A synthesis of 100 recent studies finds that water storage reservoirs emit as much greenhouse gases as Canada. Two of the authors explain how this happens.

Hydropower dams are generally thought to be a clean source of electricity. By moving water through turbines, dams can generate large amounts of electricity almost continuously and without causing air pollution.

It’s partly for these reasons that more than 3,700 hydroelectric dams are currently proposed or under construction worldwide.

But a growing body of science reveals a dark side. It turns out the reservoirs formed by dams are a significant source of greenhouse gases – particularly methane, about 34 times more potent than carbon dioxide. In the last 10 years, dozens of studies have shed light on this problem.

One is a new study, published October 5 in the journal Bioscience. Led by researchers at Washington State University in Vancouver, Washington, it synthesizes the results of 100 other studies to reveal that the world’s reservoirs may be producing as much as 1.3 percent of all greenhouse gases caused by humans. That’s more than all emissions produced by Canada.

The study considers the emissions from 267 large reservoirs around the world – the only reservoirs for which emissions have been measured. It uses these results to estimate emissions from all reservoirs – more than 1 million worldwide.

Prior research deduced that reservoirs in tropical regions are the biggest emitters. But the new study finds that isn’t necessarily true. Other factors are more important, particularly aquatic nutrient activity. This means North American and European reservoirs can also be big emitters.

To understand more, Water Deeply recently spoke with lead author Bridget Deemer, a former research associate at Washington State and now a research ecologist with the U.S. Geological Survey in Flagstaff, Ariz.; and her coauthor, John Harrison, associate professor at Washington State’s School of the Environment.

Water Deeply: How significant are these emissions, globally, compared to other sources?

John Harrison: They compare in magnitude to biomass burning for energy production. The importance of that statement is that human sources of methane to the atmosphere, such as biomass burning to produce energy, are included in the U.N. process for accounting for greenhouse gas emissions by each country. But reservoir emissions currently are not included in that process.

It’s substantial. Maybe a better comparison, from a methane perspective, is that emissions from reservoirs are comparable to rice cultivation as a source of methane, and both of those are substantial methane sources to the atmosphere.

Water Deeply: What did you learn about geographic effects? Does location matter?

Bridget Deemer: We were pretty surprised about that, because some prior work had suggested low-latitude systems were the biggest emitters, especially systems in the Amazon. But our results didn’t find that to be as important as some other factors.

Harrison: It’s not that geography isn’t important. It’s that we didn’t see that latitude was necessarily a good predictor of greenhouse gas emissions. We did see a linkage between how biologically productive reservoirs are and how much methane they emit.

Water Deeply: What do you mean by biologically productive?

Harrison: There’s a lot of organic matter that is being produced and decomposed in systems that are biologically productive.

You have the organic matter from the vegetation that’s decomposed once a reservoir is flooded, and those can provide nutrients to support algal growth. In addition, in low-oxygen conditions, nutrients can get liberated from sediments, which can support further algal growth and decomposition, leading to greenhouse gas production. Globally, fertilizer inputs to watersheds are a major source of nutrients.

We also found that chlorophyll A in a reservoir correlates with emissions. The concentration of chlorophyll A in a reservoir is an indicator of how green a body of water is, and how much algal growth there is. So systems with higher chlorophyll have higher algal concentrations.

Water Deeply: Does reservoir size or depth matter in terms of emission output?

Deemer: We didn’t find size or depth to be significant in our study. Other studies have found depth to be an important predictor of methane emission from lakes and reservoirs (with shallower sites emitting more methane), but we didn’t find that here.

Water Deeply: How does water level effect emissions?

Harrison: It’s something that we’re working to understand better now. By reducing water level, you reduce the pressure on sediments, which keeps bubbles in those sediments. And when you lower water level, bubbles can expand, their buoyancy increases, and they get released directly to the atmosphere.

Water Deeply: What is the state of the science on this? What don’t we know?

Harrison: Well, every reservoir in this study was emitting methane to the atmosphere. That said, we have a lot of work to do to better understand and predict how these systems emit greenhouse gasses to the atmosphere.

So we’re reasonably confident they are a substantial source of methane to the atmosphere. But just how big and what kinds of systems are the biggest emitters are both areas for further investigation.

Water Deeply: Given these findings, should we be concerned that there are 3,700 new dams at some stage of development globally?

Harrison: Another insight from this study is that the per-area emission of methane at reservoirs is actually about 25 percent higher than other studies have suggested. That suggests the impact of every additional reservoir is likely to be greater than people had previously thought.

All we’re suggesting with this study regarding those future dams is that this is a piece of the puzzle that needs to be considered when people are thinking about whether and where to construct additional reservoirs.

Water Deeply: So, given your results, can we still consider hydropower to be a “clean and green” source of energy?

Deemer: I think this study shows that dams as a source of energy aren’t without their greenhouse gas costs. Even though it’s a renewable source of energy, people should keep the greenhouse gas side of the picture in mind when making planning and policy decisions regarding dams.

Water Deeply: Are any governments – local or national – currently measuring reservoirs emissions as a routine practice?

Deemer: As of right now, I don’t think so, not that I know of. But I know the U.S. Environmental Protection Agency is exploring the option of including some greenhouse gas measurements in their national assessment of lakes and reservoirs. But they haven’t done that yet.

Water Deeply: Is there even an established process for measuring these emissions?

Deemer: That’s a great question. These emission measurements are actually quite challenging because of how variable emissions can be, depending on time of year, time of day, and sample location within the reservoir you’re looking at. Effective measurement approaches might vary by reservoir operational type as well. There’s a lot still to be done to kind of standardize methods that will give us numbers that we’re comfortable with in terms of the amount of uncertainty.

Water Deeply: You state in the study that you believe your emission estimates can be considered a “low end.” Why is that?

Deemer: We feel our estimate is conservative. It’s for a number of reasons, one of which is that we’re only looking at reservoir surface area, whereas we know there are some emissions associated with effects downstream of the dam, and other alternative pathways, that we just don’t have enough data on to include in the synthesis.

Water Deeply: Is it possible to mitigate these emissions, say, by operating a reservoir differently, changing project design or watershed management?

Harrison: Just knowing that reservoirs emit greenhouse gases gives us an opportunity to mitigate in other areas to compensate for those emissions, which otherwise wouldn’t be counted in national inventories.

Then, beyond that, there is this interesting relationship between biological productivity and methane emissions. So if you can prevent organic matter from getting into reservoirs or being produced in reservoirs in the first place, you might be able to both improve water quality and reduce greenhouse gas emissions.

You can reduce organic matter inputs to reservoirs by managing nutrients better on the landscape, so they don’t get into reservoirs. Or by siting reservoirs upstream of potential sources of the nutrients and organic matter that lead to greenhouse gas production. And there may be other things, too, like how you manage water level could influence greenhouse gas emissions to the atmosphere, and we’re actively working to better understand those.

Deemer: I think it’s a ripe area for future research, because these systems are human managed. So if we can identify some ways to manage at the dam that mitigate emissions, that would be pretty exciting.