A Carbon Contradiction
Reservoirs provide flood control, irrigation, drinking water and recreation for millions of people worldwide. And when positioned behind hydroelectric dams, reservoirs are also major sources of low-carbon electricity — slashing our reliance on greenhouse gas-emitting fossil fuels.
“It turns out the reservoirs are a really good place to rot organic matter,” says John Harrison, an associate professor at Washington State University’s School of the Environment in Vancouver, and one of the study’s authors. “And that makes a lot of methane. And methane is a really potent greenhouse gas. In fact, on a per-molecule basis, it’s about 35 times more potent than CO2.”
Organic matter — algae, terrestrial plant material, you name it — can collect in reservoirs in any of a few different ways.
“Organic matter that’s there when a reservoir is flooded will rot,” Harrison says. “Organic matter that flows in from upstream or grows in a reservoir … when it rots in the absence of oxygen, can be converted to methane, and then emitted into the atmosphere.”
All told, the greenhouse gas emissions from reservoirs add up — and we don’t need to pick on Canada to prove it.
“[Reservoirs are] also a similar source in magnitude to important human sources like rice cultivation, or the burning of biomass to make electricity,” Harrison says. ”So it’s a really important source, and yet it’s not counted in the current UN accounting of greenhouse gases.”
Harrison isn’t hoping that the study’s findings will discourage construction of new reservoirs. For one, although reservoirs emit greenhouse gases, he notes they can also help us mitigate the effects of climate change, by acting as buffers against cycles of flooding and drought. But he does think citizens and governments need to be more aware of the trade-offs.
“We’re trying to round out the picture of what happens when a river is dammed, so that policymakers and the general public have a better understanding of the consequences of every reservoir that’s built,” Harrison says.
That’s not to say that we can’t reduce the levels of greenhouses gases emitted by reservoirs. Harrison sees minimizing the amount of organic material in reservoirs as the best place to start.
“One thing that we’re really excited about is the potential to site reservoirs in ways that reduce greenhouse gas emissions to the atmosphere, and to manage the organic matter inputs to reservoirs that lead to those greenhouse gases eventually,” Harrison says.
*This text was updated on October 14, 2016, to reflect the following corrections: An earlier version stated that, taken together, reservoirs produce as much methane and CO2 as Canada. That line has been changed to indicate that, taken together, the world’s reservoirs have as big an impact on climate as the population of Canada. Also, John Harrison is not the lead author of the study mentioned. He is a co-author.
John Harrison is an Associate Professor in the School of the Environment at the Washington State University in Vancouver, Washington.
IRA FLATOW: And now it’s time to play good thing bad thing.
Because every story has a flip side. Reservoirs, you know, those lake like bodies of water. They provide flood control, irrigation, drinking water, electric power recreation. Beautiful and environmentally friendly, no? Well, we have such great drinking water in New York because of these reservoirs. Well, turns out maybe not.
A recent study shows that reservoirs are also an unforeseen emitter of greenhouse gases. If you put all the reservoirs together they produce as much methane and CO2 as the entire country of Canada, which is the ninth largest greenhouse gas emitting country in the world. So, why are reservoirs such a carbon contradiction?
Joining me to discuss the good and the bad is John Harrison, Associate Professor, School of Environment, at Washington State University in Vancouver, Washington. Welcome to Science Friday.
JOHN HARRISON: Thanks, Ira. It’s great to be here.
IRA FLATOW: Well, we’ve been building reservoirs for centuries. That’s because they’re very useful. So that’s got to be a good thing, right?
JOHN HARRISON: Well, exactly. Right. Yes, reservoirs do a lot of really important things for the communities in which they reside. And you mentioned a number of them. In addition to those, if climate changes the way that it’s supposed to, with more intense cycles of flooding and droughts, than reservoirs can really provide an important buffer against those changes.
IRA FLATOW: Yeah. But your research suggests that reservoirs contribute to global warming. So it’s a good thing bad thing. How does this actually happen with the reservoirs?
JOHN HARRISON: Yeah. So it turns out that reservoirs are a really good place to rot organic matter. And that makes a lot of methane. And methane is a really potent greenhouse gas. In fact, on a per molecule basis it’s about 35 times more potent than CO2.
IRA FLATOW: So anything that falls in the reservoir that’s organic is going to decompose and just fizzle out into methane.
JOHN HARRISON: That’s right. Organic matter that’s there when a reservoir is flooded will rot. Organic matter that flows in from upstream or grows in a reserve will- when it rots in the absence of oxygen can be converted to methane and then emitted to the atmosphere. And it turns out, like you said, when you add up all of the emissions of greenhouse gases from reservoirs around the world, that is a substantial source of greenhouse gas to the atmosphere.
And you mentioned Canada. We don’t have to beat up on Canada only. It’s also a similar source in magnitude to important human sources like rice cultivation or the burning of biomass to make electricity. So it’s a really important source and yet it’s not counted in the current UN accounting of greenhouse gases.
IRA FLATOW: Are you suggesting we don’t build reservoirs anymore? Is that what this research suggests?
JOHN HARRISON: No, not at all. No. No. Absolutely not. Rather, we’re trying to round out the picture of what happens when a river is dammed so that policymakers and the general public have a better understanding of the consequences of every reservoir that’s built.
IRA FLATOW: Yeah. So that we get a better idea of the emissions- the balance of greenhouse gases coming and going.
JOHN HARRISON: Yeah. Yeah. And actually, one thing they’re really excited about is the potential to site reservoirs in ways that reduce greenhouse gas emissions to the atmosphere. And to manage the organic matter inputs to reservoirs that lead to those greenhouse gases eventually.
IRA FLATOW: All the agricultural runoff and things like that. Thank you, John.
JOHN HARRISON: Yeah, sure.
IRA FLATOW: Talked to John Harrison, Associate Professor in the School of Environment at Washington State University in Vancouver, Washington.