The likelihood of a warm, dry and windy fall that could pave the way for severe fires in California and western Oregon has increased by 40% due to human-induced climate change, according to new computer models. .
The study led by Linnia Hawkins, a postdoctoral fellow at Oregon State University (OSU) College of Forestry, covered 2017 and 2018 and looked at the role climate change may have played in the extreme fire-related weather that accompanied the recent great months of September, October and November. fires in these states.
The collaboration which included David Rupp of the Oregon Climate Change Research Institute looked at weather patterns during large fires driven by strong offshore winds such as the Santa Ana and Diablo winds in California and the east wind from the west. from Oregon.
Modeling revealed that human influences on the climate actually reduced the frequency of these winds in the two years studied. But higher temperatures and drier fuels mean that the four study areas were nonetheless much more likely to have extreme fire weather in the fall than they would have been without the human-caused increases. atmospheric aerosols and carbon dioxide.
“Over the past few years, California and western Oregon have experienced their largest and most destructive wildfires on record,” Hawkins said. “The rapid and widespread growth of many of the fires was caused by strong, dry, offshore, descending autumn winds blowing over fuels that had become very parched over the summer and remained so until the fall.”
For this research, the scientists focused on conditions similar to those seen in recent catastrophic wildfires, including the Northern California Wine Country Fires in October 2017, the Campfire in November 2018, and the Northern California Wildfires. Complex Glass in September 2020; the Woolsey fire in southern California in November 2018; and Lionshead Fire in western Oregon in September 2020.
“Anthropogenic climate change has increased the likelihood of extremely hot, dry, and windy weather in the fall, but it has not necessarily increased the likelihood of fires, and these fires have not occurred because of climate change” , Hawkins said. “But these fires have provided us with archetypes of extreme offshore wind-driven autumn fire weather to study.”
To model a climate without human activity, scientists fixed atmospheric concentrations of CO2 and aerosols at mid-19th century levels. They ran thousands of simulations with current CO2 and aerosol concentrations and thousands more simulations with CO2 and aerosol concentrations set at pre-industrial levels.
The researchers then compared the likelihood of extreme fire weather in the fall – defined as conditions that, in the absence of human influence, would occur once every 20 years – between the two sets of simulations.
“We found that when CO2 and aerosols from human activity were included, the risk of extreme conditions was 40% higher in areas of California and Oregon where recent fall fires have occurred. products,” Hawkins said. “The jump was mainly due to an increase in temperature and fuel aridity and not an increase in wind speed. In fact, we found that anthropogenic climate change slightly decreased the frequency of strong, dry winds from the wide.
Hawkins points out that the 40% increase in probability is the average in the western United States and that the increase is smaller or larger in specific regions. She also points out that this study looked at extreme fire weather conditions as opposed to average conditions and only during one season of the year.
“What our research demonstrates is that anthropogenic climate change has already increased the likelihood of extreme wind-driven fire weather in the fall in the West,” she said. “With non-climatic factors such as biomass accumulation and more people living in the urban interface of wild lands in fire-prone lands, this means that the overall fire risk is increasing. Approaches such than the ones we used here can guide fire risk assessments and fire adaptation efforts.”
The National Science Foundation and the National Oceanic and Atmospheric Administration supported this research. The results were published in Geophysical Research Letters.
Sihan Li from the University of Oxford and John Abatzoglou from the University of California, Merced, also participated in the study.