Latest scientific estimates show that large dams in India are responsible for about a fifth of the India's total global warming impact. The estimates also reveal that Indian dams are the largest global warming contributors compared to all other nations. This shatters the myth that large hydro projects are a clean source of energy. This estimate by Prof Ivan Lima and colleagues from Brazil's National Institute for Space Research was recently published in a peer-reviewed scientific journal.

As per the Intergovernmental Panel on Climate Change, there are six global warming gases, of which large dams could be the source of three: methane, carbon dioxide and nitrous oxide. Methane emission from human activities is known to contribute 23% to global warming.

Methane emission from large Indian dams

This study estimates that total methane emissions from India's large dams could be 33.5 million tonnes per annum, including emissions from reservoirs (1.1 MT), spillways (13.2 MT) and turbines of hydropower dams (19.2 MT). Total generation of methane from India's reservoirs could be 45.8 MT. The difference between the figures of methane generation and emission is due to the oxidation of methane as it rises from the bottom of a reservoir to its surface.

The study had to make a number of assumptions in arriving at these estimates, as no measurements of the methane concentration or emission have been made for reservoirs in India. (Most measurements of methane emission from reservoirs have been done in Canada, Brazil and French Guyana.) Secondly, the data about the release of water from turbines and spillways of India's dams were not adequate; hence the estimates involved some further assumptions.

The study estimates that emission of methane from all the reservoirs of the world could be 120 MT per annum. This means that of the total global emissions of methane due to all human activities, the contribution from large dams alone could be 24%. This study does not include the emission of nitrous oxide and carbon dioxide from large dams. If all these are included, the global warming impact of large reservoirs would go up further.

By these estimates, the methane emission from India's dams could be 27.86% of the methane emission from all the large dams of the world, which is more than the share of any other country. Patrick McCully, director of the International Rivers Network says, "Climate policy-makers have largely overlooked the importance of dam-generated methane. The IPCC urgently needs to address this issue."

These latest round of studies should further help shatter the myth that power from large hydropower projects is clean. Indian hydropower projects are already known for their serious social and environmental impacts on the communities and environment. The fact that these projects also emit global warming gases in such significant proportion should further destroy the myth.

Looking at the available figures for dams in India, total emission of methane from Indian dams may be somewhat overestimated. Even if we assume that methane emission from Indian dams is about half of what Prof Lima et al have estimated, it is still likely to be around 17 MT per annum. Even this more conservative figure means that India's dams emit about 425 CO2 equivalent MT (considering that Global Warming Potential over 100 years of a T of methane is equivalent to GWP of 25 T of CO2, as per the latest estimates of IPCC). This, when compared to India's official emission of 1849 CO2e MT in year 2000 (which does not include emission from large dams), the contribution of methane emission from large dams is 18.7% of the total CO2 emission from India.

The science

Large dams have been known to be emitters of greenhouse gases like methane, carbon dioxide and nitrous oxide for over a decade now. The "fuel" for these gases is the rotting of the vegetation and soils flooded by reservoirs, and of the organic matter (plants, plankton, algae, etc) that flows into. Methane is produced at the bottom of the reservoirs in the anaerobic conditions prevailing there, over the lifespan of the reservoirs. The gases are released at the reservoir surface, at turbines (of hydropower projects) and spillways, and downstream of the dam.

The actual measurements by Prof Lima et al at some of the Brazil dams reveal that the biggest global warming impact of large dams could be coming from methane emissions from turbines of large hydropower projects and spillways of dams. Methane is produced at the reservoir bottom. As it rises toward the surface, part of the methane is oxidized in the water to carbon dioxide, a much less powerful greenhouse gas. But when methane-rich deep water is released at the turbines and spillways (generally from below the surface of the reservoirs), the pressure acting upon the gas due to the water column above it suddenly drops and most of the dissolved methane is released directly into the atmosphere. This degassing is a process similar to the fizzing of a newly opened bottle of soda. Researchers from INPE estimate that 95% of a dam's methane emissions are from spillways, turbines and downstream.

China and USA have more large dams than India. However, Indian dams, being situated in tropical climate, could be such big contributors to global warming. This is because the methane emissions are one or more orders of magnitude higher in tropical climate (in areas between 30 degree latitude on both sides of equator, due to higher temperature) than those from reservoirs elsewhere. This is similar to the phenomenon of lower generation of gas in winter in biogas plants in India, where too, the degradation of organic matter under anaerobic conditions lead to generation of methane. Some of the large hydropower reservoirs in Brazil (also in tropical climate zone) have been estimated to have a higher global warming impact per kilowatt-hour electricity generated than fossil fuels, including coal, according to the study.

It is true that the calculation of the warming impact of reservoirs should be based upon net emissions, which is additional emissions due to reservoirs, compared to situation without the reservoirs. Net carbon dioxide emissions from reservoirs may be significantly smaller than gross emissions. However, the difference between net and gross methane emissions is not likely to be significant.

Lima and his co-authors propose capturing methane in reservoirs and using it to fuel power plants. Lima says, "If we can generate electricity from the huge amounts of methane produced by existing tropical dams we can avoid the need to build new dams with their associated human and environmental costs."

Indian government blind to the issue

Even though the Indian government has remained silent on this issue, it cannot claim it is ignorant of this problem. Surya E Sethi, principle advisor (energy) of the Planning Commission has said he has been asking the power ministry to assess the methane emission of India's hydropower projects for five years. A study by Government of India's National Environmental Engineering Research Institute on the environmental impact of power generation noted in its report in February 2006, 'Latest studies have found that the levels of harmful greenhouse gases like methane emitted due to decomposition of vegetation submerged under water can be significant.'

However, the Indian government has not estimated the emission of global warming gases from dams in India. The Government of India's Central Electricity Authority, in its December 2006 estimation of the generation of global warming gases from the power sector in India, said the total emission for the year 2004-05 was 462.192 MT, which includes zeroemission from hydropower projects. The conservative estimation of methane emission from large dams given above is 92% of that figure. The proportionate emission of hydropower dams would be around 258 MT CO2 equivalent (since only some of the dams have hydropower component), which would increase the emission of global warming gases from the power sector by a huge 55.82%.

What needs to be done

Neither the Central Water Commission, nor the Central Electricity Authority, both premier institutes of the Government of India, have assessed the global warming impact of India's large dams and implications thereof. The minimum first step one can expect from the government is to urgently institute a credible independent scientific study of the global warming impact of dams in India, in the light of findings elsewhere. The study should include the actual measurement of methane and other GHG emission from a sample of reservoirs. While making this assessment, it should also assess as to what extent methane emitted from reservoirs and hydropower projects can be recovered for beneficial use, in the process also reducing the global warming impact of the reservoirs. While assessing power and water resources development options, the greenhouse gas emission potential of dams should be assessed, as part of the cost benefit analysis and as part of environment impact assessment.

The IPCC should initiate an independent study to assess the GHG potential of reservoirs in different parts of the world, including India. Emission of CO2 from reservoirs is already part of the mandatory reporting formats of IPCC. Reporting of methane emissions is suggested, but not mandated. The IPCC should make reporting of emission of methane from large dams mandatory.

Himanshu Thakkar (email: ht.sandrp@gmail.com) is from the South Asia Network on Dams, Rivers & People, New Delhi (www.sandrp.in)