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作者 Fred Pearce
許多水力發電計劃距離「綠色」還遠得很，比大型火力發電廠把更多的溫室效應氣體釋放到大氣中，因為，這些水力發電設備中含有許多腐敗的植物。全世界最大的大型水壩投資者，也是世界銀行支持，由科學家、工程師與環境學家組成的團體－世界水壩委員會如是說。
就在工程師們準備基於1997年所議定的京都議定書，主張應該授予水壩「清潔科技」的資格的同時，這份報告正給他們澆了一盤冷水。在本月底，委員會將在德國波昂召開會議，討論「清潔發展機制」。這機制是京都議定書中，降低全球炭排放量的目標的重要部分。他們將在會上發表他們的發現。
令人驚訝的發現是，上游的有機物質被沖刷進入水庫之中，產生了許多溫室效應氣體。當水庫蓄水時，森林的腐敗物被淹沒，只產生這些氣體的「一小部份」。即是說，當被淹沒的森林脫落敗壞時，這些氣體並沒有消失，反而是繼續進入水庫裡。
全世界水力發電用的水庫所佔面積和法國一樣大。它們釋放出二氧化碳和甲烷。停滯的水產生最糟的排放物，是由於腐化中的植物產生甲烷。這種氣體在產生溫室效應的能力上，比二氧化碳（當水中有氧時所產生的氣體）高出二十倍。所以，河流在水壩興建後，會產生更多的甲烷。
關於水庫排放出氣體的警告，在1990年代中期才浮現(新科學家，1996年五月4日，第29頁)。但最初問題只存在於少數水庫，現在卻變得更為普遍。
委員會認為「水淺、生物質量（淹沒之前）也不易被了解的熱帶水庫最危險。」。委員會指出兩個雨林中的水庫，是主要的「地球暖化器」。一個是巴西的巴比那(Balbina)，它的長度一邊只有四公尺。它的發電力是一億一千二百萬瓦特。據估計，它將在運轉的首二十年，每年產生三百萬公噸的碳。一個具有相同發電量的火力發電廠，一年產生的碳是五十萬公噸。而法屬圭亞那的「小梭（Petit-Saut）」，具有與歐洲埃里安火箭發射地相似的容積與動力，在它運作的首二十年，每年將產生十萬公噸的溫室效應氣體。
這份報告的作者，到目前為止只在四個國家裡研究了少數的水庫，所以他們相信，可能還會有更多令人不愉快的事實出現。然而，來自於水庫的排放氣體似乎是不定且無法預料的：一份研究巴西九個水庫的報告發現，它們每電力單位所排放的氣體，以五百的倍數在增長。
委員會說，「所有位於北半球與熱帶地區，已被測量的水壩水庫，都排放了數十年的溫室效應氣體。這和普遍的假設－這些氣體總值是零，形成對比。」「沒有足夠理由否定水力發電不是全球暖化的重要因素。」
雅梅•史肯那（Jamie Skinner），這位委員會在南非開普敦的環境顧問，說這份報告相當重要，因為水壩工程師與環境學家皆同意這個結論。
原文詳見:《新科學家》雜誌, 03 June 2000.)http://www.newscientist.com
環境信託基金會 (謝洵怡 譯，李育成審校)
中英對照原文詳見：http://news.ngo.org.tw/issue/water/issue-water000619.htm |
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FAR from being "green", many hydroelectric power schemes release more greenhouse gases into the atmosphere than large coal-fired power stations, because of the rotting vegetation they contain. So says the World Commission on Dams, a group of scientists, engineers
and environmentalists supported by the World Bank, the world's biggest funder of large dams.
The report comes just as engineers are arguing that dams should qualify for support as a "clean" technology under the Kyoto Protocol agreed in 1997. The commission will report its findings later this month at a meeting in Bonn to discuss the Clean Development Mechanism, a key part of the Protocol aimed at reducing carbon emissions worldwide.
One surprise finding is that organic matter washed into a reservoir from upstream generates much of the greenhouse gas. The decay of forests submerged when the reservoirs fill up creates "only a fraction" of the gas. This means that the emissions don't disappear when the flooded forest has rotted away, but may continue for the lifetime of the reservoir.
Hydroelectric reservoirs cover an area of the world the size of France. They release carbon dioxide and methane. Stagnant water produces the worst emissions because the decaying vegetation generates methane. This is 20 times as potent a greenhouse gas as CO2, which is produced when there is oxygen in the water. So a reservoir will produce more methane than the river did before the dam was built.
Warnings about the gas emissions from reservoirs surfaced in the mid-1990s (New Scientist, 4 May 1996, p 29). But what appeared at first to be a problem for a handful of reservoirs now looks much more general.
"Tropical reservoirs that are shallow and uncleared of biomass [before flooding] appear most at risk," says the commission. It names two rainforest reservoirs as major planet-warmers. One is Balbina in Brazil, which is just four metres deep in parts. Its generating capacity is 112 megawatts and it is estimated that it will produce 3 million tonnes of carbon per year over its first 20 years. A coal-fired power station of the same capacity would produce 0.5 million tonnes per year. Petit-Saut in French Guyana, which has a similar capacity and powers the launch site for Europe's Ariane rocket, will produce 0.1 million tonnes per year in its first 20 years.
The report's authors have only studied a handful of reservoirs so far, in just four countries, so they believe there may be many more offenders. They warn, however, that emissions from reservoirs seem erratic and unpredictable: one study of nine reservoirs in Brazil found that their emissions per unit of electricity vary by a factor of 500.
"Greenhouse gases are emitted for decades from all dam reservoirs in the boreal and tropical regions for which measurements have been made. This is in contrast to the widespread assumption that such emissions are zero," says the commission. "There is no justification for claiming that hydroelectricity does not contribute significantly to global warming."
Jamie Skinner, environmental adviser to the commission, which is based in Cape Town, South Africa, says the report is significant because both dam engineers and environmentalists agree on its conclusions. Fred Pearce
(From New Scientist magazine 《新科學家》雜誌, 03 June 2000.)http://www.newscientist.com/
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