The best scientific explanation: Bad trends plus bad luck.

Adam Sobel, an atmospheric scientist and director of the Initiative on Extreme Weather and Climate at Columbia University, gives an explanation:

This increasingly awful fall fire season follows hard on the previous two, in 2017 and 2018, both of which were worse than any in recent memory. Some of the same regions, and people, are being affected repeatedly in quick succession. Psychic trauma is surely compounded significantly for these residents, not to mention the firefighters on the front lines.

How should California residents think about the future, when the present is, suddenly and persistently, not only far outside their prior experience, but also, in its scale and velocity at least, beyond what science had predicted?

It’s an increasingly common experience, occurring with other kinds of events as well. Heavy rain events are becoming heavier around the world, but known climate trends can’t explain the repeated 500-or-more-year floods that Houston has seen in the last few years.

In the case of the increasingly frequent wildfire disasters in California, I argued the other day that they have multiple causes: poor maintenance by PG&E, expanded human settlement at the margins of fire-prone woodlands, and global warming. But I don’t think any of them explains either the suddenness or the persistence of the change that Californians have experienced in the last three years.

When it comes to the weather, and the climate, my views here are strongly informed by discussions with my colleague Park Williams, an expert on wildfire and climate whose research is directly relevant. That research shows that the area burned by fires each year in the summer months has increased drastically, and this is consistent with the influence expected from global warming.

But, as explained by Dr. Williams in his recent research article, and in The Times on Friday, the headline-making fires of the last three years have all occurred in fall. In that season, temperature has a role, but other factors are likely to be more important — first and foremost, the dry Diablo and Santa Ana winds.