24-hour daylight, the frozen landmass becomes a stage for the world's most spectacular seasonal change.

Slowly, the surface of the surrounding ocean begins to freeze. The ice spreads out in all directions to form an apron that doubles the size of the continent.

No ice, no home

The annual freeze-thaw cycle of sea ice sets the pace of life for Antarctica's megafauna: penguins and seals. These two species in particular are so finely tuned to the rhythm of ice, that even small variations can have a major impact on their breeding success.

And they face an uncertain future.

Emperor penguins are masters of Antarctic survival, and Weddell seals are the only air-breathing animal to make their homes under Antarctica's fast ice, or sea ice connected to the mainland. Both operate at the very limits of biology.

Emperor penguins may well never set foot on land. They forage from floating chunks of ice and breed on thick slabs of sea ice, in the depths of winter. Their life cycle is so tied up with ice, that they cannot survive without it.

Stephanie Jenouvrier, a biologist with the Woods Hole Oceanographic Institute, says sea ice is crucial during the Emperor penguins' breeding season - but also for their food supply, as it determines the abundance of krill.

"They arrive at the colony when the sea ice is forming and they leave when it is retreating again," she says.

If the sea ice breaks up too early - before the chicks have developed their waterproof plumage - they won't survive. If it breaks up too late or too slowly, the distance between the breeding colony and feeding grounds can become too large.

Therefore, changes in sea ice can have dramatic effects on penguins.

No more emperors?

If sea ice declines as predicted by climate models used by the Intergovernmental Panel on Climate Change, emperor penguins across the Antarctic continent will be in serious decline by the end of this century.

Jenouvrier is part of an international team studying an Emperor penguin colony in Terre Adelie, in east Antarctica. There, researchers have been returning year after year - for five decades - to monitor changes in population dynamics and to follow individual tagged birds from season to season.

If changes in sea ice continue to influence Emperor penguins as they have over past decades in Terre Adelie, Jenouvrier says two-thirds of all colonies will be endangered. This would result in declines of more than half the current population by the end of the century.

In a warming world, emperor penguins will be left with nowhere to go.

Jenouvrier paints a grim picture for the future of emperor penguins: "There will be no single habitat in Antarctica that can sustain growth in an Emperor penguin colony."

"All colonies will be declining," Jenouvrier says. "If they are not able to adapt to changes in sea ice, they will go extinct."

For Jenouvrier, the plight of Emperor penguins is so serious that she is calling for them to be classified as endangered to reflect the threat climate change poses to Antarctic ecosystems.

Melting away from below

Antarctica has so many different types of ice that it has its own vocabulary. Two massive ice sheets cover the continent, burying entire mountain ranges beneath them. This icy cap holds three-quarters of the planet's freshwater, frozen in a fragile equilibrium.

Along the coastline, ice sheets run out into ice shelves that float on the ocean. They may seem featureless, but are actually made up of a mix of smaller ice formations, including slow-flowing glaciers.

Some of these glaciers, particularly in parts of West Antarctica, have begun to melt from below - being eaten away by a warming ocean.

One of the mysteries of Antarctica is that land-based ice is losing mass, but the area covered by seasonal sea ice has been increasing along the eastern coastline. However, this is expected to be a temporary anomaly.

Jenouvrier says that along the Antarctic Peninsula, which has been warming faster than the rest of the continent, sea ice cover is diminishing, and a once-flourishing emperor penguin colony has already disappeared.

Back to the future

Another glimpse of what Antarctica may look like in the future comes from the past. Tim Naish, a geologist at Victoria University in Wellington, New Zealand, led an ambitious palaeo-climate project called ANDRILL.

The team drilled deep into ocean sediments off Antarctica and back in time to a geological period known as the Pliocene, when carbon dioxide levels in the atmosphere were about the same as today, but temperatures were 3 degress Celsius warmer.

Back then, he says there was no sea ice, no Ross Ice Shelf and no West Antarctic ice sheet. "That's the end game," he says. Meaning, that is what's likely if we are to maintain the current level of greenhouse gas concentrations in the atmosphere for centuries.

Bad news for seals

That's a scenario that would erase the habitat of Weddell seals, says University of Canterbury biologist Regina Eisert, who is studying Antarctica's seals and whales. "Weddell seals are fast-ice specialists - they live deep under the closed ice."

They congregate during the austral spring in areas where tides and winds have created cracks in the ice. Massive males patrol underwater marine territories, calling out to impress females with songs that sound out of this world.

Holes in the ice provide the seals' only access to air - and this is also where they haul out to bear their young. After the breeding season, the females molt, and then disperse again to feed.

"They want a lot of ice early in the season so they have a stable platform for their babies," Eisert explains. "And then they would like that ice to break out in order to make food sources accessible," she continues.

"If you have less sea ice, you'd expect the ecosystem to change in the way we're already seeing around the Antarctic Peninsula - where it provides less nutritional value," Eisert says.

If sea ice were to disappear - or even just change significantly - Eisert says Weddell seals would face invasion into their habitat by other seal species, as well as major shifts in the food web.

And this could spell extinction for another Antarctic icon.