Acting under a court-ordered deadline, on May 14, 2008, Interior Secretary Dirk Kempthorne announced a decision to list the polar bear as threatened throughout its range, and subsequently issued a final special rule for its conservation. Congress has addressed the regulations. A provision of the omnibus appropriations bill of 2009 ( H.R. 1105 ) would authorize the Secretary of the Interior to withdraw or reissue the regulation "without regard to any provision of statute or regulation." This means that no rulemaking steps would be required, such as notice and comment periods. The bill would allow the previous regulations to return to effect. A proposed amendment ( S.Amdt. 599 ) would require the Secretaries to follow full rulemaking procedures if they withdrew or re-promulgated the regulations, and require a comment period of at least 60 days. The decision to list the species under the Endangered Species Act (ESA; 16 U.S.C. §§ 1531 et seq.) highlights the intersection of two significant issues currently before Congress—climate change and species protection. Listing decisions must rest solely on the best available scientific understanding of the species. For this species, a major threat is loss of its primary habitat on the Arctic sea ice, an ecosystem that is changing rapidly. The polar bear, Ursus maritimus , is the largest terrestrial carnivore and a top predator, inhabiting circumpolar Arctic regions wherever sea ice is present for a substantial part of the year. Polar bears are well adapted to the Arctic, where ice thickness can increase or decrease rapidly as well as differ significantly from year to year and between regions. Nineteen known or identified populations of polar bears are estimated to total 20,000 to 25,000 animals ( Figure 1 ). Two of these populations occur within U.S. jurisdiction—the Southern Beaufort Sea population (shared about equally with Canada), estimated at 1,526 animals; and the Chukchi/Bering Seas population (shared with Russia), estimated at about 2,000 animals. Robust data on polar bear populations are difficult to obtain. Globally, less than one-third of the world's 19 polar bear populations are known to be declining, and more than one-third are increasing or stable. In the remaining third, data are insufficient to estimate population trends and their status has not been assessed. In the United States, the polar bear population shared with Canada in the Southern Beaufort Sea may be starting to decline. In the Chukchi Sea population, shared with the Russia, over-harvest on the Russian side is considered a problem. Elsewhere, studies suggest that polar bear numbers are declining in Western Hudson Bay; a multi-year study, following local reports of more bears being seen in the northern parts of the population range, will determine if the observed decline is the result of a change in distribution or an actual reduction in abundance. Simulations suggest that polar bear populations are also declining in Baffin Bay, Kane Basin, and Norwegian Bay. The status of polar bears in the Central Arctic Basin—where there are transient bears that normally reside in other regional populations—is unknown. Two of the most southerly polar bear populations, in Southern Hudson Bay and Davis Strait, show no evidence of population decline over the past two decades of decreasing sea ice. Much of what is known about the polar bear populations and habitat is confined to regions close to shore that have been studied during long summer days, with little known about what happens on drifting sea ice far from shore, especially in winter when there is little or no daylight. Some observers urge caution in interpreting studies of sea ice change that are based primarily on surveys of nearshore regions, rather than the drifting sea ice environment in the Central Arctic Basin, where ice may be thickest. While the Central Arctic Basin currently is marginal habitat for small numbers of transient bears from other populations, changing climate could cause this area to become more important as a refugium for polar bears. The primary food of this large predator is the ringed seal ( Phoca hispida ). A polar bear may stalk a seal by waiting quietly for one to emerge from an opening in the ice that seals make to breathe or to climb out of the water to rest. Ringed seals have a circumpolar distribution and are associated with ice for birthing and molting in the spring. Much of ringed seal habitat (especially in offshore drifting sea ice) has not been surveyed, leading to much uncertainty regarding population size and status. Current estimates of the global population numbers for ringed seals range from 2 million to as many as 7 million animals. Other polar bear prey include bearded and harp seals, juvenile walrus, beluga whales, narwhal, fish, and seabirds and their eggs. Over most of their range, polar bears remain on the sea ice year-round or spend at most only short periods on land. In October and November, male polar bears, and females that are not pregnant, head out onto sea ice where they spend the winter. Pregnant females dig large dens in snow where they give birth and spend the winter. Females seek denning sites either on the sea ice ("pelagic bears") or on mainland areas ("nearshore