Where There’s Smoke, There’s Fire

This wedding photo from California went “viral” in a visible sign of how there’s no escape from California wildfires.

Recent research from Williams et al. takes a detailed look at how fire activity has changed in California. They identify trends and study relationships with climate factors while also considering non-climate factors, for four separate regions in California as well as their whole, and within individual seasons. They live up to the high standard they set for themselves when they say,

A thorough and nuanced understanding of how, when, and where anthropogenic climate change has or has not affected wildfire in California over the past several decades is critical to guide sustainable societal decisions ranging from where to develop housing to how limited resources can be optimized for landscape management.

They make it clear that California’s wildfire situation has reached crisis levels:

Perhaps nowhere on Earth has received more attention regarding recent wildfire trends and their causes than California. One reason for the attention is that increases in statewide burned area over the last several decades were dramatically punctuated in 2017 and 2018 by particularly extreme wildfire activity with substantial loss of life and property. In 2017, modern state records were set for the largest individual wildfire (Thomas Fire: 114,078 ha) and the most structures destroyed by an individual wildfire (Tubbs Fire: 5,636 structures), which led to 22 fatalities (CalFire, 2018). The total area burned in 2017 was also nearly a state record at the time (505,293 ha), behind 2007. In 2018, state records were set for total area burned (676,312 ha), largest individual wildfire (Mendocino Complex Fire: 185,800 ha), and most destructive wildfire (Camp Fire: 18,804 structures destroyed, 85 fatalities). In these 2 years, California spent over $1.5 billion on fire suppression, far more than any previous 2‐year period (CalFire, 2018).

I’ve seen many stories in the last few days casting doubt on the impact of climate change on wildfire. Their universal aspect is a superficial and crude understanding; thorough and nuanced is not their strategy. Only by limiting themselves can they reach the conclusions they want. I prefer actual scientists doing actual science with both thoroughness and nuance.

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Studying wildfire activity in four large California regions (North Coast, Sierra Nevada, Central Coast, and South Coast) they note that wildfire burn area has increased five-fold since the early 1970s. The dominant climate-related factor they find is how dry the fuel is, and that is profoundly affected by VPD: vapor pressure deficit. It’s the difference between how much water vapor the air can hold, which depends on its temperature, and how much it does hold (its specific humidity).

VPD has increased, and that’s because of the strong upward trend in daily high temperature in California. In mainly forested regions, the North Coast and Sierra Nevada, it seems to have its strongest effect on the dryness of large fuels during summer, sometimes measured by FM1000: the 1000-hour fuel moisture. In regions with more small fuels, Central Coast and South Coast, extreme aridity (very high VPD) can suck the moisture out of small fuels with surprising quickness.

The details are indeed both thorough and nuanced, this paper is well worth reading. Unfortunately, it won’t be, by many of those who most need to know.

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