According to research conducted at Washington State University, the world’s reservoirs are an underappreciated source of greenhouse gases; they produce around 1 gigaton of carbon dioxide annually, which is 1.3% of total greenhouse gases produced by humans, and more than produced by the whole of Canada.
The study, which was published in the journal BioScience, states that reservoirs produce methane, a greenhouse gas that is 34 times more powerful than carbon dioxide over the course of a century. The methane produced by reservoirs can be compared to that produced by biomass burning or rice paddies.
According to Bridget Deemer, WSU research associate and lead author:
“We had a sense that methane might be pretty important but we were surprised that it was as important as it was. 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 drew on a large number of other studies, and is therefore to date the most comprehensive look at the link between reservoirs and greenhouse gases.
John Harrison, co-author and associate professor in the WSU Vancouver School of the Environment, said:
“Not only does it incorporate the largest number of studies; it also looks at more types of greenhouse gases than past studies. While reservoirs are often thought of as ‘green’ or carbon neutral sources of energy, a growing body of work has documented their role as greenhouse gas sources.”
Acre per acre, reservoirs emit 25% more methane than previously thought, and while they do provide critical services such as electrical power, navigation, flood control, and water, they also change the dynamics of river ecosystems and impact on fish and other life forms.
Reservoirs receive a lot of organic matter and “nutrients” like nitrogen and phosphorus from upstream rivers and are also flooded with large amounts of organic matter that produce carbon dioxide, methane and nitrous oxide.
The WSU researchers found that the total global warming effect of a reservoir depends on how biologically productive it is; more algae and nutrient rich systems produce more methane.