The Global Forest Watch report concludes that while hydro electricity releases much less carbon than power generated by fossil fuels, emerging research suggests the difference isn't as great as previously thought.
"The Canadian government ends up with one number and everybody assumes that must be the correct number," said organization spokesman Peter Lee. "Instead, there's a range of other possible, much higher, emissions based on the science."
Hydro developments release greenhouse gases when forests and plant materials are submerged by new reservoirs. As the organic material decays, the carbon stored in it is released.
The federal government, using procedures recommended by the Intergovernmental Panel on Climate Change, has reported that such emissions total 0.5 megatonnes of carbon dioxide a year.
But Lee's team, based on research on hydro developments in Quebec, suggests the real total is anywhere between seven and 13 megatonnes of carbon dioxide. Most of that is released in Quebec.
Part of the reason for the difference is that the government estimate assumes that reservoirs stop emitting carbon from submerged plants after about a decade.
"We just think it's out of date," said Lee.
"The newer literature indicates that is not the case, that there is a cutoff in emissions after 10 years. The actual net emissions extend way beyond the 10 years.
"The government should update and clean up its reporting of emissions."
Lee said there is still much to be learned about the greenhouse gas impacts of hydro reservoirs. Scientists have assumed that submerged peatlands don't decay in Canada because temperatures aren't warm enough, but nobody knows for sure.
"What happens to all these peatlands, which contain enormous quantities of carbon?" asked Lee. "It is an unknown question."
Hydro power emits as little as two per cent of the greenhouse gases produced by coal-fired generation. But hydro developments have a range of impacts from habitat destruction to their effects on First Nations.
Lee said correct and complete information is vital as Canadians make decisions about their energy future.
"Because we're potentially so important on a global scale, given our energy resources, and because we seem to want to export as many energy resources as we can, it's very important that Canadians have an understanding of the geographic distribution of these developments and the environmental trade-offs that have to be made."
By Bob Weber, The Canadian Press
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