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Public Radio's Environmental News Magazine (follow us on Google News)

The Fire and Climate Change Feedback Loop

Air Date: Week of

Wildfires are becoming more frequent and intense, lasting on average 78 days longer than they did just two decades ago. Northern Arizona University Biology Professor Bruce Hungate tells host Bruce Gellerman about research that shows a relationship between fire and the release of nitrous oxide, a potent greenhouse gas.


GELLERMAN: From the Jennifer and Ted Stanley Studios in Somerville Mass, this is Living on Earth. I'm Bruce Gellerman. Nitrous Oxide is commonly known as laughing gas, but there’s nothing funny about its effects on climate change.

Biologist Bruce Hungate accidentally discovered that during wild fires, huge amounts of nitrous oxide in the soil are released into the atmosphere. It all has to do with microscopic bugs in the soil that give off laughing gas. Bruce Hungate is a professor at Northern Arizona University.

HUNGATE: Bacteria called denitrifiers use nitrate in respiration just like humans use oxygen, and in the process, they produce nitrous oxide. Fires promote conditions in the soil that favor production of nitrous oxide by these soil microorganisms. They’re microscopic, but their impacts are global by producing this greenhouse gas. And in fact, most of the nitrous oxide in the atmosphere comes from these tiny creatures.

GELLERMAN: So how potent is nitrous oxide as a greenhouse gas, compared to, say, carbon dioxide?

HUNGATE: So on a molecule-per-molecule basis, nitrous oxide is three hundred times more potent than carbon dioxide. That’s a very potent greenhouse gas.

GELLERMAN: So you studied grasslands, right?


GELLERMAN: Would I find nitrous oxide and these little bugs that produce it in forests?

HUNGATE: Denitrifiers are everywhere. They’re in soils all around the world. And they produce nitrous oxide from these soils all around the world. And there actually have been a lot of experiments looking at the impacts of fire on nitrous oxide production from forests as well, it turns out, especially in the tropics - and often there what you see is after a fire, you get more nitrous oxide emitted from soil.

Bruce Hungate is a professor of Biology at Northern Arizona University. (Northern Arizona University.

GELLERMAN: And we didn’t know about this forest-fire and nitrous oxide relationship before?

HUNGATE: What we did know is that, in general, after fire, nitrous oxide emissions often go up. So we knew that before. What we didn’t know is how fires interact with these other components of the changing environment and in our experiment that was the real surprise.

GELLERMAN: You were running a series of experiments and you had a bunch of test plots, as I understand it, and that’s where you made your discovery.

HUNGATE: That’s right. We started this experiment back in 1998 in a grassland in California where we actually changed the physical environment around test plots to try to simulate the environment of the future. We focused on four on-going global environmental changes. More CO2 in the atmosphere, so some plots have tubes that release extra CO2 into the atmosphere around the growing plants, and also warming, we have infrared heat lamps over some plots to make them warmer. Extra nitrogen deposition - some plots get an extra dose of nitrogen simulating higher industrial activity and its effect on the atmosphere in the future. And also rainfall, some plots have sprinklers that simulate more rain. So we had each of these changes by itself, and then in every possible combination with the other global changes - it was really complex.

And then a downed power line caused a fire that burned part of it. At first we were really worried about damage to the experiment, but it turned into an opportunity. The fire burned only part of it, so we still had controls to quantify the impact of the fire along with the background of all these global environmental changes. So instead of losing the experiment, we got an even more complex experiment - very complex, but also interesting and exciting, with these new results.

GELLERMAN: So this accidental fire leads to this surprising finding that you can have accelerated global warming due to the nitrous oxide in the soil being released, essentially.

HUNGATE: Yeah, that’s exactly right, it was a surprising result. When we looked at each of these things by itself, we wouldn’t have been able to predict the result we got.

GELLERMAN: So you get this intense burst of nitrous oxide - so it’s not long lasting? Or…

HUNGATE: Well, actually it is. It was a delayed reaction. The pulse of nitrous oxide after fire lasted about three years. And that was another surprising finding, because past work on fires and nitrous oxide emissions haven’t shown quite as long lasting an effect. We think that might have to do with a combination of the global environmental changes along with the fire that really promoted nitrous oxide production.

The Wallow fire burning in Arizona. (Arizona Emergency Information Network)

GELLERMAN: So, Professor, let me play out the scenarios. So if you have a wildfire it releases this nitrous oxide into the atmosphere, it affects climate change dramatically, it gets warmer and causes the conditions for more wildfires - you’ve got a feedback loop here.

HUNGATE: That’s exactly right. It’s where climate change leads to more fires, which in turn lead to more climate change. And it’s not just nitrous oxide, these fires also produce carbon dioxide and methane, so they’re important sources of greenhouse gasses.

GELLERMAN: Whoa! Well, we’re having intense wildfires around the United States - Arizona, Texas, Florida - we’re having wildfires that are unprecedented in terms of their size and in terms of their intensity and their duration.

HUNGATE: That’s right. I’m really concerned about these fires.Wallow fire in my home state - Arizona - has burned over 450,000 acres - it’s the largest in our state’s history. But I’m also concerned because we can expect more of these large and intense fires in the future. Warming promotes fire-weather and the forests of the southwest are loaded with fuel - many and densely packed trees. That combination makes these systems especially susceptible to intense fires.

We’re playing with loaded dice with our climate system. And when we look at things like fire, having more greenhouse gasses in the atmosphere appears to make a longer fire season, a more intense fire season, more likely. The fact that we’re seeing some of those effects now is exactly consistent with what we expect from climate change.

GELLERMAN: Well, Professor, thank you so much, I really appreciate it.

HUNGATE: Thank you very much, I enjoyed it.

GELLERMAN: Bruce Hungate is Professor of Biology at Northern Arizona University - he joined us from a conference in Iceland.



Report: Global Change Could Amplify Fire Effects on Soil Greenhouse Gas Emissions

Bruce Hungate: Northern Arizona University


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