RIGHT ABOUT NOW, ATOP THE floating sea ice off the Ross Ice Shelf, emperor penguins are getting their annual chance to strut their adaptations to the crystalline cold of Antarctica. During other seasons they do their impressions of Charlie Chaplin in a too-tight tux, waddling across the ice without developing so much as a goose bump. Or they huddle together to conserve heat, never forgetting their manners: a penguin that has basked in the (relative) warmth of the center of the huddle graciously gives way to one that has been shivering on the edge. Both are fine adaptations, physiological and behavioral. But during the winter months of June through August, when not even the mercury is foolish enough to come out of the thermometer bulb, emperors prove beyond a doubt that they are built for cold. Where do the fathers incubate their egg (while Mom hunts for food)? On their feet, with a bit of belly hanging over to form a sort of pup tent: these birds are so well insulated that those are the only spots from which a few stray watts of heat can escape and warm an egg.
But now penguins may be paying for their superb fit to the frigid clime. For Antarctica, according to a mounting pile of studies, isn't as cold as it used to be. Although penguins are under siege on other fronts - a chicken virus that predisposes hatchlings to lethal infections has shown up in some emperors and Adelies - none can account for the die-offs and decreased breeding success among the Adelies, gentoos and chinstraps of the southern continent. But scientists have made a discovery that might explain penguins' problems - a discovery that reduced one veteran of Antarctic research to an astonished ""holy s---!'' Average annual air temperatures on the Antarctic Peninsula have climbed 5 degrees Fahrenheit over the last 50 years, 10 times faster than the global rate. Average midwinter temperatures there are up 9 degrees. The cause could be natural climate fluctuations. Or it could be global warming induced by the heat-trapping ""greenhouse'' gases emitted into the atmosphere. According to computer models, man-made warming will be more extreme at the poles, and show up there before it's detectable in midlatitudes.
While the cause of Antarctica's warming is up for grabs, the effects are as plain as a black-headed chinstrap on a field of snow. First, warmer air holds more moisture. In Antarctica, moisture means snow. At Adelie rookeries on five islands near Palmer Station on the peninsula, more snow has meant fewer Adelies. In 1975 scientists counted 15,200 breeding pairs on the islands; today there are 9,200. Six colonies have gone extinct. The species lays its eggs in snowless, rocky outcrops. But with more snow, more Adelies are laying clutches in what often become pools of slush, ecologist William Fraser of Montana State University told a recent Capitol Hill seminar on climate change. These birds ""lost more eggs and chicks'' to cold than those that laid in snow-free rock, Fraser reported. The smaller colonies also make easy pickings for marauding skuas, gull-like birds that have a taste for tender young penguin flesh.
A warmer Antarctica also means less sea ice. Every autumn - March or April - the southern ocean begins to ice up, freezing from the continent outward, forming a huge extension of it, kind of like a white crinoline slowly slipping below a Victorian hemline. The floating ice can cover an area twice the size of the United States. Sea ice serves at least two purposes. First, it is a refuge for krill, shrimplike crustaceans up to three inches long. Because krill both eat (single-celled phytoplankton) and are eaten (by penguins, among others), they form the pivotal link in the Antarctic food chain. ""Sea ice provides an important habitat for krill,'' says biologist Valerie Loeb of Moss Landing Marine Laboratories on Monterey Bay in California. Second, the ice - specifically, its underside - provides a home for the single-celled algae that krill devour. ""If the sea ice isn't there,'' says Loeb, ""then the algae aren't there, either.''
And sea ice, in fact, isn't there, at least not like it used to be. Over the last 50 years there have been fewer winters cold enough to produce extensive sea ice around the peninsula. Decades ago that crinoline grew to an enormous size in three or four out of every five winters. Now only one or two winters out of every five are cold enough. As a result, ""krill abundance in the Antarctic Peninsula region is down 60 to 90 percent since the early '80s,'' finds Loeb, whose research is funded by the government's Antarctic Marine Living Resources (AMLR) program. According to a study with other AMLR scientists, published in June, krill off the peninsula are being replaced by salps, gelatinous creatures that prefer open ocean - of which there is more, naturally, when sea ice is less. Penguins don't eat the indigestible blobs.
Result? According to a paper submitted to the journal Ecology, there has been ""a significant decline in fledgling survival'' since the mid-1970s. Before 1987 about 22 percent of Adelie chicks survived and returned to breed. Since then only 10 percent have, finds coauthor Wayne Trivelpiece of Montana State. The population of Adelies in Admiralty Bay has dropped at least 35 percent since 1987, he finds: from almost 10,000 to between 5,000 and 6,000. And chinstraps are also taking it, dare one say, on the chin. When not breeding or brooding, chinstraps go to sea, covering hundreds of thousands of square miles. That they, too, are now declining hints that krill abundance is down over a huge area.
Antarctica's penguins are not threatened with extinction. Most likely, if temperatures keep rising and sea ice and krill numbers keep falling, penguin populations will plummet, then stabilize. Sea-ice cover off the peninsula ""is below average'' again this winter, says fisheries biologist Roger Hewitt, administrator of the U.S. part of the AMLR program. Last week in La Jolla, Calif., he and other scientists from 13 countries that have signed the Antarctic Treaty (which governs research and resource use in Antarctica) met to figure out what the krill decline means for the continent. And climatologists are watching Antarctica to see whether it is a reliable sign of the global warming they have been predicting. The emperors, meanwhile, are still waiting patiently, their egg balanced on their feet, oblivious to the possibility that the adaptations to cold that have enabled the species to survive for millions of years may yet prove their undoing.