bears"); some individuals seem to prefer mainland sites. Both pelagic and nearshore individuals are known in all subpopulations studied. Annual rates of population increase for polar bears may be as much as 5%, with mature females reproducing once every three years (commonly twins, more rarely triplets). Large carnivorous mammals are generally considered to be most at risk of population declines and extinctions, and the minimum viable total population of polar bears has been estimated at 4,961 adults. Climate change is widely considered one of the most significant contemporary threats to biodiversity worldwide. Studies by the U.S. Geological Survey (USGS) state that two-thirds of the world's polar bear population could be lost within 50 years. A May 2002 report by the World Wildlife Fund raised public concern that polar bear populations are threatened by climate change. Scientists have observed that, in recent decades, the extent of Arctic sea ice has declined significantly, and indicate that the reduction results from climate warming: annual ice break-up in many areas is occurring earlier and freeze-up later. Arctic sea ice is experiencing a continuing decline that, it is thought, may not easily be reversed, and some models project that Arctic late summer (September) sea ice could disappear completely by mid-century. In an analysis of climate models in light of Arctic ice conditions and polar bear impacts, USGS addressed the uncertainties in the models, but stated: Climate model simulations are in universal accord that greenhouse gas increases will cause Arctic sea ice cover to decline, with the greatest reductions occurring at the end of the summer melt season.... A further consistency in climate simulations is the uneven latitudinal distribution of global warming, which always has its greatest simulated impact in the high northern latitudes. This "polar amplification" and associated sea ice decline have been consistent climate simulation features at least since the early simulation of Manabe and Souffer (1980). Since Chapman and Walsh (1993), declines in Arctic sea ice, with the largest trends in September, have also been consistently reported in observations.... However, links between climate-model predictions and threats to polar bears are characterized as tenuous by many scientists, who point to the limitations of correlational studies and the hypothetical nature of model-based predictions of environmental conditions decades into the future. Distribution patterns of some polar bear populations have changed in recent years. Greater numbers of bears are being found onshore near the Bering Sea, and in some parts of Canada, with Inuit hunters reporting more bears present on land during summer and fall. There may be several reasons for the observed changes, including changes in sea ice; those who conduct population censuses of polar bears will need to be cautious in interpreting whether apparent population variations are indicative of changing habitat use (e.g., greater numbers of bears onshore) or actual changes in population abundance. Recent studies found that mid-latitude European populations of Arctic fox became extinct at the end of the Pleistocene and did not track the habitat when it shifted to the north, suggesting that some populations of Arctic species are unable to track decreases in habitat availability and may be vulnerable to increases in global temperatures. A loss of sea ice could affect survival and reproduction of polar bears by: shortening the season during which ice is available to serve as a platform for hunting ringed seals; increasing the distance between the ice edge and land, thereby making it more difficult for nearshore female bears that prefer to den on land to reach preferred denning areas; reducing the availability of sea ice dens for gestating pelagic female bears; requiring nearshore bears to travel through fragmented sea ice and open water, which uses more energy than walking across stable ice formations; reducing the availability and accessibility of ice-dependent prey, such as ringed seals, to nearshore populations; and requiring nearshore bears to spend more time on land, thereby increasing the potential for adverse human-polar bear interactions. In addition to changing sea ice conditions, others have expressed concern that climate change could affect the integrity of polar bear den sites, as rain can destroy ice dens, exposing young polar bears to the elements prematurely. Although some scientists predict the extinction of polar bears under potential climate change scenarios, not all sea-ice changes would harm polar bears. For example, reduced sea ice thickness and coverage in far northern regions could improve polar bear habitat, by increasing the availability and accessibility of ice-dependent prey, such as ringed seals. Others remind biologists that climate-related changes to a species' distribution may not necessarily lead to changes in abundance. Three main groups of contaminants are implicated as potentially threatening polar bears—petroleum hydrocarbons, persistent organic pollutants, and heavy metals. Moreover, climate change may alter contaminant pathways through increased precipitation, increasing the potential threat to polar bears.