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PRI's Environmental News Magazine

Huge Methane Emissions from Fresh Water

Air Date: Week of January 28, 2011

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Professor John Downing still loves lakes. (Courtesy of John Downing)

Small farm ponds and other freshwater bodies sink more carbon than all the world’s oceans. But freshwater sources also release huge amounts of carbon dioxide and methane. John Downing, an ecology professor at Iowa State University, recently co-wrote an article on freshwater gas emissions. Host Bruce Gellerman asks him about how these small bodies of water make a big impact on our climate.


CURWOOD: It’s Living on Earth, I’m Steve Curwood.

GELLERMAN: And I'm Bruce Gellerman. Here’s something that might surprise you - it certainly surprised scientists. The world’s freshwater lakes, streams, ponds and swimming holes store and release immense amounts of climate changing greenhouse gases, especially methane, which is one of the most potent. Scientists found a lot more of it than they expected and their discovery is in the latest edition of Science magazine. John Downing is one of the co-authors of the article. He’s a limnologist at Iowa State University in Ames - and Professor, welcome to Living on Earth!
DOWNING: Well thank you very much it’s a pleasure to talk about this.

GELLERMAN: So, what’s a limnologist?

DOWNING: Well, a limnologist is a scientist that studies inland waters, or continental waters - lakes, ponds, rivers and streams.

GELLERMAN: Well, for this study for Science, you studied a lot of them!

DOWNING: Yeah, I think it was 400, yeah 474 I think is the number we finally ended up with. A large number of systems across the world, but there are millions of lakes and ponds, and those are hopefully representative of them.

GELLERMAN: So what did you find? You were looking for what specifically?

DOWNING: So we were trying to see how much methane, a really important greenhouse gas, might be emitted by inland waters because it hadn’t been included in really any global budget.

GELLERMAN: So where does the methane in freshwater bodies come from?

DOWNING: Well basically decomposition of organic matter that flows into them, or organic matter that’s created within them.

GELLERMAN: So you found that freshwater bodies of water have a lot of methane stored in them and that they’re releasing a lot of it. How much are we talking about here?

DOWNING: Well, it’s kind of hard to wrap your head around the numbers, because they’re usually measured in things like petagrams which is an enormously immense amount of material. But it’s equivalent to about 25 percent of all of the carbon that’s taken up and sequestered by terrestrial environments worldwide.

GELLERMAN: And terrestrial environments is the land environments, of which these ponds and lakes are part of.

DOWNING: Right. Global ecologists have sort of divided the world into three big chunks, and one of them is the ocean, another is the continents and the other is the atmosphere. What we’ve been finding over the last four years or so is that when we consider the lakes - the wet part, the aquatic part of the continental mass, they are so much more active than any piece of real-estate on the planet.

GELLERMAN: So Professor, how much of the world’s surface is freshwater?

DOWNING: It looks to be about 2.8 to three percent right now. But five years ago, the estimate was about 1.5 percent of the land surface made up of lakes and ponds.

GELLERMAN: So how is it that we didn’t know where, I guess, half the world’s fresh water was? Before not too long ago?

Professor John Downing still loves lakes. (Courtesy of John Downing)

DOWNING: A group of scientists and I, working at the National Center for Ecological Analysis and Synthesis, put together a lot of very exacting analyses of aerial and satellite photos, and began to find that there were many more small systems - small lakes and ponds than anyone had suspected.

GELLERMAN: So, to quote former president Bush, you’ve ‘mis-underestimated’ these lakes!
DOWNING: Well, I didn’t. I was pretty convinced that it would be there as was David Batsviken and Lars Tranvik and the other scientists who worked on this project. So what we’ve failed to do, really, was to consider a piece of that global budget.

GELLERMAN: When you’re talking about a global carbon budget, you’re talking about the amount of carbon that’s stored and released.
DOWNING: That’s right. And, it’s really important to know what all the major sources and sinks of carbon are. It’s sort of equivalent to running your own household budget, calculating all of your sources of incomes and expenses, and forgetting that the insurance company takes an amount out of your account each month that’s equal to 25 percent of your household income - kind of a big mistake, even a limnologist would know enough economics to make that work better!

GELLERMAN: So what happens now with this information? Does it help us, or does it hurt us?
DOWNING: Well, I think it points up a few things. It makes me feel as if we really need to pay more attention to these inland waters and what they are doing, because they are so active in everything they do that the fact that they may be only 3 percent of the land area of the planet is less important then their great activity.
So, paying attention to inland waters is very important. And then I’d say if we made this kind of an error here, what other pieces of this puzzle are missing - what other pieces might we need to fill in - and we should be paying attention to environments that are highly carbon active, even though they may be small.

GELLERMAN: So, good and bad things come from small packages.

DOWNING: Yeah, that’s in fact true. They are small, but they’re mighty. The little farm-ponds of the world probably bury or sequester as much carbon every year as the world’s oceans, and the lakes and sort of moderately sized water bodies of the earth, sequester four times the carbon that’s buried by the oceans in a year. So we have to look for not just big and obvious sources and sinks of carbon, but look at those that may be small and highly active.

GELLERMAN: Professor Downing, I was looking at your webpage, and you quote Thoreau.
DOWNING: Uh, yeah, I’ve got it right in front of me - it’s one of my favorites, it goes: “A lake is the landscape’s most beautiful and expressive feature. It is earth’s eye, looking into which the beholder measures the depth of his own nature.” Thoreau is a hero of mine, and of course, most ecologists.

GELLERMAN: It’s Earth’s eye, but I’m wondering now with your research, if it’s Earth’s black eye?
DOWNING: Not at all! We shouldn’t ever consider that lakes are bad. The rates that were measured by Batsviken and his crew of scientists from around the world - these rates of methane emission are natural and have been there a long time. Maybe if we have a black eye, it’s because we haven’t paid attention to some of earth’s most carbon-
active environments and we need to be doing that.

GELLERMAN: Well, professor Downing, it was a real pleasure, I really appreciate it.
DOWNING: Oh, what a pleasure it is for me to talk about this work. Thanks for asking me.



Read the abstract of “Freshwater Methane Emissions Offset the Continental Carbon Sink” in January’s Science magazine. Subscription required for full text

In “Lakes (Formation, Diversity, Distribution),” Downing and his colleague accounted for twice as much freshwater than had previously been estimated.

Learn more about importance of global limnology in John Downing’s treatise on the topic.


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