I grew up in southern California on the edge of the San Fernando Valley. Fires, usually fueled by Santa Ana winds on dry grasses in the hills, were a fact of life. I evacuated. I ran the pump to squirt water from the pool onto the roof. I helped neighbors on the edge of the hills water their roofs. Back in those days, in the 1970s, fires over 100,000 acres (~150 square miles) were rare. The worst fires destroyed 200 to 300 homes. Rarely, someone died. That was then.

Twelve years ago, four researchers examined a comprehensive database of all large wildfires in western United States forests since 1970. They detected a signal and concluded that “large wildlife activity increased suddenly and markedly in the mid-1980s.” Looking at weather data and land-use history, they concluded the driving factor was “increased spring and summer temperatures and an earlier spring snowmelt”. Forest management, on the other hand, had “relatively little effect”. Published in the journal Science in 2006, they had found the “force multiplier” that climate change brings to the West. We all knew that forests had been mismanaged for nearly a century, and that too many homes and towns had been built up against wildlands. But now there was a new factor driving fires—longer hotter drier summers. If you’re looking for numbers, the data show that fires really ramped up in 1987. The so-called “force multiplier” of climate change was big—about 3 or 4. That is, wildfire frequency was triple in dry years when compared to moderate years, and quadruple that in wet years. Ominously, they noted the effect was non-linear, meaning that, in warmer years, fires really increased. Thus, the multiplier was not just a constant number—it increased with temperature and lack of rain. The Berkeley Tunnel fire, which was exceptional because it killed 24 people trying to evacuate and was the first fire in California history to burn over a thousand homes (it actually burned over 2,000 homes), occurred in 1991.

In the twelve years since the Science article, mega-fires, in terms of acres burned, structures destroyed, and people killed, have gone off the charts—literally. When it comes to fires that burn more than 200,000 acres (~300 square miles), destroy more than a thousand homes, and kill dozens, if not hundreds of people, climate change is not really a “force multiplier”, it’s an on-off switch. Such fires were very rare or non-existent before the year 2000. Now, they appear to be annual.

We’ve crossed a threshold, tripped a wire. In 2017, after the astounding loss of 6,000 homes in Santa Rosa, we hoped that was an outlier, a blip in the data. A few months later, in December, when fires were previously unheard of, the Thomas Fire became the largest in state history. Barely six months later, the Carr Fire made Redding “the new Santa Rosa”. After that fire I posted a chart showing that 16 of the state’s largest 20 fires had all occurred in the past 20 years. We had a reached a “new abnormal”. Then, a few months after that, in November, when by all historical standards the fire season should be over, the Camp Fire literally wiped out the entire town of Paradise, population 26,000. Hospitals, high schools, stores, and houses, all gone. The death toll is without historical precedent. While that was burning, the Woolsey Fire became the largest and most destructive fire in the history of the Los Angeles area. At the present rate, next year the Paradise inferno will be surpassed by some hell unimaginable.

When I say the Camp Fire is the largest climate-induced mass mortality event in modern history, I’m not counting hurricanes. Hurricanes, even large hurricanes, have always occurred and always will. A Category 5 hurricane striking a major city is an inevitability. Yes, climate change has made hurricanes larger and more numerous, thus increasing the risk, but nothing like the change we’ve seen with fires in the West. Thus, attributing any one hurricane to climate change is like attributing a single specific cancer case to an environmental contaminant causing a cluster of cases.

When I say the Camp Fire was caused by climate change, I’m not saying that future destructive fires are inevitable. The fires will come, but we can do things to mitigate the

destructive aspects. A full post-mortem on the Camp Fire and other recent mega-fires is of course required. Such analysis should look beyond the political rhetoric of Trump, exclusively blaming management practices in national forests. Most of these fires in these graphs, spreading across dry grass and oak woodland, had nothing to do with forests. Furthermore, in the face of massive tree death from drought, simply removing dead trees from forests has serious feasibility limitations. To quote a forest fire expert colleague, “Yes, fuels reduction is needed same as 30 years ago, but the mills are all full from the tree mortality supply, much of the fuels are not merchantable, and we are not going to cut our way out of longer fire seasons and deadly MegaFires.”

In the short run, we can’t stop the changing climate, the record low humidity and record high dry vegetation, or the longer summers. We can’t shorten the fire season, now 80 days longer than in 1970s. But we can modify power lines, conduct preventative burns, revise urban fire perimeter requirements, and re-evaluate evacuation routes. Those things won’t turn off the switch we’ve triggered, but they might at least save some homes and lives in the coming years.