Penguin Decline in Antarctica Linked With Climate Change

By John Roach for National Geographic News Emperor penguins like it cold. Now, scientists have determined that the penguins' susceptibility to climate change accounts for a dramatic decline in their number over the past half century. Over the past 50 years, the population of Antarctic emperor penguins has declined by 50 percent. Using the longest series of data available, reseachers have shown that an abnormally long warm spell in the Southern Ocean during the late 1970s contributed to a decline in the population of emperor penguins at Terre Adelie, Antarctica. The warm spell of the late 1970s is related to the Antarctic circumpolar wave—huge masses of warm and cold water that circle Antarctica about once every eight years. In response to this cycle, Terre Adelie experiences a warming period every four or five years that generally lasts about a year. In the late 1970s, however, the warming continued for several years. Whether it was the result of natural climate variability in the Antarctic circumpolar wave cycle or an anomaly related to global warming is not possible to determine because air and sea surface temperature data from many years ago are not available. Weimerskirch thinks the unusually warm spell was probably the result of global warming. Shrinking Levels of Ice Warmer air and sea surface temperatures in the Antarctic reduce the amount of ice in the sea. This, in turn, leads to smaller populations of krill, a shrimp-like crustacean that is a staple of the emperor penguin's diet. With less food to eat, emperor penguins die. Reporting in the May 10 issue of Nature, Weimerskirch and his colleague Christophe Barbraud say this is the scenario that led to the sharp decline in the penguin population at Terre Adelie. "The population decreased because of the low rates of survival over four to five successive years," said Weimerskirch. In the early 1980s the winter air and sea surface temperatures dropped, and the emperor penguin population stabilized. Although higher levels of sea ice increase the food supply, such conditions have a negative effect on reproduction because emperor penguins hatch fewer eggs when sea ice is more extensive. After laying eggs, a female travels across the ice and out to sea to feed on krill, fish and squid that she regurgitates to feed her young. The male keeps the eggs warm until she returns. But when the sea ice is extensive, the female may be gone for months. The male eventually gives in to his hunger and abandons the egg or chick. Caution Against Generalizations Thus, as the scientists note in their paper in Nature, extensive sea ice poses a trade-off for emperor penguins. In population terms, its nutritional advantage, which favors higher survival and further reproduction, "outmatches its physical disadvantage of reducing fecundity," they write. Despite the findings that show a negative effect of global warming on emperor penguin populations, Weimerskirch cautions against making generalizations about the impacts of climate change on wildlife. For example, a reduction in the amount of sea ice is favorable to Adelie penguins, he said. On the other hand, elephant seals and some albatross species were also negatively affected by the prolonged warming period in the 1970s. Climate scientists believe that Earth's polar regions are harbingers of the effects of global warming and play a major role in regulating global climate. The Antarctic circumpolar wave, for example, is tied to episodes of drought and deluges of rain in Australia. The science, however, is still evolving. "We are progressively understanding how environmental variability affects populations," said Weimerskirch.