THE HAGUE, NOVEMBER 27 - The World Commission on Dams (WCD), recognised for independent, comprehensive and peer-reviewed work on the impacts and performance of large dams, presented its Final Report Dams and Development: A New Framework for Decision-making to the Framework Convention on Climate Change, showing how emissions of greenhouse gases from dam reservoirs compare with emissions from other energy sources, and the implications this has for assessment methods for future energy options.

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 Commission's findings were informed by the most recent research on this issue in Canada, Brazil, French Guyana and Finland.

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.

Under the Kyoto Protocol of the U.N. Convention on Climate Change, Annex I, industrialised countries have committed to reducing their emissions of greenhouse gases, including carbon dioxide and methane. One mechanism for achieving reductions is the Clean Development Mechanism (CDM), where countries can reduce emissions by purchasing emission credits from other countries that invest in projects and programs that avoid GHG emissions and produce a net global reduction in emissions. The mechanism depends, therefore, on the capacity to accurately assess the emissions avoided such that a net reduction can be verified and valued. Governments are now meeting in The Hague. In light of the Kyoto protocol, and the urgency of both stabilising the earth's climate and finding means of providing electricity, food, and water to billions of people, this evidence is important for helping countries evaluate their options for meeting development needs and reducing global warming.

Critics claim that since dam reservoirs emit GHGs, that the science is uncertain, and that dams have other significant social and environmental impacts, they should be excluded from mechanisms, such as the CDM, that would give them credit. Hydropower developers claim that dams are not only a renewable resource but also, on balance, cleaner than other options, and that therefore the credits they could generate (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. The WCD 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, and lays out a range of additional criteria and guidelines within a new framework for decision-making.

Yet the following evidence on this issue, gathered by WCD during a specially convened scientific workshop in Montreal, can help countries more accurately assess their greenhouse gas emissions reductions strategies and the global warming impacts of its development choices, including dams and reservoirs:

• 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.
  The evaluation of net greenhouse gas emissions from reservoirs following flooding requires comparison to the emissions that existed prior to flooding (i.e. the difference between the pre dam emissions from the undammed catchment and the post dam emissions). Quantification of these changes in greenhouse gas emissions, due to flooding, is complex because it requires understanding of the carbon cycle at the level of the whole watershed.
  However, the rates of GHG emission measured so far justify consideration of these emissions in evaluating individual future reservoir sites, such as hydroelectric dams (particularly in tropical regions); and in global inventories of anthropogenic changes in the sources and sinks for CO2 (carbon dioxide) and CH4 (methane).
• 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.
• 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.
• Hydropower cannot, a priori, be automatically assumed to emit less greenhouse gas than the thermal alternatives. Net emissions should be established on a case by case basis.
• As natural habitats also emit greenhouse gases it is the net change due to impoundment that should be used for assessment, and not the gross emissions from the reservoir.
• Net changes in emissions from alternative sources of energy (including thermal, wind, solar, biomass etc.) should be examined using a Life-Cycle Analysis, that includes cradle to grave assessments.
• Non-hydropower reservoirs (irrigation, storage, flood control) in the humid tropics need to be studied to examine the potential impact of net changes in greenhouse gas emissions, and more research done in temperate countries to improve understanding of reservoir emissions in this region.
• For purposes of converting a reservoir's carbon and methane emissions into a total "equivalent CO2" to compare with total CO2 emissions from thermal power plants, the currently widespread use of the 100-year Global Warming Potential (GWP) for methane can significantly under-estimate the "equivalent CO2" over the first several decades of the reservoir because methane's GWP declines significantly with a longer time horizon. In addition, significant resident time-scale uncertainties for methane and CO2 will also affect the "equivalent CO2". Other time-dependent conversion methods should be considered to improve the accuracy of comparisons.

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."

So What Does This Mean?

"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."

But even that unlocks only part of the global GHG emissions puzzle. Each reservoir's emissions will vary widely by geography, altitude, latitude, temperature, size, depth, depth of turbine intakes, dam operations, and construction procedures. All these ingredients play a role in the complex biochemical recipe that determines how much greenhouse gas emissions a specific reservoir produces over time.

Prof Asmal said "The Commission 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 in terms of GHG emission." In addition, governments will ultimately need to weigh climate impacts against other social, environmental, and economic priorities that are also addressed in the Commission's report.

Governments: Where Do We Go From Here?

In other words, 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. While complex, failure to do just that can have even more sudden, dramatic and undesirable outcomes.

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. If industrialised countries are to promote carbon credits for tropical hydropower projects, it is essential that assessing potential greenhouse gas emissions from the new reservoir be a factor in that decision, otherwise the measures may have no net beneficial effect.

The goal is to predict accurately GHG emissions from reservoirs, and compare these with Life Cycle Analysis of other options, so as to make the best possible decisions about dams among other development choices. Net emissions from reservoirs above natural background emissions would be the necessary and appropriate estimates to use in making more accurate projections of future emissions along with an understanding of carbon flows within the river basin.

Further research is required to improve current understanding of how GHGs cycle within existing basin and reservoirs and to improve capacity to predict future net emissions of new reservoir projects. Also, gross emissions of greenhouse gases are not restricted to hydropower. The effect is specific to the reservoir, not the purpose of the dam. Whether reservoirs are found to be a sufficiently important anthropogenic GHG source to merit inventory in country baselines is a matter for the Convention.