Fueled by excessive heat and drought, wildfires torched more than 4 million acres (1.6 million hectares) in California in 2020. The record-setting blazes released pollutants into the atmosphere, including more than 200 gigagrams of methane, a particularly potent greenhouse gas. Those emissions amounted to nearly 14% of the state’s total methane emissions that year, according to a new study published in Atmospheric Chemistry and Physics.
Rising methane emissions from increasingly common megafires could fuel further climate change and work against efforts to reduce greenhouse gas emissions, said biogeochemist Francesca Hopkins of the University of California, Riverside, one of the study authors.
When the Sequoia Complex and Creek fires blazed through the Sierra Nevada in August 2020, Hopkins and her colleagues took a ground-based spectrometer out into the field to measure how much carbon dioxide, carbon monoxide, and methane were present in the columns of smoke rising from the fires.
The instrument measured the difference between the amount of light that penetrated the column and the amount expected in unimpeded sunlight, which reflects the number of greenhouse gas molecules and aerosols in the smoke, Hopkins said. The researchers calculated how much methane and carbon monoxide the fires emitted relative to the amount of carbon dioxide.
Using these ratios, the group estimated California’s total wildfire methane emissions for that year from estimates of total wildfire carbon dioxide (CO2) emissions compiled by the California Air Resources Board (CARB). They also used the ratios to calculate the amount of each greenhouse gas that was emitted relative to the amount of fuel burned, metrics known as emission factors.
The study is “unique,” said Shawn Urbanski, an atmospheric scientist at the U.S. Department of Agriculture Forest Service Rocky Mountain Research Station who was not involved in the study. “It’s the first that I’m aware of where [scientists] have taken this total column measurement using this technique and measured CO2 directly, so they backed out the emission factor directly,” he said.
Bigger Blazes Bring More Methane
Methane and carbon dioxide emission factors from the Sequoia Complex and Creek fires were similar to those of other temperate forest fires in the western United States, Urbanski said. But larger fires burn more fuel, so they release more methane. “In a big fire year, methane emitted by wildfires in California is sizable compared to all the anthropogenic sources,” Urbanski explained.
This underappreciated source of methane is not considered in the CARB Scoping Plan, the state’s blueprint for reducing greenhouse gas emissions and reaching carbon neutrality. That’s in part because methane is not historically known to be a large component of wildfire emissions, wrote CARB public information officer Dave Clegern in an email.
Clegern added that the average methane emissions from wildfires between 2001 and 2022 would be closer to 1% of California’s total methane emissions in 2020.
Fires’ contribution to total methane emissions has also been small on a global scale, representing around 5% of total methane emissions between 2008 and 2017, according to an earlier study. These historic data show that methane emissions from wildfires have been minor, but the 2020 fires are “possibly representative of fires that will happen more and more in the future,” Hopkins said.
Large fires have continued to scorch California since 2020. Four of the largest 20 fires in the state’s recent history occurred the following year. In the United States, fires burned more total acres in 2021 and 2022 than the annual average in the preceding 2 decades, according to a NOAA National Centers for Environmental Information wildfire report.
Growing megafire frequency could offset efforts to reduce anthropogenic methane emissions in California, Hopkins said, causing the state to delay reaching its goal.
Similar wildfire trends are emerging around the world. So far in 2023, wildfires have burned more than 1 million hectares and displaced thousands of people in Alberta, Canada. Fires are also burning more land in boreal regions, where they are emitting record-breaking amounts of carbon dioxide, according to some research. Those fires could also be releasing more methane if the emission factors remain constant.
—Derek Smith (@djsmitty156), Science Writer