Water

In its global review of the development effectiveness of dam projects, the World Commission on Dams reported that:

'The emission of greenhouse gases (GHG) due to rotting vegetation and carbon inflows from the catchment is a recently identified ecosystem impact' and 'challenges the conventional wisdom that hydropower produces only positive atmospheric effects, such as a reduction in emissions of carbon dioxide, nitrous oxides, sulphuric oxides and particulates when compared with power generation sources that burn fossil fuels'. (page 76)

WCD recommends that GHG emissions from reservoirs need to be assessed and compared with emissions from other generations sources. The effect is highly location specific and may change significantly over time.

Although the report states that all reservoirs emit some greenhouse gases, it goes on to say 'Some values for gross GHGs are extremely low and may be ten times less than the thermal option'. Yet in some cases, particularly very shallow reservoirs in tropical regions' 'the gross emissions can be considerable'.

In its Guideline 8, WCD recommended that field studies into GHG emissions should be part of project feasibility and take into account the pre-project and with-project conditions.

Background material prepared as input to the WCD report can be found in thematic review II.2: Dams and Global Change including proceedings of a workshop in Montreal in Part III Workshop on Dam Reservoirs and Greenhouse Gases

Considerable debate on GHG emissions from reservoirs followed publication of the WCD report:

The NGO, International Rivers Network (IRN), published a paper Flooding the Land, Warming the Earth; Greenhouse Gas Emissions from dams that documents the growing evidence for the global significance of greenhouse gas emissions from dams and reservoirs.

In response, the industry coordinated by the International Hydropower Association (IHA) prepared a fact sheet Greenhouse Gas Emissions from Reservoirs that countered the claims of hydropower opponents, concluding that 'relative to typical values for hydro, coal fired generation emits about 100 times more GHG and natural gas combined cycle turbines about 40 times more'.

For a perspective on the broader issues of climate change and water resources management see IUCN publication 'Change'.

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