Wildfire - InsideClimate News

Wildfires have made and remade the California landscape for centuries. But in recent years, they have grown vaster, and the fire season itself has gotten longer. Nine of the ten most destructive fires in recorded California history have occurred since 2000, according to the California Department of Fire & Fire Protection (CalFire). The November 2018 Camp Fire, which killed 86 people and incinerated more than 18,000 structures, including virtually the entire town of Paradise, topped the grim list.

A constellation of factors has primed California to burn big. More construction is occurring in areas historically prone to wildfire, situating more people in the danger zone. More development means more utility infrastructure, such as power lines and transformers, now sit in combustible areas, including the equipment whose sparks ignited the Camp Fire.

About 85 percent of wildfires are started by human activity, as opposed to natural events such as lightning strikes, and they're more likely to happen in more populated parts of the state, according to California's Fourth Climate Change Assessment, issued in August 2018.

California's forests are also more densely packed with vegetation than a century ago, because state and federal agencies have spent decades suppressing fires rather them letting them burn and clear out vegetation. Before the Gold Rush, California forests held 50 to 70 trees an acre; by 2009, there were on average more than 400 trees per forested acre.

Already, climate change is exacerbating these vulnerabilities. The state has a natural cycle of wet winters, which nourish vegetation, and arid summers, which dry out the landscape and leave it susceptible to fire. Climate change has made California hotter, and a spate of studies has shown that the increased heat has made the state drier and more prone to drought, including an enormously destructive one that lasted from 2012 to 2016.

The lack of rain means vegetation in the state's densely packed forests can die, from grasses to shrubs and bushes to trees, said California State Climatologist Michael Anderson. But the dead vegetation remains in place, and if a spark hits it, the blaze moves in a dangerous chain, from the dry grass on the ground, up the shrubs like a ladder and into the dessicated trees, moving through the canopy as a "crown fire." Fires that spread through the trees are larger, faster and more destructive than fires that move along the ground, Anderson said. The Camp Fire, for instance, moved the length of a football field a second, as the day's high winds flung embers from burning trees hundreds of yards ahead.

A July 2019 study published by the American Geophysical Union concluded that since the early 1970s, the average area burned annually by wildfire in California has increased fivefold. That trend is driven by an 800 percent rise in the land area ignited by summertime fires "and was very likely driven by drying of fuels promoted by human‐induced warming. Warming effects were also apparent in the fall by enhancing the odds that fuels are dry when strong fall wind events occur," the study concluded.

The study homed in on the shift that has made great swaths of California far more susceptible to fire: a 1.8 degree Celsius rise in summertime average temperature in the state since the late 1800s, nearly all of it over the last 50 years. While that might not seem like much, the warmer summers make for much drier weather, and those changes, according to the study, have an "exponential" impact on wildfire.

"Human‐caused warming has already significantly enhanced wildfire activity in California, particularly in the forests of the Sierra Nevada and North Coast, and will likely continue to do so in the coming decades," the study said.

The state's fourth climate assessment pointed to a 2018 study that projected under high greenhouse gas emissions, the average annual area burned in California would increase by 77 percent by 2100 from what it was around 1990. The massive fires of 2017 and 2018, including the Camp Fire, are a sign of things to come.

Speaking of the future of wildfire in California, the lead author of the 2018 study and professor at University of California-Merced, LeRoy Westerling said: "We don't know how big a really big fire can get."

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