Arctic Wildfires Are Burning An Important Carbon Sink

IRA FLATOW: The California wildfires are the ones that we often hear about, but wildfires occur in many other spots, including in forests above the US-Canadian border and near the Arctic Circle. A group of researchers wanted to know how these fires affect the northern forests and how this impacted their ability to store carbon, and of course, that means their influence on climate change. The results were published in the journal Nature– Climate Change. My next guest is one of the authors on that study, Jonathan Wang, a post-doctoral scholar in Earth System Science at the University of California Irvine. Welcome to Science Friday.

JONATHAN WANG: Hi, Ira. Thanks.

IRA FLATOW: Jon, what are these northern fires like, and how are they different from the ones in California?

JONATHAN WANG: The forests in the Far North are actually quite different from what we’re used to seeing in California. So for example, in California, the fires tend to be– they can be pretty big, but they’re usually suppressed. Humans go in and try and prevent the fires from raging too large and causing damage to property and to livelihoods, whereas in the Far North, the forests are so vast and remote that these fires usually don’t get attention for a long time. And so they’re just allowed to burn for quite a long time and over quite a large area, I would say, like, two or three times the size of the fires that we see in California. They occur in the Far North pretty regularly.

IRA FLATOW: How do they get started? I know in California, we talk about bad power lines or people dropping a cigarette butt. But in these northern fires, how do they get started?

JONATHAN WANG: Yeah, so the northern fires, they’re typically started by lightning strikes, because there aren’t that many people living up north, and so there aren’t as many opportunities for campfires or power lines to really cause damage. And so what climate change has been doing– it has been increasing the likelihood or the incidence of lightning strikes in general. And so that’s been leading, in part, to some of the increased occurrence of large fires in the Far North.

IRA FLATOW: So then how are these fires impacting these forests?

JONATHAN WANG: So the forests in Canada and Alaska, they’re primarily dominated by this species of trees called the black spruce. And so these black spruce trees are pretty dry. They’re pretty flammable.

When fires occur throughout a forest in the North, it tends to burn pretty severely, tends to almost completely torch the forest. And so any carbon that was stored in the above-ground components– so we’re talking, like, in the leaves or the branches or the trunks of these trees– they’re totally annihilated. And at the same time, carbon that’s stored in the soil burns as well. And so the Far North is unique in having very dense carbon stores in its soils. And so these increasing fires are starting to threaten these soil stores.

One interesting aspect, though, that we’ve found is that after a fire occurs, there’s a lot of, let’s say, regrowth or chance for regeneration after the fire. And so our study looked at trying to understand how this balance between post-fire regrowth and initial fire-induced loss– how they add up, how they balance. And we’re finding that because fires are increasing in size and extent and intensity in the Far North, forests aren’t getting a chance to recover all the way before the fires strike again.

IRA FLATOW: Just a quick note– you’re listening to Science Friday. And so what is the impact of all of this on carbon sequestration, the absorption of CO2, and climate change?

JONATHAN WANG: In the Far North, we’ve seen that there’s a lot of so-called greening. And we think that because of climate change, because of warming or increased CO2 fertilizing the forest, that there’s hope that there might be a net absorption or sequestration of carbon dioxide into northern ecosystems. But what we’re finding is that these forests that have been affected by these fires, that the carbon emissions from these fires are really offsetting nearly half, a third to a half, of the carbon gained through just normal growth throughout the northern forests. And so these fires really impose, I guess, a strong suppression of the carbon sequestration potential of the Far North.

IRA FLATOW: These northern forests, are they typical of northern forests around the rest of the world and having the same effect?

JONATHAN WANG: The forests in Canada and Alaska are, as I mentioned, primarily black spruce. Now, the other Far Northern forests, for example, in Siberia or in Scandinavia, they’re dominated more in Siberia by a tree species called larch. And so these trees are slightly less flammable, so there’s maybe some hope there that the fires don’t impact them as much.

Now, that being said, we’ve noticed in the past couple years that Siberia has experienced some of its most dramatic fire years. We’ve even seen fires survive through the winter and reignite in the spring. It’s unclear, still, how much the fire impact in other parts of the Far North are influencing the carbon cycle.

IRA FLATOW: What surprised you most about what you found in your research?

JONATHAN WANG: I guess I was surprised by how limited the sequestration of carbon was in the Far North. As I mentioned, there’s this notion that the Far North is greening, and we see this through certain satellite-based studies, or certain modeling studies are suggesting that the Far North should be greening pretty rapidly. And so I was expecting to see a much larger carbon sink.

What we saw was that about 400 teragrams of biomass was grown in the Far North during these last 30 or so years. I was really thinking it would be much higher than that, maybe more like 1,000 rather than 400. I was just surprised by how– in fact, it felt like the Far North is almost neutral compared to what I was expecting.

IRA FLATOW: You mentioned before that you were surprised by the carbon sequestration that you found, or the lack of it. You were surprised by the rate of greening that was not happening at the rate that people thought, scientists thought. What does this all mean, then, for climate change models or predictions that may take this into account, and maybe erroneously?

JONATHAN WANG: Yeah, so we actually, in our study, we compared our carbon sequestration numbers with earth system models where these computer-based models would simulate over the same time and the same area. And those models tended to overestimate the growth of biomass by a factor of three. It’s a lot bigger, and what we found is that a large reason for that is that these models do a pretty poor job of simulating fire in the Far North. Most of the models didn’t even bother to have fire as an explicit process, and the ones that did totally did not capture the rate or trends of burning that was occurring in the study domain.

IRA FLATOW: And so that means we have to remodel, right?

JONATHAN WANG: Yeah, that means we have to improve our models. Our understanding of fire is really, as you might guess, driven by what happens in temperate ecosystems like in California. But like I mentioned, the fires in the Far North are pretty different. And so if we are expecting the Far North to sequester carbon and to sort of try and maybe offset some of our fossil fuel emissions, we may have to revise how we feel about how much work the Far North might do in helping us prevent this climate crisis.

IRA FLATOW: That’s really interesting and good to know. Thank you for bringing that to our attention.

JONATHAN WANG: Thank you.

IRA FLATOW: Jonathan Wang, a post-doctoral scholar in Earth System Science at the University of California Irvine.

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