A huge new study is making a very bold and very alarming claim for mankind: the huge water reservoirs we have in our country may actually be contributing to global warming, rather than being a solution to the problem with clean hydropower electricity. Hydropower has long been put forth as a potential clean energy source that, along with other “green” sources of energy, could reduce our dependence on dirty coal and oil.

But this new study, which is based on an analysis of more than 200 studies, has found that these reservoirs are actually contributing to greenhouse gas emissions, and not a small amount either: scientists estimate that they produce about a gigaton of carbon dioxide each year into the atmosphere, which would be more than all annual greenhouse gas emission by Canada, according to a statement from Washington State University.

How is this happening? Apparently, when authorities flood an area that didn’t previously have a lake, it pulls out organic material in the soil, which water-borne microbes then munch on and then excrete in the form of methane and carbon dioxide. Also, since reservoirs are typically connected to a river source, new organic material continues to flow in, increasing the rate of carbon dioxide production by the microbes.

“We had a sense that methane might be pretty important but we were surprised that it was as important as it was,” said Bridget Deemer, WSU research associate and lead author. “It’s contributing right around 80 percent of the total global warming impact of all those gases from reservoirs. It’s a pretty important piece of the budget.”

The BioScience analysis, which drew on scores of other studies, is the largest and most comprehensive look to date at the link between reservoirs and greenhouse gases, John Harrison, co-author and associate professor in the WSU Vancouver School of the Environment, said in the statement.