Renew On Line (UK) 32
Extracts from the July-August
2001 edition of Renew
|Welcome Archives Bulletin|
Hydropower and Greenhouse Gasses
As we noted in Renew 129, the World Commission on Dams has published a major report on the impacts and performance of large dams, Dams and Development: A New Framework for Decision-making. It looks at the emissions of greenhouse gases (GHSs) from dam reservoirs, due to the rotting of biomass trapped when the reservoir is filled and then brought down stream subsequently, and compares these emissions with those from other energy sources.
After two years work, the WCD concluded on the greenhouse gas issue that "All large dams and natural lakes in the boreal and tropical regions that have been measured emit greenhouse gases some values for gross emissions are extremely low, and may be ten times less than the thermal option. Yet in some cases the gross emissions can be considerable, and possibly greater than the thermal alternatives".
The WDC note that the flooded biomass alone does not explain the observed gas emissions. Carbon is flowing into the reservoir from the entire basin upstream, and other development and resource management activities in the basin can increase or decrease future carbon inputs to the reservoir.
The Commissions findings were informed by the most recent research on this issue in Canada, Brazil, French Guyana and Finland. It notes that Greenhouse gases (GHGs, methane and/or carbon dioxide) are emitted for decades from all 30 dam reservoirs in the boreal and tropic regions for which measurements have been made. This is in contrast to the widespread assumption (e.g. IPCC scenarios, or International Energy Agency) that such emissions are zero or negligible.
Dams and Development calls for "explicit assessment of future net greenhouse gas emissions of a project" through full Life Cycle Assessments to compare available options. For the hydropower option, the WCD proposes guidelines to assist in these assessments. These include measuring carbon flows in the natural pre-impoundment watershed and assessing how these will change following dam construction. Critics claim that since dam reservoirs emit GHGs and that dams have other significant social and environmental impacts, they should be excluded from support mechanisms designed to reduce emissions, such as the Clean Development Mechanism (CDM) proposed at Kyoto.
By contrast, Hydropower developers claim that dams are not only a renewable resource but also, on balance, cleaner than other options, and that therefore the emission credits they could generate under schemes like the CDM (currently estimated at $10 per tonne or more) could make investments in dams more attractive by potentially generating millions of dollars in carbon credits per hydroelectric dam, per year.
For it part, the WCD seems to be sitting on the fence to some extent. It notes that the level of GHG emissions is merely one criterion in determining the overall economic, social, and environmental sustainability of energy and development options.
The WDC adds, a simplistic, universally applicable "good/bad, clean/dirty" verdict on the impact of dams on climate change will remain out of reach until countries devote time, resources, and scientific attention to these variables.
Summarising their position, Jamie Skinner, Senior Advisor in the WCD secretariat said, "In short, the World Commission on Dams found that while dams may produce net greenhouse gas emissions, that fact alone cannot be removed from the context of place, scale, time or how the dam compares to the other options available in any given country. Further, the science on this issue is still young, and additional research is needed before any proposed new hydro projects could be definitively classed as "cleaner" in terms of GHGs than their thermal equivalents and therefore eligible for carbon credits".
Reasonably enough, the WDC point out that it is a site specific issue, requiring site by site assessment. It notes that in boreal climates (like Canada and Scandinavia) available studies so far suggest that emissions from hydropower reservoirs are low. For Brazil, of ten dams studied, emissions vary from dam to dam with a 500-fold difference between lowest and highest. The lowest emissions are on similar levels to Canadian lakes and reservoirs, the highest annual gross emissions reach the ranges of thermal energy plants, although life cycle assessment, and determination of net emissions, is needed before definitive comparisons can be made.
That seems fair enough- and there may also be some possible way of ameliorating the problem, as we note in our Technology Section. Even so, big dams in the tropics are likely to be seen as increasingly problematic.
"The discovery that a dam reservoir emits greenhouse gas emissions is, by itself, insufficient to determine its ultimate contribution to global warming", said Commission Chair, Prof Kader Asmal. "What should matter to governments and the international community concerned about climate change and GHGs is the critical net change that their decisions will bring, and whether the project selected to meet energy needs will in fact emit less than other alternatives considered".
The Commission, he said, recommends that governments, energy developers, and Climate Convention authorities will need to establish the baseline level of emissions produced by a given river basin before a dam is introduced, and then compare the net change with emissions and warming impacts of other irrigation, energy and water supply options. The final choice may be not so clean, cleaner, or cleanest in terms of GHG emissions". In addition, the WDC says that governments will ultimately need to weigh climate impacts against other social, environmental, and economic priorities that are also addressed in the Commission's report.
The WCD see the problem as both complex scientifically, and tragic in strategic terms - for the cause of limiting Climate Change. Preliminary indications are that GHG emissions from reservoirs are likely to be of more concern in tropical countries. Yet these are the very same countries targeted to reduce the global warming impact of economic development in the coming decades, and where most dams are likely to be built to meet emerging water and energy needs.
For their part, anti-dam campaigners will however be delighted by the WCD report- they see dams as expensive and dangerous irrelevancies. That view will also be strengthen by the recent revelation (noted in New Scientist - 18 Nov 2000) by managers of the $4 bn Xiaolangdi dam on China's Yellow River, part-funded by the World Bank, that they could not find buyers for the electricity in China's newly liberalised power market.
Material abstracted from the WCD web site www.dams.org