Portable solar spectrometers measuring alongside a higher resolution solar spectrometer. Researchers performed side-by-side measurements before and after field campaigns to ensure instrument stability and check data quality. Photo taken at NASA's Armstrong Flight Research Center in Edwards, California. Credit: Frausto-Vicencio/UCR
Scientists are turning to novel detection techniques in the fight against a vicious climate-related cycle. In the past few years, climate change has been fueling the severity, range and frequency of wildfires, especially in the Western U.S. These wildfires emit methane, a super-potent greenhouse gas—that accelerates said climate change.
One issue begets the other—and now researchers at the University of California Riverside (UCR) have discovered the problematic cycle is worse than previously thought.
The state of California tracks methane emissions for climate goals, but it does not collected data on natural sources of methane—like those that come from wildfires. If California did track such numbers, the state would have seen that wildfires were the third largest source of methane in the state in 2020.
“The fires in 2020 emitted what would have been 14 percent of the state’s methane budget if it was being tracked,” said study co-author Francesca Hopkins, an environmental sciences professor at UCR. “Fires are getting bigger and more intense, and correspondingly, more emissions are coming from them.”
Novel detection technique
Wildfire emissions have primarily been characterized by in situ laboratory measurements and air-based field observations, such as space-based instruments, aircraft and satellites. With air-based techniques, however, observations of trace gases are limited due to poor spatiotemporal coverage and the aerosol burden from smoke plumes.
Remote sensing techniques are considered more accurate as they can capture an integrated plume from the fire that includes different burning phases. Specifically, Hopkins and her team say ground-based solar spectrometers are an ideal alternative to precisely and accurately measure fire emissions at regional scales, and can even complement older air-based techniques.
“The plume, or atmospheric column, is like a mixed signal of the whole fire, capturing the active as well as the smoldering phases,” Hopkins said. “That makes these measurements unique.”
Rather than using a laser like other instruments do, a solar FTIR spectrometer uses the sun as a light source. Gases in the plume absorb and then emit the sun’s heat energy, allowing analysis of the quantity of aerosols as well as any present methane, carbon and carbon dioxide.
FTIR analysis
For the study, the researchers placed the solar FTIR spectrometer downwind of the Sequoia Lightning Fire Complex—a wildfire that would go on to kill 10 to 14% of the large sequoias in the Sierra Nevada and become the largest recorded fire in a giant sequoia grove.
According to the results, nearly 20 gigagrams of methane was emitted by the Sequoia Lightning Fire Complex. (For reference, one gigagram is 1,000 metric tons.) Extrapolating out their data, Hopkins and her team then calculated the methane contribution from California’s top 20 fires of 2020. They estimated the amount to be about 214 gigagrams—14 percent of the state’s total methane budget and the third largest source of the greenhouse gas in the state, behind only agriculture and industry.
The researchers say the amount of methane emissions from the 2020 fires was more than 7x the average of wildfires in the previous 19 years. And while 2020 may have been exceptional in terms of wildfires, scientists only expect larger and more frequent megafires in the coming years as Earth deals with a rapidly changing climate.
“California has been way ahead on the [methane] issue,” Hopkins said. “We’re really hoping the state can limit the methane emissions under our control to reduce short-term global warming and its worst effects, despite the extra emissions coming from these fires.”