NEW DELHI: Latest scientific estimates show that large dams in India are responsible for about a fifth of the countries' total global warming impact.

The estimates also reveal that Indian dams are the largest global warming contributors compared to all other nations. Brazil comes second with the emission of methane from its reservoirs being 21.8 million tonnes per annum, which is 18.13 per cent of the global figure. This estimate by Ivan Lima and colleagues from Brazil 's National Institute for Space Research (INPE) was recently published in a peer-reviewed journal, according to the South Asia Network on Dams, Rivers and People.

The study titled, "Methane emission from Indian Large Dams" estimates that total emissions from India's large dams could be around 33.5 million tonnes per annum, including those 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.

Oxidation

"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," says the report.

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

The methane emission from India 's dams is estimated at 27.86 per cent of the methane emission from all the large dams of the world, which is more than the share of any other country of the world.

Not mandated

"It is unfortunate that Lima 's study has come too late to be included in the recent reports from the Inter-governmental Panel on Climate Change (IPCC)," said Patrick McCully, Director of the International Rivers Network. Emission of carbon dioxide from reservoirs is already part of the mandatory reporting formats of IPCC. Reporting of methane emissions is suggested, but not mandated.

Hydropower myth

These latest round of studies help shatter the myth that power from large hydropower projects was "clean."

Indian hydropower projects are already known for their serious social and environmental impact on the communities and environment.

The fact that these projects also emit global warming gases in such significant proportion should further destroy the myth, pointed out Himanshu Thakkar of the South Asia Network on Dams, Rivers and People.

"The Indian Government has been blind to this issue so far, even though it has been known for more than a decade now that reservoirs in tropical climate are significant source of global warming gases. Neither the Central Water Commission, nor the Central Electricity Authority, has assessed the global warming impact of India's large dams and implications there of," he said.

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.)

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.).

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.

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