Meltdown? Sea ice may be thawing, which could mean disruption of life at Earth's polar ends.(Frozen Ocean)

After four months of fasting while warming eggs on their toes in the deep cold of an Antarctic winter, male emperor penguins trek as many as 300 kilometers (186 mi.) over sea ice to feed in polynyas, large bodies of ice-enclosed water. On the way, some may stop to snack in tidal cracks and seal breathing holes.

At the other end of the Earth, female hooded seals give birth in spring on the deteriorating ice pack of the North Atlantic. They stay on the ice for just four days, delivering to their pups milk that is 60 percent fat. Four days is believed to be the shortest time any newborn mammal nurses. The speeded-up process allows seals to take advantage of a pupping platform floating in a sea full of food yet not be caught dependent on the ice platform when it breaks up.

With these and other recent findings, biologists are adding to a store of knowledge about the web of life at the frozen surface of the polar seas. Organisms ranging from minute algae to the 30 million crabeater seals that breed on Antarctic ice floes not only cope with sea ice, they have evolved to exploit it. "A whole array of species uses sea ice," says retired marine-mammal biologist John J. Burns, formerly of the Alaska Department of Fish and Game. "It's an incredibly complex and beautiful ecosystem."

It may also be a threatened ecosystem. Some experts on the subject of global climate change are predicting massive dwindling of the ice within a relatively short period of time. University of Washington climatologist Richard Moritz, for one, is of the opinion that the Arctic's year-round ice pack will dis- appear in 50 years if atmospheric warming continues (seasonal ice will con- tinue to form each winter). Other experts guess it may take 100 or 200 years.

Moritz is director of a research project called SHEBA, for Surface Heat Budget of the Arctic Ocean. Last October, an international group of 50 SHEBA researchers froze a research vessel, a Canadian icebreaker, into Arctic pack ice hundreds of kilometers north of Alaska. Their mission as they drift in the ice is to gather information that will help climatologists make better com- puter models of climate change and its effect on sea ice.

For one year, the SHEBA scientists are intensely studying ice 50 kilometers (31 mi.) in every direction around the ship, as well as the water below and air above the ice. They are measuring everything from the size of snow parti- cles on the ice to the stress produced by a new crack. Among their questions: …