Scientists Recant Tipping Point Theory That Doomed the Polar Bear: In the December issue of Nature, Steven C. Amstrup of the US Geological Survey and others issued a letter revising the model and the expected fate of the polar bear. This strikes at the heart of the decision to list the bear on the ESA as well as the arguments of the Convention on Biological Diversity (CBD) that the bear should have been listed as endangered instead of threatened. (Nature, Vol. 468 at 955, entitled Greenhouse gas mitigation can reduce sea-ice loss and increase polar bear persistence.) Amstrup is the high priest behind the polar bear listing, so his second thoughts are illuminating. He is joined in the letter by co-authors Eric T. DeWeaver, David C. Douglas, Bruce G. Marcot, George M. Durner, Cecilia M. Bitz and David A. Baily.
On the basis of projected losses of their essential sea-ice habitats, a United States Geological Survey research team concluded in 2007 that two-thirds of the world’s polar bears could disappear by mid-century if greenhouse gas emissions continue, the letter reports. “A key question was whether temperature increases lead to proportional losses of sea-ice habitat, or whether sea-ice cover crosses a tipping point and irreversibly collapses when temperature reaches a critical threshold.” “It has been proposed that existing greenhouse gas emissions already have committed the earth to temperatures that will rise above the tipping point for loss of perennial Arctic sea ice.” That perception that nothing can be done was at the root of the listing on the ESA, as demonstrated when the general media proclaimed polar bear were “irreversibly doomed.”
This letter states that new model outcomes show “that increased habitat retention under greenhouse gas mitigation means that polar bears could persist throughout the century in greater numbers and more areas than the business-as-usual cases. Our general circulation model outcomes did not reveal thresholds leading to irreversible loss of ice; instead, a linear relationship between global mean surface air temperature and sea ice habitat substantiated the hypothesis that sea-ice thermodynamics can overcome albedo feedbacks proposed to cause sea ice tipping points. Our outcomes indicate that rapid summer ice loss in models and observations... (don’t represent) tipping-point behavior.”
“Instead of tipping-point behavior, recent observations and model outcomes illustrate great natural variability.... Future polar bear population sizes in at least the Convergent Ice Ecoregion and the Archipelago Ecoregion...could be equivalent to or even larger than at present.” With greenhouse gas mitigation combined with the best on-the-ground management practices, extinction was not the most probable outcome in any ecoregion.” Overall, if greenhouse gases “are mitigated polar bear can be maintained at sustainable numbers throughout the century.”
This remodeling and resultant probability will prove interesting if the listing is remanded by the court, as we expect. The listing was in part based upon “doomsday” predictions, which is more alarming for what it reflects about our federal agencies and their agendas than the status of polar bear. Oral arguments in the listing case were in February. A decision is expected at any time.