A decline in the population of emperor penguins appears likely this century as climate change reduces the extent of Antarctic sea ice, according to a detailed projection published this week. The study, led by the Woods Hole Oceanographic Institution (WHOI), with co-authors from the National Center for Atmospheric Research (NCAR) and other organizations, focuses on a much-observed colony of emperor penguins in Terre Adelie, Antarctica.

The authors conclude that the number of breeding pairs may fall by about 80 percent by 2100.

"The projected decreases in sea ice may fundamentally alter the Antarctic environment in ways that threaten this population of penguins," says NCAR scientist Marika Holland, a co-author of the study.

The study uses a set of sophistical computer simulations of climate as well as a statistical model of penguin demographics. Building on previous work, it examines how the sea ice may vary at key times during the year such as during egg laying, incubation, rearing chicks, and non- breeding season, as well as the potential influence of sea ice concentrations on males and females.

The authors stress that their projections contain large uncertainties, because of the difficulties in projecting both climate change and the response of penguins. However, almost all of their computer simulations pointed to a significant decline in the colony at Terre Adelie, a coastal region of Antarctica where French scientists have conducted penguin observations for more than 50 years.

"Our best projections show roughly 500 to 600 breeding pairs remaining by the year 2100," says lead author Stephanie Jenouvrier, a WHOI biologist. "Today, the population size is around 3,000 breeding pairs."

She noted that another penguin population, the Dion Islets penguin colony close to the West Antarctic Peninsula, has disappeared, possibly because of a decline in Antarctic sea ice.

The new research represents a major collaboration between biologists and climate scientists to assess the potential impacts of climate change on a much-studied species.

Published this week in the journal Global Change Biology, the study was funded in part by the National Science Foundation, NCAR's sponsor. Other funders include WHOI; the French National Agency for Research (ANR) program on biodiversity; the ANR REMIGE program (Behavioral and Demographic Responses of Indian Ocean Marine Top Predators to Global Environmental Changes); the Zone Research Workshop for the Antarctic and Subantarctic Environment (ZATA); the Paul Emilie Victor Institute (IPEV); Alexander von Humboldt Foundation; Marie-Curie European Fellowship; and the U.S. Cooperative Institute for Research in Environmental Sciences visiting fellowship.

Vulnerable emperors of the ice
At nearly four feet tall, emperors are the largest species of penguin. They are vulnerable to changes in sea ice, where they breed and raise their young almost exclusively. If that ice breaks up and disappears early in the breeding season, massive breeding failure may occur, Jenouvrier says.

Disappearing sea ice may also affect the penguins' food sources. They feed primarily on fish, squid, and krill, a shrimplike animal that feeds on zooplankton and phytoplankton that grow on the underside of ice. If the ice goes, Jenouvrier says, so too will the plankton, causing a ripple effect through the food web that may starve the various species that penguins rely on as prey.

To project how the extent of sea ice in the region will change this century, Holland and another co-author, Julienne Stroeve, a sea ice specialist from the National Snow and Ice Data Center, evaluated 20 of the world's leading computer-based climate models. They selected the five models that most closely reproduced changes in actual Antarctic sea ice cover during the 20th century.

"When a computer simulation performs well in reproducing past climate conditions, that suggests its projections of future climate conditions are more reliable," Holland says.

The team evaluated simulations from each of the 20 climate models. The simulations were based on a scenario of moderate growth in greenhouse gas emissions during this century. The moderate growth scenario portrays future reliance by society on a combination of greenhouse-gas emitting fossil fuels as well as renewable energy sources.

The simulations showed a decline in sea ice coverage across a large region by Terre Adelie at key times in the penguin breeding cycle, although they differed in the details.

Jenouvrier used the output from the climate models to determine how changes in temperature and sea ice might affect the emperor penguin population at Terre Adelie, studying such details as how the sea ice was likely to vary during breeding season and how it could affect chicks, breeding pairs, and non-breeding adults. She found that if global temperatures continue to rise at their current rate-causing sea ice in the region to shrink-penguin population numbers most likely will diminish slowly until about 2040, after which they would decline at a much steeper rate as sea ice coverage drops below a usable threshold.

The authors say that more research is needed to determine whether emperor penguins may be able to adapt to changing conditions or disperse to regions where the sea ice is more habitable.

Human reliance on the Antarctic
Rising temperature in the Antarctic isn't just a penguin problem, according to Hal Caswell, a senior mathematical biologist at WHOI and collaborator on the study. As sea ice coverage continues to shrink, the resulting changes in the Antarctic marine environment will affect other species, and may affect humans as well.

"We rely on the functioning of those ecosystems," he says. "We eat fish that come from the Antarctic. We rely on nutrient cycles that involve species in the oceans all over the world. Understanding the effects of climate change on predators at the top of marine food chains-like emperor penguins-is in our best interest, because it helps us understand ecosystems that provide important services to us."

Also co-authoring the study were Christophe Barbraud and Henri Weimerskirch of the Centre d'Etudes Biologiques de Chize, in France, and Mark Serreze of the National Snow and Ice Data Center in the United States. Title: Effects of climate change on an emperor penguin population: analysis of coupled demographic and climate models Authors: Stephanie Jenouvrier, Marika Holland, Julienne Stroeve, Christophe Barbraud, Henri Weimerskirch, Mark Serreze, and Hal Caswell Journal: Global Change Biology