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Wildfires could grow exponentially as climate warms, study warns

Wildfires could grow exponentially in the next 40 years as the climate continues to heat up. That is one conclusion of a new study which highlights the robust links between climate change and wildfires in western North America, with a focus on California.

From 1972 to 2018, California has experienced a five‐fold increase in area burned year round, with an eight-fold increase during summer.  

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Area burned by California wildfires in thousands of square kilometers, 1972-2018. Specific regions studied are at upper left. Adapted from Williams et al., 2019

The two largest fires in California since 1932 (the year accurate records date back to) occurred the last two years, the Mendocino Complex fire in 2018 and the Thomas Fire in 2017. 

The spike in larger fires really ramped up around the beginning of this century. Sixteen of the 20 largest  fires in California history have occurred in the last 20 years, according to the California Department of Forestry and Fire Protection. 

Scientists seek to understand a growing threat, "firenados"

But there has been some controversy about the reasons behind these more massive fires. In a November tweet, President Trump blamed poor forest management.

While it is true that there are various intermingling factors at work, the team of scientists in this new study conclude that the main cause is our warming climate. According to their research, the spike in large wildfires is most likely due to the drier conditions caused by warmer temperatures. Warm air has more energy and evaporates more moisture, leaving forest cover parched and flammable. 

Since 1896, average summer temperatures in California have risen 3.25 degrees Fahrenheit. Recently the pace of the warming has quickened, with three-quarters of that increase occurring since the 1970s. Though it's only a few degrees difference, it has resulted in a growing moisture deficit in the atmosphere.

The moisture deficit — or vapor pressure deficit, as scientists like to call it — is simply the difference between how much moisture the air can hold versus how much moisture it is actual holding. As temperatures rise, that spread increases and the larger deficit leads to lower humidity. Low humidity is a key ingredient for wildfires.

The team's research shows there's a strong relationship between the extent of summer forest burned and the summer moisture deficit in a given year. According to the paper, this fact "strongly suggests that nearly all of the increase in summer forest‐fire area during 1972–2018 was driven by increased vapor pressure deficit."

California wildfires driven by weather and climate conditions

CBS News reached out to the lead author, Park Williams, a bioclimatologist at Columbia University's Lamont-Doherty Earth Observatory. We asked him about two main critiques about the role climate change plays in these recent larger wildfires in California.

This first is: It's already dry enough for wildfires half the year and brush can dry out immediately, independent of the warming climate. In other words, a little extra drying shouldn't make much difference.

Williams says if that argument were true, "we wouldn't see the strong interannual correlation between California's area burned and the atmospheric vapor-pressure deficit." Put another way, the area burned would not so closely mirror the ups and downs of the moisture deficit each summer. 

Next we asked about historical evidence that wildfires also burned very large, and total area burned was seemingly just as high, back in the 1800s and early 1900s. 

 Williams replied, "Anybody else who knows anything about fire history in the western U.S. would be correct in noting that the recent increase in fire activity is to some degree a return to conditions more common in the 1800s, before Euro-American settlement led to the near elimination of fire from the landscape." That is to say, fires were bigger back then because fire suppression was lacking. After the 1920s the size of the fires decreased dramatically as settlers took control. Only in recent decades did fire size start to spike again.

That said, Williams does agree that the recent increase in wildfire activity is not solely due to warming. "I do believe that by allowing forests to grow artificially dense by fighting fire for a century and also by producing a near constant supply of ignitions, we have set the table in many western forests for the effect of warming to be extra potent," he explains.

It should be noted that fluctuations in large-scale natural oscillations in both the Pacific and Atlantic Oceans also factor into the ebb and flow of moisture, heat and wildfire activity in the West. There are plenty of other intermingling factors. 

However, given the large upward trend in recent decades, Williams goes back the strongest evidence his research bears out: "Based on the statistical analyses we have done, it simply cannot be accurately said that warming has not been the proximate driver of the increases in forest fire size."

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