Many scientists believe there is strong evidence to suggest that the Earth's atmosphere is warming, and that human activities - chiefly the burning of fossil fuels - are a significant cause.
Hydro-electric power, often proclaimed as one of the greenest ways of generating energy, can be more polluting than coal.
This is because the reservoirs that power the dams can trap rotting vegetation, which emits large amounts of greenhouse gases.
But establishing just how much pollution a particular reservoir will cause is very difficult, because they vary widely.
The announcement of this cause of gas emission comes shortly before the next round of negotiations on how to tackle it.
The discovery of the full greenhouse potential of hydro-electric schemes, reported in New Scientist magazine, was made by the World Commission on Dams (WCD).
Clean claim
The commission, a group of scientists, engineers and environmentalists, is supported by the World Conservation Union and the World Bank, which is the single biggest funder of large dams.
Supporters of dams argue that they should qualify for support as a clean technology under the 1997 Kyoto Protocol on tackling climate change.
Many scientists believe there is strong evidence to suggest that the Earth's atmosphere is warming, and that human activities - chiefly the burning of fossil fuels - are a significant cause.
The WCD will report its findings at a meeting in the German city of Bonn in June to discuss the Clean Development Mechanism, a key part of the Kyoto Protocol.
It says the decay of forests submerged when reservoirs are flooded produces only a fraction of the two gases involved, carbon dioxide (CO2) and methane.
Much more is produced by organic matter washed into the reservoir from further upstream, a process which may continue for the lifetime of the reservoir.
Methane is 20 times more potent as a greenhouse gas than CO2. It is given off by stagnant water, while running water, which contains oxygen, produces CO2.
Extensive problem
So a reservoir will produce more methane than the river which ran before the dam was built.
The WCD, which believes the problem is far more widespread than first thought, says the reservoirs that appear most at risk are shallow ones in the tropics which have not been cleared of biomass before they were flooded.
It says one particular cause for concern is the Balbina reservoir in Brazil, which in parts is only four metres deep.
Its generating capacity is 112 megawatts, and it is estimated to produce three million tonnes of CO2 annually over its first 20 years.
A coal-fired power station of the same capacity would produce 0.35m tonnes of CO2 a year.
In French Guyana, the Petit-Saut reservoir, which has a similar capacity and powers the launch site for Europe's Ariane rocket, is expected to produce 0.9m tonnes of CO2 annually in its first 20 years.
The WCD has looked at only a few dams in four countries, and thinks there will be many similar examples.
But it says one study of nine Brazilian reservoirs found their greenhouse emissions varied per unit of electricity by a factor of 500.
It says: "There is no justification for claiming that hydro-electricity does not contribute significantly to global warming."