The summer of 2008 was a tough fire season for California. A July heat wave coupled with dry lightning storms to spark blazes across northern and central regions of the state, killing 23 people and causing tens of millions of dollars in property damage.
The extent of the fire's effect didn't end there, though. For days after the flames had died down, a pall of smoke hung over the Central Valley, spiking air pollution levels and prompting authorities in the Bay Area to caution the young and elderly against venturing outside.
Fire sows destruction with dramatic force. Smoke, on the other hand, can have a longer-lasting, farther-reaching effect on humans, animals and even vegetation. Focusing on California, new research in the journal Environmental Science and Technology suggests smoke and fire will likely increase in tandem as the West dries and warms under climate change.
Building on past studies that examined how future hydrology, forest composition and population growth might exacerbate fire-related property damage, the new work explores how similar patterns might affect the air quality in a climate-changed future. Emissions scenarios from the Intergovernmental Panel on Climate Change provided a basis for the study's warming trends.
While the presence of humans, particularly in proximity to nature, can lead to more ignition events, it appears a changing climate will drive most of the increases in fire and smoke-related activity, said Matthew Hurteau, a researcher at the Department of Ecosystem Science and Management at Pennsylvania State University and lead author of the report. That's because humans tend to concentrate in sparsely forested areas like grass and scrublands, where fires are less severe, he said.
And it's in those densely forested, sparsely populated regions that most smoke will be generated, he said.
Big potential jump in emissions
"If you're looking for emissions, rather than just intensity of the fire, then vegetation comes into play," he said. Dense forests have more biomass to burn and therefore release more emissions than grass- or shrub-dominated landscapes, he explained.
Under the high-severity emissions pathway laid out by the IPCC -- in which temperatures would likely increase by 3.5 to 4 degrees Celsius by the end of the century -- emissions from biomass burning in California should increase between 19 and 101 percent, according to the study.
That wide range of possibility stems from the differing fire severity of the state's various landscapes, Hurteau said. "Where you fall in that range is going to be a function of the type of vegetation the fire is burning through and the type of fire burning," he said.
Forest fires produce a range of harmful pollutants, some of which are themselves short-term climate forcers. More concerning in the short term, however, are the effects these materials can have on people and their environment.
According to the report, from the years 2005 to 2010, biomass burning accounted for roughly 18 percent of California's man-made carbon monoxide, 21 percent of its nitrogen oxide and 34 percent of its small particulate matter.
Controlled burns could cut emissions
"For large fire years, the emissions from biomass burning can equate to much more," the report notes.
Carbon monoxide and nitrogen oxides can be toxic in large doses, and small particulates can cause irritation or breathing difficulty or exacerbate respiratory problems. Non-methane organic compounds produced by fires can react with nitrogen oxides to form ozone, a pollutant that has been shown to impede plant growth and slow photosynthesis.
"A real takeaway from this for me is that, under the high-emissions scenario, we're seeing yet another societal impact from climate change," Hurteau said. "It may only be indirectly that climate is influencing these fires, but they in turn are affecting us with their emissions."
Hurteau's other takeaway from the study might strike some as paradoxical -- he wants to see more fires.
Given how much of California is fire-adapted, he said, it has become imperative that fire be allowed to return to the landscape under controlled conditions. While this practice will certainly generate some smoke of its own, controlled burning as a forest management technique tends to produce less emissions than spontaneous wildfires, he said.
"Prescribed fires tend to be smaller than, say, last year's Rim Fire," he said. "You're burning under your own conditions, so typically later in the year, when temperatures are cooler and there's more moisture in the wood." Some of the fuel is burned up, he said, but much of it holds onto its carbon.
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