Dams have helped nations save water, avoid flooding and produce electricity but researchers at Washington State University (WSU) just revealed that reservoirs are also responsible for emitting the equivalent of roughly 1 gigaton of carbon dioxide a year, around 1.3% of all greenhouse gases produced by humans.

In a study published by the university, scientists highlight that dams are a source of methane, a greenhouse gas that is 34 times stronger than carbon dioxide over the course of a century. The researchers pointed out that unlike what was previously believed, reservoirs are not “green” or carbon neutral sources of energy.

“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, a research associate at the university 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.”

John Harrison, co-author and associate professor at WSU stressed that “acre per acre, reservoirs emit 25 percent more methane than previously thought.”

Unaware of their role in global warming, many cities have built dams to save both water and energy such as Tarbla Dam in the city of Topi, Pakistan, Aswan Dam in the City of Aswan, Egypt, or Ataturk Dam on the boarder of Adıyaman Province and Şanlıurfa Province. The United States and Canada are also known for their dams such as Fort Peck Damn, Oahe Dam, Oroville Dam, Gardiner Dam and W. A. C. Bennett Dam.