Hydroelectric dams and other reservoirs around the world are emitting the equivalent of nearly a gigaton of carbon dioxide per year, contributing 1.3% of total global emissions, according to a new study to be published next week in the journal Bioscience.

Past research had pointed to a risk of methane releases from hydro reservoirs, and the 10 authors behind the Bioscience study synthesized much of that work in their assessment of the million dams delivering electricity generation, irrigation, flood control, and other services in communities around the world. The study extrapolated global data from studies of 267 reservoirs spanning 30,000 square miles.

“The emissions are largely in the form of methane, a greenhouse gas with a relatively short life in the atmosphere but a very strong short-term warming effect,” the Washington Post reports. “Scientists are increasingly finding that although we have begun to curb some emissions of carbon dioxide, the principal greenhouse gas, we are still thwarted by methane, which comes from a diversity of sources that range from oil and gas operations to cows.”

The research team determined that methane accounts for 79% of the CO2 equivalent in reservoir releases, followed by carbon dioxide itself at 17% and nitrous oxide at 4%.

“There’s been kind of an explosion in research into efforts to estimate emissions from reservoirs,” said lead author Bridget Deemer of Washington State University. We found that the estimates of methane emissions per area of reservoir are about 25% higher than previously thought, which we think is significant given the global boom in dam construction” for hydro generation and other purposes.