A new report from the International Rivers Network (IRN) "Fizzy Science: Loosening the Hydro Industry's grip on Greenhouse Gas Emissions Research" provides a compelling argument for the international Inter-Governmental Panel on Climate Change of the UN (IPCC) to undertake further Independent and careful studies for a Special Report that will inform the international community on reservoir emissions of Greenhouse Gases.

The Special Report would update and strengthen the existing 2005 guidelines on the methodologies to be followed to measure reservoir emissions and submit reports to the UN under the United Nations Framework Convention on Climate Change (UNFCCC).

Are dams and the large reservoirs they create conducive to climate change mitigation? Are they really providers of clean or at least cleaner, energy at a low environmental price? Are they the clean and green answer to the desperate race to "developed" status prosperity and wealth that they are promoted as being? The report from IRN suggests otherwise.

The IRN report includes the proposed Tipaimukh Dam, along with Tehri, Sardar Sarovar, Polavaram and Indira Sagar (Narmada) dams, amongst 18 dams worldwide to evaluate their power density. The relationship between power capacity and area flooded has been termed "power density," expressed in watts per square meter. Reservoir emissions of Greenhouse Gases generally decline with increasing power density.

The table below power densities for the five Indian dams that have recently been completed or are under construction or proposed.


The Tipaimukh Dam, along with the Narmada Dams and the Polavaram on the Godavari River, has a low power density, and therefore is taken as high greenhouse gas emission reservoir.

One of the most compelling arguments used in favour of large dam construction in the critical environment of climate cha-nge impacts is that they supply "clean" or at least, "cleaner" (than fossil fuel combustion) energy. This claim has been disputed on many grounds, from the impacts of mass human displacement and its emissions consequences to the carbon emissions of large dam construction itself and of course the growing evidence of land use change on macro and micro climate.

Adding to this, the actual emissions of large reservoirs is now being indicted as far from clean in itself. According to the IRN report, "Writing in the September 2004 issue of Climatic Change, Fearnside used a comparison with the fizzing of a newly opened bottle of Coke to explain the massive surge of methane emission that can occur by "degassing" when water is discharged under pressure at hydropower dams.

According to Fearnside's calculations, degassing emissions from several large hydro dams in the Brazilian Amazon make these plants much larger contributors to global warming than fossil-fuel alternatives.

"Hydro proponents repeatedly imply that taking net emissions into account would always greatly reduce the apparent climate impact from reservoirs when only surface emissions are assessed. Hydro-Quebec, for example, claims that net emissions are probably 30-50% lower than gross. The reality is much more complex'.

The ecosystems flooded by reservoirs are a mosaic of sources and sinks of carbon dioxide, methane and nitrous oxide. Natural lakes and rivers are usually sources of C02 and CH4, Tropical soils may be either sources or sinks of N20. Changes in local weather conditions mean that the same ecosystem can be a source one year, a sink the next.

"A team of Brazilian researchers led by Elizabeth Sikar has calculated fluxes of greenhouse gases before and after construction of Manso and Serra da Mesa dams in the Brazilian cerrado (savanna) ecosystems. Based on measurements taken in March 2004 (and not necessarily representative of other months or other years), the areas flooded by these dams would both have been C02 sources before impoundment. C02 emissions from Serra da Mesa reservoir were slightly lower than pre-dam emissions, and Manso reservoir was acting as a sink for carbon dioxide.

Net C02 emissions from both reservoirs were thus negative. Sikar et al found that the area flooded by Serra da Mesa had been a small source of CH4 before dam construction, and that of Manso had been a small methane sink. At both sites the reservoirs created significant methane sources, Serra da Mesa emitting 100 times more CH4 than before impoundment (not including degassing emissions), Both reservoirs turned nitrous oxide sinks into sources.

The climate impact of the net N20 emissions at Serra da Mesa were almost two-thirds that of its net CH4 emissions. This is a potentially significant finding as N20 emissions have rarely been measured at tropical reservoirs and are usually assumed to be, negligible."

