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Overall, our analysis highlights the potential for large reductions in Polar Bear abundance if sea-ice loss continues over the long-term, which is forecasted by climate models and other studies (IPCC 2013). It also highlights the large amount of uncertainty in even simple statistical projections of Polar Bear subpopulations, and the sensitivity of these projections to plausible alternative assumptions. Across the six scenarios in Table 4 that projected Polar Bear abundance forward in time using the median and 95th percentile of GL, the median probability of a reduction in the mean global population size greater than 30% was approximately 0.71 (range 0.20-0.95). The median probability of a reduction greater than 50% was approximately 0.07 (range 0-0.35), and the probability of a reduction greater than 80% was negligible. The IUCN Red List Guidelines suggest that assessors consider nearly the full range of uncertainty in potential outcomes, and adopt a precautionary but realistic attitude toward risk tolerance (Section 3.2.3, IUCN 2014). In light of the significant probability, across scenarios, of a reduction greater than 30%, and the relatively low probability of a reduction greater than 50%, we conclude that Polar Bears currently warrant listing as Vulnerable under criterion A3c (IUCN 2014).
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Estimating Polar Bear abundance is expensive and difficult because the animals often occur at low densities in remote habitats. Although abundance estimates have generally improved in recent decades (Obbard et al. 2010), information remains poor or outdated for some subpopulations. Summing across the most recent estimates for the 19 subpopulations (Table 3 in the Supplementary Material) results in a total of approximately 26,000 Polar Bears ( 95% CI = 22,000-31,000 )
Does this link discuss the population in the 70s? Regardless, there have been significant conservation efforts since the 70s (as your two links both discuss) and population data across time periods is difficult to compare since methodologies have been refined and improved, even in the last few years.