II.2 Dams and global change
Scoping Paper - Terms of Reference

1.0 BACKGROUND

1.1 Introduction

1. There is growing evidence of global warming that has been linked to greenhouse gas (GHG) emissions into the atmosphere, often from fossil fuels. If nothing is done, the world average temperature is expected to increase from between 1.5 degrees to 4.5 degrees celsius over the next 60 years. This is expected to lead to changes in rainfall patterns, rising sea levels and the increased occurrence of major storm events as the energy flows in the sea and atmosphere increase. Around half the global warming is created by the energy sector and 80 per cent of this stems from the 22 billion tonnes of CO₂ released into the atmosphere each year. Electricity generation from fossil fuels is responsible for 25 per cent of the total global warming effect.

2. Dams relate to global change in two ways. On the one hand, the role of dams in global energy policies in the age of CO₂ reduction programs could be a positive one, as hydropower offsets thermal generation and could therefore potentially help to reduce the GHG emissions of the electricity sector. Dams can also contribute to offset changes in hydrological patterns, be it as storage reservoirs in regions of decreased precipitation or as flood control devices in regions of increased precipitation.

3. On the other hand, although hydro is generally seen as a power generation technology with a very low net GHG emission level, recent studies claim that hydropower reservoirs may in fact emit substantial amounts of methane into the air. In terms of global warming, methane is 22-24 times as potent as carbon dioxide. With the imminent prospect of trading of emission rights it is essential to establish the baselines from which power generation options can be assessed and their consequences for global change evaluated.

1.2 The main issues

4. Methane is emitted from reservoirs that are stratified and where the bottom layers are anoxic, leading to degradation of biomass through anaerobic processes. Where the water is well oxygenated, degradation of biomass generates CO₂ not methane. Reservoirs that risk being potent emitters of CO₂ per KWh generated are those lying in warm latitudes, where vegetation was not cleared before flooding, and where the reservoirs are extensive and stratified with anoxic layers. Methane or CO2 emissions tend to decline during the lifetime of a dam, so total emissions per KWh generated are sensitive to the duration of the dam lifetime. Published research indicates that even including methane emissions, total GHG per KWh generated from hydropower is still at least half that from the least polluting thermal alternatives, although there is not always consensus on the economic valuation of these emissions. One purpose of this review is to clarify the values that are best used for such assessments.

5. Dam planning and operation might also be affected by the uncertainties linked to a changing climate. For example, in areas of climate-induced deforestation there may be increased soil erosion and sedimentation in reservoirs. Increased temperatures will increase evaporation from reservoirs, which could result in reduced water and energy benefits. In some regions increased precipitation infers increased energy benefits if the storage capacity exists and if annual distribution of rainfall is favourable. Otherwise increased storm events may mean higher flood flows affecting downstream populations and potential consequences for dam safety.. In some areas, decreased precipitation may reduce energy benefits, or provide less water for irrigation and other uses, increasing competition for a dwindling resource. Under any of these scenarios it seems evident that the past analysis of hydrological flow patterns is not sufficient to guarantee predicted benefits over the long lifespan of some dams in the absence of a risk assessment of how climate change might affect run-off in future. It is possible that some projects may not generate their predicted benefits as a consequence of rainfall variation in future, or that operating rules could be revised if flows or seasonal variation increases.

2.0 SCOPE OF WORK

6. This review will seek expert advice on the scientific evidence available to clarify these concerns, and provide guidance for future dam-planning and management work. The following will be addressed:

• When methane emissions are taken into account what are the relative contributions of different energy options to global warming?
• To what extent, should greenhouse gases emissions be factored into energy planning, through which methods/procedures, and using which historic and present values ?
• How will the effects of global warming on the hydrological cycle through increased frequency of extreme storm events or modified seasonal rainfall patterns affect dams in different regions of the world?
• How should this be considered in the planning, design and operation of present and future dams? How can the improved long-term weather forecasts, which are partly the result of research work in global warming, be used to operate existing and future dam projects more efficiently?

3.0 LINKAGE

3.1 Case studies

8. This thematic review will define the values (or range of values if there is dispute) to be used in each of the case studies to calculate the value of the thermal emissions avoided through adoption of a hydropower option.

3.2 Linkage to other thematic reviews

9. The changing resource base due to climate change clearly has an impact on options assessment and on the feasibility of new projects and there are linkages here toV.1 the planning process and to options assessments. There may also be linkage with IV.5 Operations, if dams have to modify their operating rules as a result of the changing resource base.

3.3 Linkage to output

10. The product of this review will be linked to two WCD outputs, namely (i) under Criteria and Guidelines, identify the relevant emerging issues that should be incorporated in the dams project cycle; and (ii) identify methods and procedures for assessment of global impacts (e.g. to calculate GHG emissions of dams) for the Framework of options assessment and decision making.

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