Greenhouse gas emissions have now been measured at more than a hundred reservoirs, mostly in North America and Brazil. Despite the many methodological and analytical disagreements between independent researchers and those linked to the hydro industry, there are many points on which there is no serious dispute.

These include:

• All reservoirs produce methane (CH4) and C02. Reservoirs are also sou-rces of the potent greenhouse gas nitrous oxide (N20). A small number of reservoirs in boreal and temperate zones have been found to be sinks for C02 and N2O
• The gases are released via diffusion across the water surface and in bubbles that rise from the reservoir bottom. There can also be significant emissions, especially at dams in the tropics, from the degassing of water released through turbines and spillways. When water from below the surface of the reservoir is discharged at the dam, the pressure acting upon it suddenly drops and - according to the chemical principle of Henry's Law - it is able to hold less dissolved gas. Degassing emissions are also due to the greater air/water interface created when water is pulverized at the bottom of a spillway or, as at Petit Saul by a weir immediately downstream of the dam built to aerate the oxygen-depleted reservoir water and prevent it wiping out aquatic life downstream.
• The major component of the warming impact of boreal reservoirs is diffusive C02; the major component of the warming impact from the surfaces of tropical reservoirs is methane bubbles.

• The gases are formed by the decomposition in the reservoir of dissolved and particulate organic carbon. The main sources of this carbon - the "fuel" for the reservoir emissions - are the vegetation and soils flooded when the reservoir is first filled, the organic matter washed into the reservoir from upstream (which may be from natural or farmed ecosystems, or sewage from cities), the plankton and aquatic plants which grow and die in the reservoir, and the vegetation that grows on the "drawdown" land temporarily exposed during low reservoir periods. Reservoirs absorb atmospheric C02 due to photosynthesis by plankton and aquatic plants; this uptake can occasionally exceed C02 emissions.
• Methane emissions occur due to bacteria that decompose organic matter in oxygen-poor water. The bottom layer of water in tropical reservoirs tends to be seriously depleted of oxygen. Some methane bubbles are oxidized to carbon dioxide as they rise to the reservoir surface - thus shallow tropical reservoirs where bubbles have less time to become oxidized tend to have the highest methane emissions.
• Emissions per unit of area flooded are much higher from tropical reservoirs than from those in boreal zones, which are in turn generally higher than those in temperate zones.
• Reservoirs emit greenhouse gases over their lifetime. There is an initial high pulse of emissions in the first few years after reservoir filling because of the huge amounts of carbon in the biomass and soils in the area flooded. Emissions generally appear to decline over subsequent decades. The actual rate of decline varies widely between individual reservoirs and climate zones. Some reservoirs fail to show any clear decline, and researchers have sometimes recorded increased emissions over time when sampling the same reservoir several years apart.
• Emission levels vary widely between reservoirs depending upon such factors as the area and type of ecosystems flooded, reservoir depth and shape, the local climate, the duration of winter ice-cover, the area of the reservoir covered in aquatic plants, water quality (especially pH and nutrient content), the way in which the dam is operated, and the ecological, physical and socio-economic characteristics of the dammed river basin. Among the factors influencing degassing emissions are the concentrations of methane at different reservoir depths, the depth of turbine and spillway intakes, and the type of spillway design.
• Surface emissions vary widely among different parts of the same reservoir (largely due to changes in depth, exposure to wind and sun, and growth of aquatic plants), and from year to year, season to season, and between night and day. This greatly complicates efforts to develop reliable whole-reservoir estimates from a limited set of samples measured at specific points in the reservoir during specific time periods. Confidence in the measurements themselves is also hampered by the different results obtained through different measuring equipment and techniques, and disagreements over which measuring methods are most appropriate. Factors affecting degassing emission volumes include variations in the volume of water discharged, and the proportion of turbined water versus that which is spilled.
• Calculation of the warming impact of reservoirs should be based upon net emissions. This requires adjusting measurements of gross emissions at the reservoir surface and dam outlets to allow for whatever sinks and sources of greenhouse gases existed in the reservoir zone before submergence the uptake of carbon through reservoir photosynthesis, and the impact of the reservoir upon the pre-dam flows of carbon throughout the wider watershed.