from What’s Natural? column

published in Pacifica Tribune, December 4, 2019

Why Worse Wildfires? Part 2

Figure 1 Managing forest ground fuels

Why worse wildfires? The short answer is more humans cause more wildfire ignitions in altered landscapes. Since 1970, California’s population doubled, adding 20 million people. As more human habitat was developed, the increasingly disturbed landscape quickly became covered in easily ignitable invasive grasses (see part 1). To protect human habitat, fires were suppressed and ground fuels increased. Development also expanded a vulnerable electric grid. Furthermore, more people increased the probability of careless fires and more innocent accidents. And sadly, a larger population added more arsonists.

During a typically warm and dry July day, a rancher was innocently driving a stake into the ground to plug a wasp’s nest. Surrounded by dry grass, the hammer’s spark ignited a devastating inferno named the Ranch Fire. Despite sensationalist’s hype, global warming had not made the grass drier. Grass becomes highly combustible in just a few hours of dry weather. And like most of northern California, there has been no warming trend for maximum summertime tempertures. Based on Western Regional Climate Center data, maximum summer temperatures in the Mendocino area had cooled by 3°F since the 1930s. The rapidly spreading Ranch Fire soon merged with a different fire to form the Mendocino Complex Fire, California’s largest documented fire.

Similarly, a highway accident sparked roadside grasses that kindled northern California’s 7th largest fire, the Carr Fire.

Careless fires cannot be considered accidents and offenders should be held accountable. A hunter’s illegal and improperly attended campfire caused the August 2013 Rim Fire, centered around Yosemite National Park. It was California’s 5th largest fire.

Governments and utility companies should likewise be held accountable for carelessly maintaining our electric grids. An electric spark ignited California’s deadliest fire, the Camp Fire which destroyed the town of Paradise and killed 85 people. As a 2018 research paper estimates, “Since the year 2000 there’ve been a half-million acres burned due to powerline-ignited fires, which is five times more than we saw in the previous 20 years.”

More disturbing is the number of fires started by arson. According to the U.S. Fire Administration, nationally, as in California, one in every five brush, grass, or forest fires since 2007 were intentionally set. Arsonists have been recently charged for some of California’s 2019 fires. Arson accounted for 55% of Kentucky’s fires and is the leading cause of Florida’s fires. Because arson is so difficult to prove, arson statistics are probably underestimated. So, experts in Australia combine arson and “suspicious” fires to argue half of Australia’s fires were likely intentionally set. That means each year 31,000 Australian bushfires are intentionally ignited. And as in the American west, Australia’s bush fires have been increasingly fueled by invasive grasses like Buffel grass.

Wildfires caused by natural lightning ignitions, peak during the summer months of July and August, and become virtually non-existent in the autumn and winter. In contrast, human ignitions have created year-long fire seasons. Counter-intuitively, California experiences the most dangerous fire weather during the cooler and wetter seasons. As seasonally cold air settles in over the high mountain deserts in autumn and winter, episodes of high winds, known as the Santa Ana and Diablo winds, flow downslope. Sinking air warms 5°F for every 1000-foot drop in elevation so these downslope winds can raise lowland temperatures 25°F in just a few hours. That warming causes relative humidity to fall, so these winds rapidly suck moisture out of whatever vegetation it passes over. In combination with faster spreading embers, fires burn 2 to 3 times more area during high wind events.

Under natural conditions, seasonally extreme winds never coincided with the season of abundant lightning. But due to human ignitions there has been an increased probability of more ignitions occurring during strong cool-weather winds. California’s 2nd biggest fire, the Thomas fire, was ignited in December by a downed power line during high winds. The third largest fire, the Cedar Fire was ignited in October by a lost hunter who carelessly lit a signal fire. California’s deadliest fire, the Camp Fire, was ignited by a powerline and fiercely spread due to a November high wind event.

Climate change does not ignite fires. Climate change does not affect how quickly dead grasses and bushes can dry. Climate change may affect the winds, but any warming, natural or human, would reduce those extreme winds. Regards California’s worst fires, a US Geological Survey’s wildfire expert states, “Some will argue that it’s climate change, but there is no evidence that it is. It’s the fact that somebody ignites a fire during an extreme [wind] event.”

Jim Steele is Director emeritus of San Francisco State’s Sierra Nevada Field Campus and authored Landscapes and Cycles: An Environmentalist’s Journey to Climate Skepticism