Whiplash: How Big Swings in Precipitation Fueled the L.A. Fires

Elizabeth Kolbert: You predicted a lot of what has played out over the last week in LA. What did you see that made you particularly worried?

Daniel Swain: It was pretty clear about a week in advance that a strong, dry windstorm was at least a possibility across Southern California. What really matters for fire risk is the overlap between critically dry vegetation conditions and those [wintertime] Santa Ana winds. The winds, you can kind of view it as an act of nature — kind of random, not really any climate change links that I can see. But the overlap, which is really what matters, that’s where the climate change connection comes in, which is the fact that we do know that we’re seeing hotter summers and drier autumns.

Kolbert: You’ve noted that Southern California isn’t just getting drier. It’s seeing wider swings between wet and dry years. Is that a factor in the fires?

Swain: What we essentially had were two exceptionally wet winters in a row in coastal Southern California, each of which was not necessarily record-breaking in its own right, but together were essentially the wettest two-year period observed in some parts. And that matters because a lot of what’s burning is not forest, but it’s grass and brush, chaparral, the quintessential stuff of Southern California.

So, after two wet years, there was a tremendous amount of additional grass growth. In grasslands, there’s about twice as much biomass as a result of those wet conditions as there usually would be, which then literally becomes fuel for the fire. And then September came and we had a record-breaking heat wave. That helps set the stage by bringing about a period of exceptional drying for the heavier fuels, the trees and the heavier brush.

But the most striking thing over the past three or four months is that it just hasn’t rained in L.A. or anywhere meaningfully in Southern California. We’re talking about a widespread part of Southern California that since May has seen under two-tenths of an inch of rainfall, which is now the driest start to the season on record for most of Southern California.

Kolbert: Let’s talk a little bit about the idea of “hydroclimate whiplash” — sudden large or frequent swings between very dry and very wet conditions — which you wrote about recently in the journal Nature Reviews Earth & Environment.

Swain: The underlying idea is that the water-vapor holding capacity of the atmosphere increases exponentially as a function of temperature.

And in that article, my coauthors and I use the analogy of the expanding atmospheric sponge to describe how this works, which is that not only do you raise how intense precipitation can become, but then in the reverse direction, that increased water-vapor holding capacity also means that when the atmosphere is not saturated, it actually has an increased thirstiness, if you will. It means, in this context, the landscape could become drier faster. The “thirsty” atmosphere steals water away from plants and from humans and animals fast, unless you’re delivering more water consistently.

So that’s the expanding atmospheric sponge effect. It really drives the majority of what we’re talking about when it comes to hydroclimate whiplash. Local wind pattern and weather pattern changes can accentuate or offset to different degrees. But, overall, that’s the main mechanism.

Kolbert: And whiplash refers to the difference between a very dry period and a very wet?

Swain: Essentially the rapid or high-magnitude swings between wet and dry, and vice versa, in some broad sense.

Kolbert: And do we see that happening pretty much across the globe?

Swain: Overall, it appears [whiplash] has already increased globally. The more robust finding is that by the time we get to 2 degrees C of warming, or even 1.5 degrees of warming, this should emerge much more clearly across virtually all populated areas, with rather few exceptions.

It really does feel like this is one of the signatures of global warming that will be nearly universal in places where people actually live.

Kolbert: So in the case of LA, where you had all this growth of brush from the wet years, should there have been a focus on getting rid of it as soon as possible so that the fuel doesn’t build up? Is there any practical thing that could have been done to reduce the fire risk?

Swain: Since last spring? Essentially, no. It’s the whole landscape. It’s every freeway median, everybody’s backyard, every city, county, regional park, everything, everywhere. So the scope of it is vast. Obviously, in the long run, at scale across the American West, one of the most promising interventions is scaling up prescribed burning and cultural burning. Because we know that there is, at least in the forested regions, a clear deficit of fire, that the ecosystems would probably benefit from more fire. And if we could achieve more of that burning on our own terms, we’d see fewer of the destructive fires.

That’s less obviously true of chaparral [in Southern California]. It’s not easy to do safely because chaparral burns very intensely. And if it’s nestled in between people’s homes, and then these parks that are right up against suburbs, there is some real risk.

Kolbert: Is there anything that you think should be done differently in future cases like this?

Swain: One of the benefits of having a really good weather prediction is that there was a very high level of concern in the days leading up to the event. And so firefighting resources were prepositioned from around the state. They drove to L.A. County and sat in parking lots near where they thought fires might break out. The aircraft were moved to firefighting airport facilities in Southern California in anticipation of this wind event. So I don’t think that there really could have been a lot more of that.

People complain about the water supply and then they forget that what happens when you put several hundred fire engines all drawing water through hoses from the same water main. There’s only so much water that can flow through the main at one time. And once the fires started burning structure to structure, think about what’s in each of those structures: water pipes, people’s kitchens and bathrooms and laundry rooms. When those houses and commercial buildings burned, all of those pipes melted or burst. Now you have thousands and thousands of major water leaks at the same time that you have hundreds of fire engines drawing upon the city supply.

Overall, I don’t see this as a failure of firefighting or planning. I just see it as an indication of the limits of what you can achieve when conditions are this extreme.

Kolbert: California Governor Gavin Newsom has already signed an executive order making it easier for people to rebuild. It’s likely we’re going to get this rush to rebuild. I wonder whether you think that makes sense or whether we should be pausing for a pretty radical rethink here.

Swain: I mean, obviously it’s difficult to tell people who just lost everything to hold on a minute, but if we don’t do that, then we’re just going to rebuild in a way that allows this to happen again in exactly the same places.

In the wake of the Camp Fire [in Northern California in 2018], that was an entire community essentially wiped off the map, and most people have not returned. But a lot of the buildings that were rebuilt were rebuilt the same way they were built before they burned down. I was up there earlier this year, and there’s a whole lot of structures in that burn footprint that have wooden fences and dense brush running right up against the house.

Should people be forced to move away from the high-risk zones? Well, good luck with that in a place like California where a quarter of the population lives in those places. Where are you going to put all those millions of people?

I do think the question is a bit different in cases where a whole community has been wiped out by fire, and there’s a question about rebuilding. These handful of properties right on the margin [of wildland in Los Angeles,] you know it’s going to happen again in those places. Should the city buy out those properties and make it a park with a fire break in it instead? That’s still a difficult question because people have lived there. Individuals own property, and that’s tricky.

Kolbert: If you had to name some things you should do when you rebuild to reduce fire risk both for yourself and your neighbors, what would they be?

Swain: There’s some things that are sort of no-brainers, such as structures having clearance around them from obviously flammable stuff, whether that’s dry brush or overhanging trees or wooden fences or wooden decking. I made my parents put ember-resistant vents on their home. That’s a pretty high-yield, low-cost intervention that a lot of people can just do themselves for the cost of purchasing fire-resistant vents.

Kolbert: We just recently got the 2024 global climate data. It was the hottest year on record, 1.5 degrees C over the preindustrial average. Looking into the future, beyond 1.5 degrees, what happens in L.A. and Southern California?

Swain: All I can say is that the past 10 or 15 years in California really have been a poster child of what we expect to see more of in the future, which are these really wide swings between wet and dry. We will see more whiplash in California. Not every winter will bring extreme winter fires. But it’s not a question of what does the average year look like, it’s what do the bad years look like? And the bad fire years are clearly getting worse. The driest years are clearly getting drier and hotter.