How Did the Los Angeles Fires Get So Out of Control?

The fires in and around Los Angeles have already claimed dozens of lives, destroyed thousands of homes, and led to evacuation orders for hundreds of thousands of people. The economic damage is projected to be as much as a hundred and fifty billion dollars. Daniel Swain is a climate scientist at U.C.L.A. and the University of California Agriculture and Natural Resources who studies extreme weather events and their connection to climate change. He and I recently spoke by phone. Our conversation, edited for length and clarity, is below. In it, we discuss what really caused these fires to rage out of control, how he tries to communicate the role that climate change plays in natural disasters, and how the Los Angeles area became so vulnerable to catastrophe.

What makes these fires unique other than the extent of the damage they have caused?

I do think the sheer scope of the damage in terms of the number of structures lost and the economic losses is what jumps out the most. That said, there are other aspects of the over-all situation that are very different from anything we’ve seen before, too.

Let’s start with what’s not unusual, since I think that’s helpful for setting the stage. It is not unusual for strong dry winds to occur in the mountains and valleys near Los Angeles in January. This particular event was notably strong and did extend across more of the high-population and lower-valley regions than usual. But if you were to pick a time of year when you would expect to see strong dry winds, it would be now.

These are the Santa Ana winds you’re speaking of?

Yeah, these are Santa Ana-like. These were a little bit different from a traditional Santa Ana event because they were driven by a slightly different low-pressure system in a slightly different spot. That is partly why the winds are coming more from the north than from the east, and also why they were stronger than usual and reached deeper into the valleys than usual. But if you can think of it as being Santa Ana or Santa Ana-adjacent, it’s the same general idea. We only see winds that strong every five or ten years or so. So it’s notable for sure, but it’s far from unprecedented in its own right.

The preconditions, though, were drastically more unusual, bordering on unprecedented—specifically how dry conditions have been. And that is essentially quantified by looking at how much rain has or has not fallen in the Los Angeles area or San Diego area. And what we find is that this is now either the driest or second-driest start to the season on record throughout Southern California, going back a hundred years. In modern history, it has not been this dry this late in the ostensible rainy season.

That is something that really sets the stage for these fires, because had these same winds occurred following, say, an inch or two of rain so far in the season, even if that’s below average, it’s still a good soaking. If that had occurred, we wouldn’t be seeing the fires that we’re currently seeing. We wouldn’t have that explosively dry vegetation. It essentially has not rained in Los Angeles since last spring—in many areas, about a tenth of an inch or less, which is insignificant from a wildfire perspective.

On top of that, the inland parts of Southern California—the mountains, the elevated plateaus, and the desert regions—experienced their hottest summer on record. The city of Los Angeles did not, to be clear. But then, in early September, even the city of Los Angeles and really the entire Los Angeles basin did experience a record-breaking heat wave. And that was actually associated with the major wildfire outbreak at the time, if you recall, in early September. Many structures were destroyed. It remained unusually warm and hasn’t rained at all since then. That is the real anomaly here, with the winds being sort of a second-order anomaly.

Anyone can understand why a dry twig is easier to burn. But is that too simplistic a way to think about why dryness leads to fires that can get out of control? Is there another level on which it is dangerous?

There are different levels, although I think the basic intuition gets you a good part of the way there. Imagine trying to light a damp log for a campfire. It’s not going to happen. There’s this binary switch, where if vegetation is too wet, you can’t get combustion at all. There won’t be any flame. You light a match and it’ll sputter out. You can’t start your campfire.

Then there’s another tier of dryness, where the wood might be not damp, but also not particularly dry. It’s somewhere in the middle. And if you’re very good at starting a campfire, you can probably get that wood going with some preheating, right? So if you get some twigs and some grass, the wood will eventually catch because the flame below it kind of dries it out enough.

But there’s also another level of dryness where the vegetation is extremely receptive to a spark. And not only does it make it more likely to ignite in the first place but it also greatly increases the actual intensity of the subsequent combustion or the fire. And by intensity, I literally mean the amount of thermal energy that it outputs. That matters a lot because it dictates not just how intense the flames are and how much damage they can cause in situ but it also affects how quickly those flames can spread. The more intense the fire generally, the faster potential spread it can have. And also, it increases the propensity of the fire to start to generate its own localized weather conditions which can sort of self-amplify.

So there’s these self-reinforcing, vicious-cycle feedbacks. Vegetation can be too wet to burn at all, in which case the risk of wildfire approaches zero, right? And then, there’s this other condition where it’s dry enough to burn, but maybe it won’t burn very intensely or very readily. And then, there is what’s known as critically dry vegetation, which will burn essentially at the drop of a hat and burn with great intensity, facilitating all of these cascading positive feedbacks in terms of increasingly extreme and exotic fire behavior, which are of course compounded if you have hurricane-force winds acting on those critically dry fuels, as we did this week.

I have seen you mention something called “atmospheric thirstiness,” but wasn’t sure what you meant. Is that connected to what you’re talking about?

It is, and the main connection between increasing wildfire risk and climate change is actually through this same process, the aridity or the dryness of the vegetation itself, which is of course what eventually becomes fuel for wildfires. By definition, a wildfire is a fire that’s burning vegetation. And there are really two ways to dry out vegetation pretty quickly. One of them is to, on the supply side, not deliver any water. In other words, if there’s a lack of rain or snow, lack of precipitation, the soil becomes dry and the plants don’t really have water to work with in the first place.

There’s also another way, which is even if there’s plenty of precipitation, but if there’s an excess of evaporation and transpiration—evaporation being the passive part of the process and transpiration being the part of the process that happens when water passes through living plants and comes out the stomata in the leaves—there is something known as atmospheric evaporative demand. That’s the technical term for thirstiness, and it essentially represents the propensity of air to extract water or evaporate water from surfaces or from plants. Surfaces can mean bodies of water or it can be the soil. Soil usually has at least some water in it, and so therefore water that can potentially be evaporated, and will be evaporated at an increasing rate the greater the evaporative demand or thirstiness.