US Geological Survey, Alaska Science Center, 4210 University Drive, Anchorage, Alaska 99508, USA.
Nature. 2010 Dec 16;468(7326):955-8. doi: 10.1038/nature09653.
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 (Ursus maritimus) could disappear by mid-century if business-as-usual greenhouse gas emissions continue. That projection, however, did not consider the possible benefits of greenhouse gas mitigation. A key question is 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. Such a tipping point would mean future greenhouse gas mitigation would confer no conservation benefits to polar bears. Here we show, using a general circulation model, that substantially more sea-ice habitat would be retained if greenhouse gas rise is mitigated. We also show, with Bayesian network model outcomes, that increased habitat retention under greenhouse gas mitigation means that polar bears could persist throughout the century in greater numbers and more areas than in the business-as-usual case. 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 losses in models and observations represent increased volatility of a thinning sea-ice cover, rather than tipping-point behaviour. Mitigation-driven Bayesian network outcomes show that previously predicted declines in polar bear distribution and numbers are not unavoidable. Because polar bears are sentinels of the Arctic marine ecosystem and trends in their sea-ice habitats foreshadow future global changes, mitigating greenhouse gas emissions to improve polar bear status would have conservation benefits throughout and beyond the Arctic.
基于对其重要海冰栖息地损失的预测，美国地质调查局的一个研究小组在 2007 年得出结论，如果商业性温室气体排放继续下去，到本世纪中叶，世界上三分之二的北极熊（Ursus maritimus）可能会消失。然而，这一预测并没有考虑温室气体减排的可能好处。一个关键问题是，温度升高是否会导致海冰栖息地成比例地减少，或者海冰覆盖是否会在温度达到临界阈值时越过一个临界点而不可逆转地崩溃。这样的临界点将意味着未来的温室气体减排不会给北极熊带来任何保护益处。在这里，我们使用一个通用循环模型表明，如果温室气体排放得到缓解，将会保留更多的海冰栖息地。我们还通过贝叶斯网络模型的结果表明，在温室气体减排下增加栖息地的保留意味着，与商业性排放情景相比，北极熊在本世纪能够在更多的地区和更多的数量中生存下来。我们的通用循环模型结果没有显示出导致冰不可逆损失的阈值；相反，全球平均地表空气温度与海冰栖息地之间的线性关系证实了这样一个假设，即海冰热力学可以克服被认为导致海冰临界点的反照率反馈。我们的结果表明，模型和观测中夏季海冰迅速流失代表了海冰覆盖变薄的波动性增加，而不是临界点行为。减排驱动的贝叶斯网络结果表明，以前预测的北极熊分布和数量的下降并非不可避免。由于北极熊是北极海洋生态系统的哨兵，它们的海冰栖息地的趋势预示着未来的全球变化，因此，减少温室气体排放以改善北极熊的状况将在整个北极地区及其以外地区带来保护益处。
