北极熊只生活在北半球的海洋地区，这些地区被海冰覆盖的时间足够长、使它们有充足的机会去捕捉其海洋哺乳动物猎物。夏季北极海冰最近的减少与北极熊种群数量的下降巧合，而美国地质调查局2007年的一份报告预测，在正常情况下，北极熊本世纪末就可能从它们的所有生活范围内灭绝。一些观察家甚至提出，北极海冰也许已经越过了一个临界点，过了这个点之后，生境也许将无法恢复。但是现在，一项新的分析表明，拯救北极熊还为时不晚。最近所观察到的夏季海冰的快速消失也许反映了一个变薄的海冰覆盖层挥发性的增加，而不是代表着一个临界点。温室气体减排有可能使海冰的消失停止并保护北极生态系统。本期封面所示为2009年夏天一只北极熊正从斯瓦尔巴群岛附近一处海冰所剩无几的地方走过。（生物谷Bioon.com）
生物谷推荐原文出处：
Nature doi:10.1038/nature09653
Greenhouse gas mitigation can reduce sea-ice loss and increase polar bear persistence
Steven C. Amstrup,Eric T. DeWeaver,David C. Douglas,Bruce G. Marcot,George M. Durner,Cecilia M. Bitz& David A. Bailey
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 continue1, 2, 3. 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 threshold4, 5, 6. 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 model7, 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 case3. Our general circulation model outcomes did not reveal thresholds leading to irreversible loss of ice6; 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 points5, 6, 8. Our outcomes indicate that rapid summer ice losses in models9 and observations6, 10 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 numbers3 are not unavoidable. Because polar bears are sentinels of the Arctic marine ecosystem11 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 Arctic12.
