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.
"We knew since the 1980s that emperor penguins had declined, but it is only today, because of the improvements of our knowledge in the climate-ocean processes, that we have been able to understand why they have decreased," said Henri Weimerskirch of the French National Center for Scientific Research in Villers en Bois, France.
The warm spell of the late 1970s is related to the Antarctic circumpolar wavehuge 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.
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