北極熊只生活在北半球的海洋地區(qū),，這些地區(qū)被海冰覆蓋的時間足夠長、使它們有充足的機會去捕捉其海洋哺乳動物獵物,。夏季北極海冰最近的減少與北極熊種群數量的下降巧合,，而美國地質調查局2007年的一份報告預測，在正常情況下,，北極熊本世紀末就可能從它們的所有生活范圍內滅絕,。一些觀察家甚至提出,，北極海冰也許已經越過了一個臨界點，過了這個點之后,，生境也許將無法恢復,。但是現(xiàn)在，一項新的分析表明,，拯救北極熊還為時不晚,。最近所觀察到的夏季海冰的快速消失也許反映了一個變薄的海冰覆蓋層揮發(fā)性的增加，而不是代表著一個臨界點,。溫室氣體減排有可能使海冰的消失停止并保護北極生態(tài)系統(tǒng),。本期封面所示為2009年夏天一只北極熊正從斯瓦爾巴群島附近一處海冰所剩無幾的地方走過。（生物谷Bioon.com）
生物谷推薦原文出處：
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.
