But if a new study is correct, there’s a big problem: There might be more greenhouse gases going into the atmosphere than we thought. That would mean an even larger need to cut.

The new paper, slated to be published next week in BioScience, confirms a significant volume of greenhouse gas emissions coming from a little-considered place: Man-made reservoirs, held behind some 1 million dams around the world and created for the purposes of electricity generation, irrigation, and other human needs. In the study, 10 authors from U.S., Canadian, Chinese, Brazilian, and Dutch universities and institutions have synthesized a considerable body of prior research on the subject to conclude that these reservoirs may be emitting just shy of a gigaton, or billion tons, of annual carbon dioxide equivalents. That would mean they contributed 1.3 percent of the global total.

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Moreover, 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. 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 new research concludes that methane accounted for 79 percent of carbon dioxide equivalent emissions from reservoirs, while the other two greenhouse gases, carbon dioxide and nitrous oxide, accounted for 17 percent and 4 percent.

“There’s been kind of an explosion in research into efforts to estimate emissions from reservoirs,” said Bridget Deemer, the study’s first author and a researcher with Washington State University. “So we synthesized all known estimates from reservoirs globally, for hydropower and other functions, like flood control and irrigation.”

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“And we found that the estimates of methane emissions per area of reservoir are about 25 percent higher than previously thought, which we think is significant given the global boom in dam construction, which is currently underway,” she continued.

As Deemer’s words suggest, the study does not single out dams used to generate electricity — it focuses on all reservoirs, including those that are created for other purposes. It drew on studies on 267 reservoirs around the world, which together have a surface area of close to 30,000 square miles, to extrapolate global data.

Reservoirs are a classic instance of how major human alteration’s to the Earth’s landscape can have unexpected effects. Flooding large areas of Earth can set off new chemical processes as tiny microorganisms break down organic matter in the water, sometimes doing so in the absence of oxygen — a process that leads to methane as a byproduct. One reason this happens is that the flooded areas initially contain lots of organic life in the form of trees and grasses.

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Meanwhile, as nutrients like nitrogen and phosphorus flow into reservoirs from rivers — being poured in by human agriculture and waste streams — these can further drive algal growth in reservoirs, giving microorganisms even more material to break down. The study finds that for these reasons, reservoirs emit more methane than “natural lakes, ponds, rivers, or wetlands.”

“If oxygen is around, then methane gets converted back to CO2,” said John Harrison, another of the study’s authors, and also a researcher at Washington State. “If oxygen isn’t present, it can get emitted back to the atmosphere as methane.”And flooded areas, he said, are more likely to be depleted of oxygen. A similar process occurs in rice paddies, which are also a major source of methane emissions.

In fact, Harrison said that based on the new study, it appears that reservoir emissions and rice paddy emissions are of about the same magnitude on a global scale — but rice paddy emissions have been taken into account for some time. Reservoir emissions often have not.

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‘There are inventory compilers in each country that are responsible for compiling information about greenhouse gases to the atmosphere,” Harrison explained. “The [United Nations’ Intergovernmental Panel on Climate Change] writes the guidance, the cookbook that’s supposed to be used by these inventory compilers, and that guidance currently includes reservoirs only as an appendix, not an official part of any nation’s inventory. But that is likely to change as those guidelines get revised over the next two years.”

The research, said Deemer, complicates the idea that hydropower is a carbon-neutral source of energy, although she stresses that the authors aren’t saying that they’re against using large bodies of water to generate energy through dams. Rather, they’re arguing that the greenhouse gas calculus has to be included in evaluating such projects.

This problem is not an entirely new one: A major 2000 study in BioScience raised this issue, and the International Hydropower Association on its website acknowledges that “While hydropower is a very low-carbon technology, it is known that some reservoirs in certain conditions can release quantities of methane, a greenhouse gas. Reservoirs can also, in other circumstances, act as carbon sinks.”

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But what is new about the current study is its synthesis of a large number of studies since 2000, and the determination that these emissions add up to something that is big enough to be taken seriously as part of the global carbon budget. It also finds that while some reservoirs are indeed “sinks” for carbon dioxide or nitrous oxide — meaning, they take up more of these gases than they emit — that was not true for methane.

The authors acknowledge the study does not represent a full “life cycle analysis” of reservoirs, taking into account how much carbon was stored (or emitted from) lands prior to their being flooded, and also what happens after reservoirs are decommissioned. Nor does it attempt to weigh the methane emissions from reservoirs used to generate hydropower against the amount of greenhouse gas emissions that would presumably be created if that electricity was instead generated by burning coal or natural gas.

But it clearly suggests a need to take these emissions seriously, and conduct further research.

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“We’re trying to provide policymakers and the public with a more complete picture of the consequences of damming a river,” said Harrison.

Correction: A prior title of this article suggested that methane emissions from reservoirs are a “key new source of greenhouse gases.” In fact, scientific budgets of global methane emissions have included reservoir emissions in the category of lakes and rivers, according to Harrison. The new research, however, does suggest that reservoir emissions may have been underestimated in such budgets.

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