A “fireside chat” with Woodwell’s fire and climate experts

Drs. Machado, Rogers, and Zobel talk about the links between fire and climate change

Sarah Ruiz, Science Writer

We see the same headlines every year now. Fires—in the Arctic, in the Amazon rainforest, edging dangerously close to human communities. Forest fires play an important role in the climate change story—as both a symptom of warming temperatures and a cause of them. Woodwell Climate Science Writer, Sarah Ruiz, recently sat down with three of the Center’s leading experts on climate and fire to unravel that relationship— covering topics including the way fire differs between ecosystems, what is “fire weather,” how is climate change altering fire regimes, and how do we combat the issue to keep carbon locked away in forests and soils. Below is an excerpt from that conversation.

SARAH RUIZ: Brendan, you work primarily in boreal forests, where fires are a natural part of the landscape, correct?

DR. BRENDAN ROGERS: Yes, that’s right. So even though boreal forests are in the north and they’re cold and damp for a lot of the year, the surface vegetation in the soil, the soil organic matter, can dry out pretty dramatically in the summer. This fuel, the term for it in fire science, often just takes one single ignition source to generate a pretty large wildfire. Humans certainly ignite fires, but still most of the burned area in boreal forests is coming from lightning ignitions.

Fire is also an important natural process in boreal forests. Many of the fires are what we call stand replacing—meaning they’re high intensity, they kill most of the trees, at least in Alaska and Canada. This initiates the process of forest succession, with often different types of vegetation, and tree species playing pretty key ecological roles. But fire regimes are changing and intensifying with climate change, taking us outside the range of what we would consider our natural variability that we’ve seen in these systems for millennia.

SR: Now, Manoela, you work in the Amazon rainforest, where fire is never a natural part of the landscape. Can you explain what kind of role fire plays in a tropical rainforest?

DR. MANOELA MACHADO: The Amazon biome did not evolve with fire pressure selecting for strategies of survival, which means that the plants are not adapted to this disturbance. Fire is a very powerful tool used to transform the landscape and has been used for millennia. Traditional and Indigenous communities still use it for agricultural purposes, but that’s not the fire that we see on the news, making headlines of “fire crisis in the Amazon.”

Those catastrophic events with lots of smoke in the atmosphere, they’re normally related to deforestation fires, which are fires used after clear cutting to clear out biomass and use the land for cattle ranching and other agricultural purposes. Those fires can escape into forest areas. So the ignition sources are always human—there are no natural ignition sources in the Amazon forest.

SR: With climate change, these dynamics are shifting in many places, as drier and hotter conditions make it easier for fires to spark. Zach, could you talk to us a little bit about what makes a forest susceptible to fire, and how climate change might be affecting that?

DR. ZACH ZOBEL: Fire weather is a given set of atmospheric parameters that indicate—if there was an ignition source—fire would be able to grow and spread rapidly. What we do is we model what is known as the fire weather index. This index consists of four different atmospheric variables, and those are: temperature (the hotter it is, the more likely vegetation is going to dry out quicker); relative humidity (the lower the humidity, the more rapidly vegetation can dry out); precipitation, both backward looking (“has it rained a lot recently?”) and today; and wind speed, because once a fire starts, if the wind is adequately high, that’s when it’s going to spread.

We take those variables out of the climate models, and we model it—what it looks like historically, versus what it’s going to look like in the future. And what we find is that in several fire regimes, in most of them actually, these “high fire risk days” are starting to rapidly increase.

We see it especially in the Mediterranean, Brazil, eastern Australia, the Western United States, in several parts of Africa. Over the next 30 years, we think these high fire risk days are going to increase on the order of a couple of weeks in some locations like the Western US, to upwards of one to two months in the Mediterranean and Brazil. And that’s pretty significant, when you think about how historically these days only occurred maybe one week a year.

SR: So what are some of the risk outcomes posed by those more frequent, intense fires, globally?

BR: More frequent intense fires are changing the ecology of many boreal forests, and in some cases, leading to transition from forest to grassland or shrubland, which of course impacts the resident animals. But there are also large impacts on humans. The smoke from large wildfire seasons is a direct threat to human health, and rural and especially Indigenous communities often feel the largest impacts. Additionally, in areas of permafrost, which is ground that is frozen year after year, fires can lead to permafrost thaw for many years. That can often destabilize the ground leading to ground collapse, presenting a hazard to people that are living in these areas.

MM: I think the Amazon has many similarities with the Arctic, despite being very different environments. Aside from not being natural, fires have become a recurrent issue that coincides with the dry season, which then creates what we call the burning season. Any fire is damaging to an environment that is not adapted to it. So there’s the immediate release of huge amounts of carbon when that biomass is burning, and there’s the delayed mortality that understory fires cause, so emissions of carbon continue after a fire as well. That can cause a shift in species composition.

And fire also begets fire, which means that forest canopy that is disrupted allows more wind and sun to penetrate the forest, which creates drier microclimates. And tree mortality increases the fuels on the forest floor as well. So a degraded forest becomes even more vulnerable to future burning. As Brendon mentioned as well, there are several studies linking the burning season with higher hospitalization rates of people with respiratory illnesses as well.

SR: How much of the fire headlines we have seen in recent years—the 2019–2020 Australian wildfires, the incredibly destructive Camp Fire in California—is on par with what fire weather models have shown?

ZZ: When we talk with our partners, we always show them how rapidly the climate models are viewing this increase in fire weather days, but we definitely caveat it by saying, “Here’s what the observations are showing us. The climate models aren’t even keeping up with how quickly wildfire risk days are increasing.” This is the best case scenario for the next 30 years, and the best case scenario is scary enough…

Increased fires not only have immediate ecological and safety impacts, they also represent a significant and growing risk to our ability to achieve our climate goals, which would bring fires back to a manageable level. Forests are one of our most valuable carbon sinks and keeping them healthy and standing is essential to curbing warming.

header image: Fire in the Pantanal region of Brazil. / photo by Manoela Machado