北冰洋冰層大面積融化 70%是人爲造成大綱

Study finds only 30% of radical loss of summer sea ice is due to natural variability in Atlantic – and it will probably get worse.

The radical decline in sea ice around the Arctic is at least 70% due to human-induced climate change, according to a new study, and may even be up to 95% down to humans – rather higher than scientists had previously thought.

The loss of ice around the Arctic has adverse effects on wildlife and also opens up new northern sea routes and opportunities to drill for oil and gas under the newly accessible sea bed.

The reduction has been accelerating since the 1990s and many scientists believe the Arctic may become ice-free in the summers later this century, possibly as early as the late 2020s.

"Since the 1970s, there's been a 40% decrease in the summer sea ice extent," said Jonny Day, a climate scientist at the National Centre for Atmospheric Science at the University of Reading, who led the latest study.

"We were trying to determine how much of this was due to natural variability and therefore imply what aspect is due to man-made climate change as well."

To test the ideas, Day carried out several computer-based simulations of how the climate around the Arctic might have fluctuated since 1979 without the input of greenhouse gases from human activity.

He found that a climate system called the Atlantic multi-decadal oscillation (AMO) was a dominant source of variability in ice extent. The AMO is a cycle of warming and cooling in the North Atlantic that repeats every 65 to 80 years – it has been in a warming phase since the mid-1970s.

Comparing the models with actual observations, Day was able to work out what contribution the natural systems had made to what researchers have observed from satellite data.

"We could only attribute as much as 30% [of the Arctic ice loss] to the AMO," he said. "Which implies that the rest is due to something else, and this is most likely going to be man-made global change."

Previous studies had indicated that around half of the loss was due to man-made climate change and that the other half was due to natural variability.

The research is published online in the journal Environmental Research Letters.研究發現，北極海冰夏季大面積融化可能會進一步加劇，而這其中只有30%由大西洋的自然環境變化所造成。

根據一項最新研究，北冰洋海冰的大面積融化至少有70%都是由人類活動引起的氣候變化造成，而且這一比例可能還會升至95%——比科學家之前預測的還要更高。

北極周圍冰蓋的融化對野生動物的影響十分不利，此外海冰的融化還將使北部海域得以開闢新的航線，有機會開採新近得以勘探的海底油氣。

自上世紀90年代起北極冰蓋便已開始加速融化，很多科學家都認爲本世紀後期北極夏季將沒有冰蓋，也許這一情形在本世紀20年代末便會發生。

英國瑞丁大學國家大氣科學中心氣象學家約翰尼•戴是這項新研究的帶頭人，他說：“20世紀70年代以來，北極夏季海冰已經減少了40%。”

“我們正試着確定這種情況多大程度上是由自然環境的變化造成的，從而得出哪些方面是由人爲氣候變化造成。”

爲了驗證這一想法，戴利用計算機技術，模擬了在沒有人類排放溫室氣體的情況下，北極地區的氣候自1979年以來是如何發生變化的。

他發現一個名叫大西洋多年代際震盪的系統是追溯冰層變化的一個主要來源。該系統是北大西洋以65到80年爲週期的冷暖週期——根據該系統數據顯示，自上世紀70年代中期以來，該地區便處於暖週期。

將該模型與實際觀察對比後，戴找出了該自然系統對研究者觀測衛星數據所獲得結果的影響。

“我們最多隻能把北極冰蓋融化的30%歸咎於AMO系統。”他說：“這就意味着剩下的70%都是其它原因造成的，而這些原因最有可能就是人類活動引起的全球氣候變化。”

前人研究已經表明，北極冰層融化的50%是人爲氣候變化造成的，另一半則是由於自然環境的變化。

這一研究結果發佈在《環境研究快報》雜誌的網站上。