Caught In A Melting World - Adelie penguins may be the canaries in the coal mines of global warming.

FOUR LITTLE HEADS pop up simultaneously in a jagged pool of blue-black water surrounded by ice as far as the eye can see. They are Adelie penguins, and the ice defines their existence.

Like little torpedoes, the birds-just over 2 feet long-leap about excitedly in tight circles, rocketing in and out of the water, perfectly at ease in this frigid sea that surrounds the shores of Antarctica. Not only at ease, but at home. The seasonal freezing and thawing, spreading and shrinking, of the ocean's surface is the world they know. In recent years, as Earth's climate has warmed, it has also become a world in rapid and disturbing change.

The Adelies' home is a place made up of great plates of frozen water. Floes half a football field in size crunch and grind against one another as powerful ocean currents and opposing winds shove and shift the entire mass in enormous swirls and gyres. Under this constant motion, cracks and fissures open and close, and tidal eddies force open large pools. This is what the Adelies use to transit between the landscape above the ice, which they need for air to breathe, and the one below, which provides them with food.

Their food is tied, literally, to the frozen ocean. Within spongy layers of sea ice, microscopic algae bloom in profusion as sunlight floods in from above. When the sea ice melts with the onset of summer, the algae, aptly called ice algae, escape into the water, where they are grazed by dense swarms of krill-a type of shrimplike crustacean. The krill, in turn, are the Adelie penguins' primary food. To eat them, Adelies spend their entire lives on, around or beneath the Antarctic pack ice.

It is now late spring in the Ross Sea, some 1,800 miles due south of New Zealand. Three more penguins erupt from the water as I watch, emerging through the 6-foot-thick layer that covers the rich waters where they have been feeding. By following cracks and leads, the penguins have been able to push farther and farther into the heart of the pack ice, for these penguins are on an urgent mission.

As the seven heads appear together at the surface, I hear their harsh …