What's Driving Our Larger, More Destructive Fires?
This article is part of our four-part series “Forests, Fires, and People,” presented by the Institute for Science and Policy and the Center for Collaborative Conservation, with support from Gates Family Foundation and in partnership with the Denver Museum of Nature and Science, the Colorado Forest Restoration Institute, the Southern Rockies Fire Science Network, the Colorado State Forest Service, and the Climate Adaptation Partnership. Find all series episodes on our YouTube channel.
Wildfires are a natural phenomenon and have long played a key role in promoting the broader ecological health of forests. But for humans, living amidst fire is a delicate balancing act. After a century of suppressing any wildfires and population growth driving more settlement in wildland areas, the risk of catastrophe from any given blaze has only increased. On top of that, climate change has created hotter, drier conditions, turning already burn-prone western forests into a tinder box of available fuel just waiting for ignition. The result: a new era of larger, more destructive megafires that threaten homes and lives like never before.
In the first episode of our series, Tim Brown, Research Professor of Climatology at the Desert Research Institute, and Camille Stevens-Rumann, Assistant Professor of Forest & Rangeland Stewardship in Colorado State University’s Warner College of Nature Resources, discussed fire behavior and climate factors with Institute for Science & Policy Director Kristan Uhlenbrock and Center for Collaborative Conservation Director John Sanderson. Below are some of the key takeaways from the discussion. Watch the full recording here.
Fires have predictable ingredients, each with big impacts
CAMILLE STEVENS-RUMANN: The classic “fire triangle” is made up of the weather and the climate, fuels and landscape aspects that are contributing to how that fire might burn. In a lot of the fires that have happened in the past, even those large ones from Colorado like the Hayman fire, a burn predominantly in lower elevation forests where we had a lot of fire suppression activities that really changed what those forests look like.
One of the really interesting and somewhat concerning things that happened in 2020 was that these fires burned through much higher elevation forests and interacted with bark beetle outbreaks, as well as forests that were historically much more dense. And so the type of fuel those fires burned through is really not something we've seen here in Colorado up until 2020, at least on this scale. And not only were there lots of fuels, but also lots of that fuel got dried out through many months of a hot, dry summer.
And then the final aspect is that wind. You can't do much as a firefighter when you're dealing with a 60 mile per hour wind. There's just not very many actions that can be taken reasonably to try to stop a fire when those kind of synergies happen.
TIM BROWN: In the Medicine Bow National Forest, a spruce fir forest, the fuel dynamics are probably very different than they would have been even 20 years ago before all these trees had died. Also, it presents a very big risk for firefighters as well. You don't want to be standing amongst all those dead trees that are now even more weakened by burning.
In the Arapaho-Roosevelt, you see a kind of intermix of live and dead trees. These trees are all very close to one another, and historically, this is how these forests would have been. This isn't a product of bad management or lack of management, this is what these ecosystems look like. So these kind of cyclical mortality events have happened in the past.
It's very difficult to get in and actually cut logs, remove that fuel, change the likelihood of a fire going over the top. No matter how much you try to build those fire lines through this, very little is going to happen when you have 60 mile per hour winds because of those climate and weather conditions.
Climate change has increased the probability of disastrous fires
TB: There's a difference between weather and climate, and like Stephen Colbert said once upon a time, climate is 1,000 little weathers. It’s that longer term condition that enables fire, so the drying of the fuels, the curing of fuels...versus the weather which is a driver.
Colorado, and actually much of the West, has been in moderate to severe to even extreme drought. So this has been going on for a while, and this was setting up the dry conditions for fuels because this would be also related to reduce soil moisture, which is the reduced water availability for plants to uptake.
A good analogy to think about how these two work together is to imagine a Christmas tree in your living room. You have a bucket of water underneath the tree and that tree is uptaking that water. But at the same time, the air in the room is warm and probably dry. And so that's taking that moisture right back out of the tree. And sometimes what happens is that that evaporative demand – or, taking that moisture out of the tree - happens at a lot greater rate than what the tree is able to uptake. So the tree will become very stressed by that. And ultimately what that does is increase fuel.
There are four elements that go into this evaporative demand: temperature, relative humidity, wind, and solar radiation. And so it was anomalous warm temperatures and a dry atmosphere that's really rapidly increasing the fuel flammability. And so if a fire is going to ignite in this, the vegetation is primed. And so that was the setup for the three big fires of 2020.
