India's greenhouse gas emissions could be 40 percent higher than official estimates if methane released from dams is taken into account, according to a new study.

Methane -- about 23 times more powerful than carbon dioxide in terms of the amount of heat it traps -- is released from reservoirs, spillways and turbines of hydropower dams as a result of rotting carbon-containing vegetation.

But India, already one of the world's top polluters, has never measured methane emissions from its 4,500 large dams and has therefore never taken it into account in official data.

According to a study by scientists from Brazil's National Institute for Space Research, methane equivalent of 825 million tonnes of carbon dioxide is released annually by India's dams.

"I am quite positive that surface methane emission estimations are correctly estimated," said Ivan B.T. Lima, lead author of "Methane Emissions from Large Dams as Renewable Energy Resources: A Developing Nation Perspective."

"I am confident that Indian dams might be altering atmospheric methane although not precisely to what extent," Lima told Reuters in an e-mail interview.

India's carbon emissions, which excluded the contribution of methane from dams, were around 1,890 million tonnes in 2000, according to the World Resources Institute, a Washington-based environmental think-tank.