極地海冰消失　北極熊喪棲地將成瀕絕動物

一組美加研究團隊昨天提出研究報告指出，全球暖化已對北極熊產生顯著且負面的效應。北極熊在極地海冰的原始棲地，稍早因氣候暖化而破裂分離，這對於北極熊的生存與繁衍造成了最直接的衝擊。

北極熊靠海冰作為狩獵平台，捕捉海豹和其他海洋哺乳動物為食；當海面浮冰日益減少，北極熊禁食時間便愈來愈長。禁食影響了牠們生育能力，造成北極熊數目減少。

先前曾有遊客表示，他們看到北極熊出沒次數比以往多，這應該表示北極熊數量在增加。但研究人員駁斥了這個說法。

溫室氣體導致全球暖化，這項人為破壞氣候的後果之一，就是北極海冰每年提早開始溶化，縮短母北極熊春季狩獵時間，造成哈德遜海灣西部的母熊族群平均體重持續減輕。母熊體重減輕不但影響自身生育能力，育幼力亦受波及，造成幼熊生存率減低。

這份研究報告作者之一、同時也是加拿大環境部野生動物保護局資深科學家史德林（Ian Stirling）表示：「1980年，哈德遜海灣西部的成熟母北極熊平均體重是650磅，然而2004年平均體重降為507磅，整整減輕143磅！」

1992年發表於加拿大動物學期刊的一篇論文早已指出，母熊體重一旦低於416磅，次年春天便不會生育；再者，北極熊體重愈輕，愈可能為覓食而出沒於人類居住區，這便說明了旅客比較容易看到北極熊的原因。 

「我們擔心的是，倘若海冰時期持續縮短，北極熊不得不縮短狩獵季節；而從夏季到早秋的融冰時期，北極熊就被迫待在陸地地區更長的時間了。」美國太空總署高達德飛行中心科學家，也是此份報告作者之一帕金森（Claire Parkinson）指出：「北極熊囤積的體脂肪無法供應長期陸地生活所需能量，這明顯會威脅到北極熊的健康，長遠來看，甚至影響物種存續。」

根據史德林和帕金森利用衛星持續觀察發現，北極海冰覆蓋面積至少從1978年便開始縮小。美國太空總署另外兩份報告同步顯示，北極海冰目前融化速率之快，超越過去30年任何時期。 

Diminishing Arctic Sea Ice Endangering Polar Bears

WASHINGTON, DC, September 14, 2006 (ENS)

Global warming is having a significant and negative effect on polar bears, U.S. and Canadian researchers said Wednesday. A new study finds that Arctic sea ice in prime polar bear habitat is breaking up earlier, with direct impacts on the species' ability to survive and reproduce.

Sea ice provides polar bears with a vital platform from which to hunt seals and other marine mammals for food. The reduction in sea ice is forcing Arctic polar bears to fast for longer and longer periods, with adverse consequences for the species' survival.

The researchers say the findings reject the claim that increased polar bear sightings by Arctic tourists indicate the species' population is increasing.

The findings suggests that progressively earlier breakup of the Arctic sea ice - stimulated by climate warming due largely to increased human emissions of greenhouse gases - shortens the spring hunting season for female polar bears in western Hudson Bay and is likely responsible for the continuing fall in the average weight of these bears. As females become lighter, their ability to reproduce and the survival of their young decline.

"In 1980 the average weight of adult females in western Hudson Bay was 650 pounds," said study coauthor Ian Stirling, a senior scientist with the Canadian Wildlife Service. "Their average weight in 2004 was just 507 pounds - a 143-pound reduction."

A 1992 study in the Canadian Journal of Zoology indicated that no females weighing less than 416 pounds gave birth the following spring. Furthermore, as the bears become thinner, they are more likely to push into human settlements for food, giving the impression that the population is increasing.

"Our concern is that if the length of the sea ice season continues to decrease, then the polar bears will have shorter periods on the ice, when they can feed, and longer periods on the land, during the open-water season in summer and early fall," said study coauthor Claire Parkinson, a scientist at NASA's Goddard Space Flight Center. "Their stored fat from life on the ice will likely not provide enough nourishment for the fasting period on land, posing a clear danger to their health and, in the long term, possibly to their species."

Sea-ice cover in the regions studied by Stirling and Parkinson has decreased since at least 1978, the beginning of consistent satellite monitoring. Two other studies released Wednesday by NASA scientists confirm that Arctic sea ice is melting at a dramatic rate, faster than at any time in the last three decades.

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