A forest’s elevation and characteristics matter, but megafires could change the dynamic
CSR: A lot of these places that we've had large fires have been lower down in elevation, these kind of ponderosa pine-dominated forests. You can think about the Hayman or the Hyde Park fire or even the Four Mile fire being kind of this type of forest type that historically would have been more open and probably had more active fires that were low to moderate severity - burning fairly frequently, not killing all of the trees, keeping a low density forest on the landscape.
As you move like up in elevation, you transition into these different forest types. I think Colorado is one of those places where this transition is probably more clear and those lines are more defined than other states. The next one is lodgepole pine, which historically would have had a higher intensity, higher severity fire regime that those trees are very adapted to.
As you move even higher in elevation up to spruce fir forests, we end up with forests that are really dense closed canopy forests with a mix of species, lots of fuel on the ground. In some cases, those burned on a 175-year timeframe, up to several hundred years. And that’s what we're getting at when we think about what was limiting versus enhancing these fires, is that a lot of times what stopped fires at those higher elevation forests before is that the climate and the weather wasn't conducive. They were moist, the snow would have stayed on those sites for much longer. Think about those years that we've had where you can go backcountry skiing in June, you're still tromping around at high elevation through a foot of snow. Well, that didn't happen really this past year.
As you move up that mountain, you go from a fuel limited system to a climate limited system, meaning that at the bottom - historically, at least - those forests would have been more open and the fire would have been limited by the fact that there’s just not enough fuel to carry that fire every year or be that extreme. Whereas, at high elevations, it's really been limited by climate in the past, and that's why we're concerned about the pattern that we saw this year with that evaporative demand change and things like that, because we're suddenly dealing with a system that we would have thought the fires would have been mediated by climate are now mediated by that large amount of fuel that's up there.
Drought doesn't help
TB: It's not particularly unprecedented to have fire periods even into October. In terms of the drought monitor, I think if we were to take that back several centuries, sure, we're gonna see periods of dry like that across the western U.S. Maybe we're in or starting to be in something called a mega drought, where it's actually a century-scale kind of period of predominantly dry conditions. That doesn't mean there won't be wet years or even a series of wet years. But overall, yeah, it could be more on the dry side.
The West is naturally dry. Fires are a natural occurrence in the West, so these things are going to align pretty frequently. The beetle kill, for example, I mean that's going to be partly driven by climate.
CSR: I think we do see alignment with hot and dry conditions leading to pretty widespread fires. Even on a centuries long scale, alpine lakes across Colorado show a pretty distinct pattern of dry conditions, leading to pretty large fires that have been in many cases actually led to differences in forest structure or a switch from a forest of vegetation to an alpine meadow in some cases.
Humans and lightning are the primary sources of ignition
TB: For most of Colorado, it would be predominantly lightning-started fires at these elevations.
CSR: We put out about 98% of fires that start, so it's really only that 2% of fires that make it past that first 24 hour period, or initial attack period to that to be these large events. As you think about the amount of resources and people that we have that we employ in the United States to fight fires, they're, they're pretty effective, even though what we see on the news does not necessarily demonstrate that.
And it really has less to do with the people and more to do with those like weather conditions that are driving that 2%. When the conditions are so extreme, it doesn't matter how many people you put on it. You’re probably not going to be extremely effective in putting it out.
TB: From my perspective, I think of it as the confluence of climate, people and fuels. Camille's talked about the fuels a bit, and the role of 100 years of suppression to help build up fuel availability from a climate perspective. There’s no question that the West is warming. And there's also no question about how that warming exacerbates the conditions, because the warmer it gets, that will draw moisture out of the lake, water supplies out of the ground, out of the vegetation, and so it's a feedback process. And so we know that that's going on. And then from the people perspective, it’s humans that are causing the warming. So it all comes full circle. You might imagine making arguments that all this fire activity was all human-driven, really.
Fire season is now close to year-round
CSR: Another factor is the fire season length, just the pure length of time that fires are starting is increasing. In the past, even when you heard about California fires being a bad year, it would be just a short period of time and now we hear it from January to December, almost. We see that the snow is melting earlier, resulting in the drying of the fuels, which makes them available and ready to burn sooner and that fire season is stopping later. That kind of lengthening was one of the things that was really shocking about last year's fires, even for me. I was like, well, okay, it's August, I still expect some fires. September, it's kind of late but, yeah, maybe we're still within the fire season. But when we're having those big fire runs and the East Troublesome fire in October, that's pretty rare for this type of forest and really across Colorado. We don't usually see those huge fire spreads when we're moving that far in the fall.
