Summary/Abstract
In 2008, projections that up to two-thirds of the world’s polar bears could disappear by mid-century led to polar bears becoming the first species listed under the US Endangered Species Act (ESA) because of threats from anthropogenic climate warming. Updated analyses corroborated the 2008 projections and showed a linear but inverse relationship between Arctic sea ice extent and global mean temperature. Despite the relationship between warming and sea ice loss, absence of a quantitative link between anthropogenic greenhouse gas (GHG) emissions, sea ice loss, and declining polar bear vital rates has foiled full ESA implementation for polar bears. By quantifying the relationship between anthropogenic GHG emissions and polar bear population levels, the authors show that sensitivities to cumulative anthropogenic emissions explain observed population trends, allow estimation of demographic impacts from new emissions sources, and enable ESA procedures to assess global warming impacts of proposed actions—along with impacts on the ground.