Fossil fuels, agriculture and transportation are examples of well-known sources when it comes to greenhouse gas emissions and a changing climate, but a new study suggests there may be an additional silent contributor in our midst. Environmental scientists tracking greenhouse gases rising from the world's reservoirs say they produce the equivalent of around one gigaton of carbon dioxide each year, more than all of Canada, and a figure not currently taken into account when sizing up our environmental footprint.

Scientists have known for some time that reservoirs play a role in global warming. The difference between these and natural bodies of water is in what lies beneath, with the construction of manmade reservoirs usually flooding soil and vegetation rich with organic matter. When these nutrients are decomposed by microbes, they are transformed into carbon dioxide, methane and nitrous oxide and flow upwards into the atmosphere.

Researchers have monitored this phenomenon over the last 15 years or so, but a paper to be published this week by scientists at Washington State University will be the largest and most comprehensive to date. The team has gone over the previous literature and synthesized a large number of studies, concluding that not only are these emissions equal to 1.3 percent of the global total, but the particularly potent greenhouse gas methane is a bigger part of the picture than previously suspected.

"We had a sense that methane might be pretty important but we were surprised that it was as important as it was," says 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."

While the concentration of methane in the atmosphere is outweighed by carbon dioxide, it is much better at trapping heat beneath the Earth's atmosphere and therefore poses a bigger threat to the climate than CO2 does, pound-for-pound. We know that methane can arise from cows, biomass burning, leaks and reservoirs, but the way it escapes from the latter has so far made its presence hard to quantify.

"Methane is less soluble in water than are the other greenhouse gases included in this study (carbon dioxide and nitrous oxide)," Deemer tells us. "Because of this, a large fraction of methane emission can occur as bubbles. If these bubbles aren't captured by measurements, reservoir methane flux can be underestimated."

The scientists say that these bubbles can make up over 95 percent of methane emissions from some systems, but their study is the first to take them into account. The upshot of this is that reservoirs are emitting 25 percent more methane than we previously thought. But on a more positive note, the researchers tell us that this new knowledge may help us come up with ways to lessen the impact.

"Because reservoirs are human designed and human operated, there may be an opportunity for greenhouse gas mitigation at both the planning and the operation stages," says Deemer. "The results of our study suggest that reservoirs sited in locations downstream of 'nutrient' inputs will produce more methane than those receiving fewer nutrient inputs. Large nutrient inputs to waterways are often associated with human activities like food and energy production. It is also possible that reducing nutrient inputs to existing reservoirs could reduce methane emissions, but this remains to be tested in the field."

The team's research is scheduled to be published in the journal BioScience.