The other thing we've seen in the literature that we can contribute specifically to or correlate to climate change is things like fire size potentially increasing. This is somewhat of a small window that we have data on. We're not talking about long historic fires but at least in the last 100 years, we see larger fires, and even over the last 30 years, we see an increase in the amount of high severity fires where we see 100% tree mortality - just a black moonscape.
Wind-driven fires move fast, posing challenges for prediction
TB: Wind is usually, but not always, the factor in rapid spread. I don’t know right off the top of my head, but I think I heard that the Camp fire in California basically burned a football field a second. It's not the wind speed that's hard to predict, it’s what are the fuels are going to be, the moisture of those fuels...all these different pieces need to be modeled and combined in a way that you can predict a spread rate given a wind speed. Dry cold fronts with strong winds were major factors in all three of the big fires last year in Colorado.
CSR: The East Troublesome fire moved at 6,000 acres an hour, which is pretty incredible. I think the other thing that we haven't talked about in regards to wind and weather in those big fire events is that to a certain degree, these fires, once they reach a certain size, start to create their own weather. So on top of driving more landscape-scale patterns like driving wind direction and wind speed, we start to see a lot of interplay and weird weather dynamics that happen. That’s kind of what makes it unpredictable in the moment, that fire itself is going to create the weather dynamics and push the fire in different ways.
Is it unusual for fires to jump the Continental Divide like we saw last year?
CSR: I think we have seen droughts before with fuel dynamics and weather dynamics that could have caused a fire to jump the Continental Divide. But that's pretty rare. If you have a fire that's able to jump the Continental Divide, you're probably not going to be effective at even saving hundreds or thousands of acres. We do have some previous evidence that that happened. Going back longer in history, it's a little bit harder to know that information. We have point data. But we don't have a great sense of how often that may have happened before.
There are still a lot of unknowns
CSR: A lot of my work is looking after the fire happens, how these ecosystems are going to recover. That’s very much an unknown with climate change. For a long time, we expected these ecosystems to come back as forests and that they would naturally regenerate. And that's something we just don't have a good sense of, especially across all these forest types. How do we plant, what do we plant? What's a site suitable for now versus what it was suitable for even 10 or 20 years ago? So I think that will continue to be a big issue, especially as we just deal with the fires from 2020.
In relation to subsequent fires, we know with climate change that there's a likelihood of increasing fires, but we don't know for a fact that fuel will become limiting in some of these places. You have to have fuel for there to be a fire and at some point, if it's burning all the time, you won't continue to have fire spread. That’s one thing we haven’t yet unraveled.
TB: To me, what really sticks out is understanding this holistic and synergistic nature between the fuels, the fluid dynamics of fire, the smoke, and the impacts on human health. The second area I'm keen is the relationship between the dynamics of soil moisture and this evaporative demand by the air. That's been used since the 1970s as “fire danger.” Those Smokey the Bear signs that you see alongside roads entering parks and forests - that's fire danger. And the third thing is really a mix between physical science and social science, and that is, I think there needs to be some changes and improvements to the red flag warning system that the National Weather Service uses. It was initially built for firefighting agencies but this is now also very much used publicly. What should that system be like, particularly as these fires are more intense.
Science and technology are important. But there’s also the societal and political will once you have that information to implement those actions. And so, what are you going to do with that information as an agency or as a community?
CSR: I think one of the big things that we're going to have to tackle in an increasingly flammable world is how we use fire to our advantage. We can only do so many treatments, We can only do so many prescribed fires. We should be using fire. We have many land stewards that worked on this across North America that used fire very effectively for many uses. And I think we're going to have to be comfortable with more fire use as we go forward if we really want to be effective in mitigating the potential for these large fires.
I think that comes back to that other 98% of fires that we put out right now. Those can be used. And I think there's a lot of science around that on where and how and under what conditions is that appropriate given the wildland-urban interface. It comes down to an acknowledgement that fire is going to be a part of this ecosystem. It doesn't matter how many bad fire years we had, we're still going to have more fires. And so it's how we can harness them ourselves versus allowing them to be the manipulators of the landscape.
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