The phrase "canaries in the coal mine" has long been understood to reference an environmental early warning system, alluding to the observation that changes in the behavior or even death of caged canaries could reliably alert working miners to the possible presence of lethal underground gases. Long-term ecological research in the Western Antarctic Peninsula (WAP) has revealed an analogous situation in which Adélie penguins (Pygoscelis adeliae) represent the canaries in the coal mine, with changes in their populations providing some of the earliest evidence that regional climate warming is negatively impacting the marine ecosystem on which this and other species depend (Fraser et al., 1992).
Adélie penguins are long-lived (up to 20 years), wide-ranging (although flightless, annual migrations may cover 3,000 km) top predators that breed on land but feed at sea. As a result, their life histories not only integrate the effects of environmental variability over large space and time scales, but also across both marine and terrestrial habitats. What this means in contemporary ecological language is simply that the behavior, breeding biology, and feeding ecology of this species can, in effect, provide a window into how the WAP marine ecosystem functions, including, importantly, how it is responding to climate change.
Adélie penguins are a true polar species, which means a species in which some aspect of their life history is critically dependent on the availability of sea ice. For these penguins, it is winter sea ice that is essential, and indeed we now recognize that without it their ability to survive the harsh Antarctic winter greatly diminishes. This occurs for two reasons: one is that during winter the krill and fish on which the penguins depend tend to concentrate in only a few key marine locations; the other is that access to these locations is dependent on the formation and expansion of sea ice during winter. Thus, sea ice functions as a critical platform from which Adélie penguins forage; without it, access to the prey on which they depend becomes increasingly difficult and winter survival more challenging (Erdman et al., 2011). The WAP winter sea ice season has decreased by an astonishing 86 days since 1978, nearly three months (PAL Bullet #2), which as shown in Fig. 1 has had a catastrophic effect on Adélie penguin populations.
Because sea ice acts as a cap on the ocean during winter, its gradual disappearance has resulted in more open water and, due to more evaporation to the atmosphere, increasing cloud cover and snow precipitation over the WAP. Although Adélie penguins can tolerate some snow cover on their breeding colonies, PAL field and experimental data indicate that snow cover is now on average deeper and persists longer, especially on south-facing landscapes that are not effectively scoured by the region’s predominant northern winds during storm events (Patterson et al., 2003). What happens when these deeper snow accumulations melt in the spring at the height of the penguins’ breeding season is illustrated in Fig. 2. These flooding events can have devastating effects on eggs and chicks, and indeed if repeated over time will lead to the complete extinction of breeding colonies. Since 1974, 68% of Adélie penguin breeding colonies within the PAL study region have gone extinct; 82% of these extinctions are associated with increasing snow and spring melt on south-facing landscapes.
One of the great difficulties in understanding and predicting how ecosystems might respond to climate change is that we cannot always identify related uncertainties and, therefore, their consequences. The effects of snow on Adélie penguins illustrates this critical point well, in that it represents a process with serious ecological implications that was not anticipated even as data first suggested that diminishing sea ice due to climate change was negatively impacting the WAP marine ecosystem. Although identifying these uncertainties will remain a formidable interdisciplinary challenge in climate change science, there is no question that long-term research offers the best venue for informing researchers and society.