Drier Autumns Are Fueling Deadly California Wildfires

California’s wildfire season is running out of superlatives. The wildfires tearing across the state are breaking record after dismal record—most destructive, deadliest—with no sign of abating soon.

Daniel Swain, a climate scientist at UCLA and the National Center for Atmospheric Research, is among the scientists researching how the nature of how climate change can impact a fire—as heat sucks moisture from plants, rainfall patterns change, and winds drive fires in new ways.

As firefighters struggle to gain control of the three wildfires devastating California, InsideClimate News talked with Swain about the role that climate change is playing in this season’s particularly dangerous fires, and what the future has in store for the state.

This interview was edited for length and clarity.

What are the main causes of wildfires in California?

In a state with 30-something million people, with millions and millions of acres of open land, chaparral and open spaces, there are going to be fires started occasionally. It can be as mundane as a spark from a lawnmower or someone getting a flat tire and running on their rims.

Getting that out of the way, given that there are always going to be people who start fires by accident, the real question is: What happens to those fires once there’s a spark? What happens next? That’s where these other factors come into play like climate change and changing patterns of urban development.

Does it creep along, slowly burning the low grass? Or does it immediately take off very fast, consume all the vegetation completely, jump up into the tree canopies, hopscotching from tree to tree and jumping barriers like rivers and roads? You can have a fire that starts in the same place with the same vegetation, that under different conditions could do either of those things.

Part of what we’ve been seeing with all of these big fires is there are two big weather or climate factors at play—very strong offshore winds, which in California can be extremely strong in localized canyons, especially along the western slopes of the Sierra Nevada and the canyons of coastal areas, where both of the fires are currently burning. Those really strong winds can push a fire extremely quickly.

The other part is what the vegetation is actually like. How dry is it? If it is at summerlike dryness levels, as it is now, then that fire is going to behave much differently than if we had experienced the rain that we typically experience come beginning of November.

There is the primary climate connection. What’s really happening is an indirect effect, but a powerful one through the dryness of the landscape and dryness of the vegetation.

As things get warmer, it’s not just warmer sometimes. It’s warmer all the time. The amount of evaporated water coming out of living plants goes up pretty dramatically even at a couple of degrees. That affects how flammable the vegetation is, which not only affects if a spark catches but also how the vegetation burns. How hot does it burn? Does it completely combust or just smolder? That kind of question ends up cascading back up to the large-scale wildfires. The whole character of the wildfire can change.

How is climate change shifting California’s rainy season, and what impact does that have on wildfires?

This is something that’s partly based on recent research and partly based on observation. In California—and this is a quirk of California climatology—we have a drought every year. It’s called summer. Unlike most places, it does not rain here during summer. Everything dries out. The fire season typically peaks in early autumn and that’s because California has this weird thing where it gets warmer along the coast in autumn because the fog goes away. And the long dry summer has dried out all the vegetation so it’s most dry come late September or October. And it stays that dry until the rainy season develops, which more often is late October or November. Usually by the time November comes around, it has rained considerably.

This year, that hasn’t happened. We haven’t had that rain really anywhere in California, but particularly in the vicinity of the Camp Fire [which swept through the town of Paradise] it was very unusual. That particular four or five inches [that usually comes in autumn] matters so much because it comes at the end of this dry season. The vegetation becomes much less dry and will perk up, and you dramatically reduce the fire risk.

The firefighters often call it a season-ending event. As soon as it rains a couple inches, typically everyone can pack up and go home. We haven’t had that season-ending rain yet anywhere in California this year. A similar thing happened last year, especially in southern California where the then-largest fire burned in late December.

What does the recent data show in California? And how are these changes impacting overall rainfall?

We are starting to see a trend towards drier autumns in California. It’s somewhat new, it’s just emerging from the noise, one might say, but it is actually there. This year is going to add another data point in that direction.

It matches climate projections. There has long been an expectation that California’s so-called shoulder season precipitation would probably decrease—that’s autumn and spring. Now what we’re starting to see is, especially in the autumn, that process now appears to be underway. It’s both an emerging observation but also a projection for the future, a future that maybe isn’t really the future any more.

That actually doesn’t necessarily mean the overall amount of precipitation is decreasing. There’s a growing overall concentration of water in the rainy season. Our research suggests that concentration will be a pretty strong indicator of California’s future climate.

You’ve made the point that it’s problematic to ask whether climate change causes a specific event. Why is that?

In any sort of natural system there’s never really, in any context, a singular cause of anything.

It depends how you define causation, which then is a non-trivial task. It ends up being more meaningful to say, look, we’re going to have fires no matter what. Whether they’re caused naturally by lightning, by totally innocent human error or by more malicious human intent. It doesn’t really matter what started the fire. But the question is, what factors contribute to what happened after the fire starts. The real question is not so much what caused it, because ultimately it doesn’t really matter. The question is what made it as bad as it was.

Then you can get an answer that, yes, there is a link between wildfire behavior intensity and climate change.

As climate change progresses, what is expected to happen with wildfire season?

When it comes to wildfire trends, the last five years in California have really been something else. It’s really been hard to watch. it’s pretty rare to see such large, dramatic step changes as what we’ve seen in California in the last five to 10 years. We’ve broken every record, and we’ve broken them several times. Largest, most destructive, deadliest—all of these have now been set and, I think, set again.

There is no question that we are seeing dramatically larger, more destructive and deadlier fires in California. The question is why? I think climate change is a major part of the conversation, but it isn’t the only part of the conversation. Climate change is increasing, human encroachment and development in high risk zones, all of these things are happening simultaneously and amplifying each other’s effects. But there’s no question that things have reached a fever pitch in California with wildfires.

I hope at a minimum we can stop breaking records for the number of lives lost. But in terms of the size of the fires and the number of structures lost, I think we unfortunately probably haven’t seen the end of this trend.