Polar Bears and Climate Change: Certainties, Uncertainties, and Hope in A Warming World.

Abstract

—The spatiotemporal distribution of arctic sea ice has been in decline for decades. Polar Bears are dependent on the sea ice for access to their marine mammal prey. Observed sea ice declines have been linked to reduced body condition and stature, altered feeding and movement patterns, reduced survival, and population declines in Polar Bears. Based upon projected future sea ice losses, a US Geological Survey (USGS) research team concluded in 2007 that twothirds of the world’s Polar Bear population could disappear by mid-century. Yet, adverse effects of changing sea ice have not occurred simultaneously across the Polar Bear’s circumpolar range, and projections of future sea ice change differ among regions. Some Polar Bear populations are already in real trouble while those in historically colder regions of the Arctic now may be benefiting from a milder climate. Natural variation in weather and climate means we cannot predict exactly when many critical thresholds will be exceeded. We cannot predict, for example, the first year that reduced access to sea ice will prevent female Polar Bears in a particular region from reaching their traditional denning areas or from achieving weight gains necessary for reproduction. But, we were able to confidently project the distant worldwide future for this iconic species because the fundamental laws of physics guarantee that without mitigating the rise in greenhouse gases we ultimately will exceed these and other critical thresholds. Because their sea ice platform is literally “melting away” the relationship between habitat change and animal welfare may be more clear-cut for Polar Bears than for many other species. Nonetheless, the concept of exceedence thresholds is an important part of understanding the ultimate challenges that anthropogenic climate change brings to all species and ecosystems. Received 14 March 2011, accepted 20 April