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環境特派記者 艾力克斯•柯比
水力發電向來被認為是最環保的發電方法之一，現在卻可能比燃煤發電更具破壞力。
原因是水庫會留住腐敗的植物，這些腐敗的植物釋放出大量的溫室氣體。
但要針對一座水庫提出確切的環境污染評估，十分困難，因為水庫之間的差異很大。
這項造成氣候變化的聲明將在下一波反對會議召開之前公布。
來自新科學家雜誌的一篇由世界水壩委員會﹝WCD﹞所提出的報告發現，水力發電有導致溫室效應的高潛在性。
環保訴求
該委員會係由一群科學家，工程師，及環保學家組成，接受世界保育聯盟(World Conservation Union)以及世界銀行(World Bank)的贊助。世界銀行是大型水壩的最大贊助者。
贊成建築水壩的人士強調他們符合贊助的條件，根據1997年聯合國氣候變遷網要公約之京都議定書，水壩是屬於環保科技。
許多科學家相信已有充分的證據顯示地球的溫度正逐漸升高，其主要肇因正是人類的活動，尤其是燃燒石化燃料產生廢氣所致。
世界水壩委員會將在六月份於德國波昂舉辦的一項會議中提出報告，並針對京都協定中的關鍵議題─環保﹝清潔﹞發展機制進行討論。
據該委員會所說，水庫注水後遭淹沒的森林所釋放出的兩種溫室氣體(二氧化碳和甲烷)的量，只佔一小部分。
從水庫上游沖刷下來的有機物質製造的溫室氣體佔絕大部分，而這過程將隨著水庫的存在而繼續下去。
停滯死水會產生甲烷，其溫室效應又比二氧化碳強二十倍，至於流動的水含有大量氧氣所以只產生二氧化碳。
延伸的問題
所以, 水壩興建後會比興建前的河流製造出更多的甲烷。
世界水壩委員會認為問題比他們最初想的更為廣泛，他們說在熱帶地區的一些水位較淺尤其在放水之前未清除範圍內生物相的水庫最具風險。
該委員會並指出一個最值得關注的例子就是巴西的巴比那水庫，水庫內部份地方的水位只有四公尺深。
該水庫的發電量是112兆瓦特，預計在最初的二十年間，每年將製造出三百萬公噸的二氧化碳。
而一座相同發電量的燃煤發電廠，一年僅會製造出35萬公噸的二氧化碳。
位在法屬圭亞那的小澤（音譯佩提邵特）水庫，擁有類似的發電量，供應歐洲阿里安火箭發射區的電力，估計其最初的二十年間，每年製造的二氧化碳約為90公噸。
天南地北的差異
世界水壩委員會只探察了四個國家中的幾座水壩，以為能歸納出許多相似點。
然而在針對巴西境內九座水壩進行的研究報告顯示，各水壩每單位發電量的溫室氣體排洩量，卻有截然不同的差異。
『無人能辯稱水力發電不是造成全球暖化的重要因素』該委員會如是說。
原文詳見: http://news.bbc.co.uk/hi/english/
sci/tech/newsid_771000/771465.stm
版權歸屬 International Rivers Network，環境信託基金會 (張正慈 譯，楊同正審校)
中英對照全文詳見：http://news.ngo.org.tw/issue/water/
issue-water000612.htm |
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By environment correspondent Alex Kirby
Hydro-electric power, often proclaimed as one of the greenest ways ofgenerating energy, can be more polluting than coal.
This is because the reservoirs that power the dams can trap rotting vegetation, which emits large amounts of greenhouse gases.
But establishing just how much pollution a particular reservoir will cause is very difficult, because they vary widely.
The announcement of this cause of climate change comes shortly before the next round of negotiations on how to tackle it.
The discovery of the full greenhouse potential of hydro-electric schemes, published in New Scientist magazine, was made by the World Commission on Dams
(WCD).
Clean claim
The commission, a group of scientists, engineers and environmentalists, is supported by the World Conservation Union and the World Bank, which is the single biggest funder of large dams.
Supporters of dams argue that they should qualify for support as a clean technology under the 1997 Kyoto Protocol on tackling climate change.
Many scientists believe there is strong evidence to suggest that the earth's atmosphere is warming, and that human activities - chiefly the burning of fossil fuels - are a significant cause.
The WCD will report its findings at a meeting in the German city of Bonn in June to discuss the Clean Development Mechanism, a key part of the Kyoto Protocol.
It says the decay of forests submerged when reservoirs are flooded produces only a fraction of the two gases involved, carbon dioxide (CO2) and methane.
Much more is produced by organic matter washed into the reservoir from further upstream, a process which may continue for the lifetime of the reservoir.
Methane is 20 times more potent as a greenhouse gas than CO2. It is given off by stagnant water, while running water, which contains oxygen, produces CO2.
Extensive problem
So a reservoir will produce more methane than the river which ran before the dam was built.
The WCD, which believes the problem is far more widespread than first thought, says the reservoirs that appear most at risk are shallow ones in the tropics which have not been cleared of biomass before they were flooded.
It says one particular cause for concern is the Balbina reservoir in Brazil, which in parts is only four metres deep.
Its generating capacity is 112 megawatts, and it is estimated to produce three million tonnes of CO2 annually over its first 20 years.
A coal-fired power station of the same capacity would produce 0.35m tonnes of CO2 a year.
In French Guyana the Petit-Saut reservoir, which has a similar capacity and powers the launch site for Europe's Ariane rocket, is expected to produce 0.9m tonnes of CO2 annually in its first 20 years.
Huge variation
The WCD has looked at only a few dams in four countries, and thinks there will be many similar examples.
But it says one study of nine Brazilian reservoirs found their greenhouse emissions varied per unit of electricity by a factor of 500.
It says: "There is no justification for claiming that hydro-electricity does not contribute significantly to global warming."
http://news.bbc.co.uk/hi/english/
sci/tech/newsid_771000/771465.stm
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