According to new computer models, the risk of hot, dry, windy autumn weather in California and western Oregon, which can lay the stage for devastating fires, has increased by 40% as a result of human-caused climate change.
The study, led by Linnia Hawkins of Oregon State University, looked at the effect climate change may have played in the extreme fire weather conditions that accompanied recent big fires in those states in September, October, and November.
The team, which included David Rupp of the Oregon Climate Change Research Institute, looked at the weather conditions during large fires caused by strong offshore winds like as the Santa Ana and Diablo winds in California and the East wind in western Oregon.
In the two years analyzed, the modeling revealed that human-induced climate change reduced the frequency of those winds.
However, due to higher temperatures and drier fuels, the four research sites were considerably more likely to experience intense autumn fire conditions than they would have been if human-caused increases in atmospheric aerosols and carbon dioxide were not present.
In those states, significant flames occurred in September, October, and November.
“Over the last handful of years, California and western Oregon have experienced their largest and most destructive wildfires ever recorded,” said Hawkins, a postdoctoral researcher in the OSU College of Forestry.
“The rapid and extensive growth of many of the fires was driven by strong, dry, offshore, downs lope autumn winds blowing across fuels that had become very parched over the summer and stayed that way into fall.”
The scientists focused their research on conditions similar to those seen during recent catastrophic fires, such as the Wine Country fires in October 2017, the Camp Fire in November 2018, and the North Complex Glass fires in September 2020 in Northern California; the Woolsey Fire in November 2018 in Southern California; and the Lionshead Fire in September 2020 in western Oregon.
“Anthropogenic climate change has increased the likelihood of extremely hot, dry and windy weather in autumn, but it has not necessarily increased the likelihood of fire, nor did these fires occur because of climate change,” Hawkins said. “But those fires provided archetypes of extreme offshore-wind-driven autumn fire weather for us to study.”
Over the last handful of years, California and western Oregon have experienced their largest and most destructive wildfires ever recorded. The rapid and extensive growth of many of the fires was driven by strong, dry, offshore, downs lope autumn winds blowing across fuels that had become very parched over the summer and stayed that way into fall.
Linnia Hawkins
The scientists used mid-nineteenth-century CO2 and aerosol concentrations in the atmosphere to simulate a climate without human activity. They ran thousands of simulations using current CO2 and aerosol concentrations, as well as thousands more using pre-industrial CO2 and aerosol concentrations.
The researchers then examined the chance of extreme autumn fire weather conditions between the two ensembles of simulations, defined as conditions that would occur once every 20 years absent human impact.
“We found that when CO2 and aerosols from human activity were included, the chance of extreme conditions was 40% higher in those areas of California and Oregon where recent autumn fires have occurred,” Hawkins said. “The jump was mainly because of an increase in temperature and fuel aridity and not an increase in wind speeds. In fact, we found anthropogenic climate change slightly decreased the frequency of strong, dry, offshore winds.”
Hawkins adds that the 40% increase in likelihood is the average across the western United States, with lesser or bigger increases in certain places. She also emphasizes that this research looked at extreme fire weather circumstances rather than usual conditions, and it only looked at one season of the year.
“What our research demonstrates is that anthropogenic climate change has already increased the likelihood of autumn wind-driven extreme fire weather conditions in the West,” she said. “Together with non-climatic factors like biomass accumulation and more and more people living in the wildland urban interface in fire-prone lands, that means overall fire risk is going up. Approaches such as we used here can guide fire risk assessments and fire adaptation efforts.”
This study was funded by the National Science Foundation and the National Oceanic and Atmospheric Administration. Geophysical Research Letters published the findings. The study included Sihan Li of the University of Oxford and John Abatzoglou of the University of California, Merced.
