Global warming at the center of recent California wildfires – “We show that nearly all of the observed increase in burned area in California over the past half-century is attributable to human-caused climate change”

(A) Time series of summer (May to September) forest fire burned area (BA, in red) and spring to summer (April to October) maximum near surface temperature (TSMAX; in black) from 1971 to 2021 in California; (B) observed versus out-of-sample 10-fold predicted changes in BA. Vertical gray lines indicate 2.5th and 97.5th percentiles of 10,000 different predictions. Colors indicate the decade of each sample. The Inset shows a map of California with the domain of interest shaded in gray. Graphic: Turcu, et al., 2023 / PNAS — (A) Time series of summer (May to September) forest fire burned area (BA, in red) and spring to summer (April to October) maximum near surface temperature (TSMAX; in black) from 1971 to 2021 in California; (B) observed versus out-of-sample 10-fold predicted changes in BA. Vertical gray lines indicate 2.5th and 97.5th percentiles of 10,000 different predictions. Colors indicate the decade of each sample. The Inset shows a map of California with the domain of interest shaded in gray. Graphic: Turcu, *et al*., 2023 / *PNAS*

By Anne M. Stark
12 June 2023 (LLNL) – Summer wildfire seasons in California routinely break records. The average summer burn area in forests in northern and central portions of the state have increased fivefold between 1996 and 2021 compared to between 1971 and 1995.

Although the drivers of increased temperature and dryness are known, the contribution of human-caused climate change to wildfire activity, relative to natural climate variation, is unclear.

However, a new study by a Lawrence Livermore National Laboratory (LLNL) scientist and collaborators shows that nearly all the recent increase in summer wildfire burned area is attributable to human-caused (anthropogenic) climate change. Anthropogenic simulations yielded burn areas an average of 172% higher than natural variation simulations. The research appears in the Proceedings of the National Academy of Sciences.

The team, led by Marco Turco from the University of Murcia, Spain, modeled the climate drivers of summer wildfire activity in California, both with natural climate variation alone and with anthropogenic climate change effects.

Beginning in 2001, the team found, the drivers of increasing summer burn area are all anthropogenic, with no detected natural forcing component of enhanced burn area. Although repeated wildfires consume fuel and limit the activity of subsequent fires in the same area, the team estimated that the next decades — between 2031 and 2050 — may see a further increase in average summer burn area in California of up to 52% more than current average burn areas.

“The results show the role of human-caused climate change in driving fire activity and highlight the need for protective adaptations against summer wildfire seasons,” said LLNL scientist Don Lucas, a co-author of the study.

Forest fire area projections in California using static and dynamic models. (A) Burned areas simulated from CMIP6 climate models and the static climate-fire model (historical forcing until 2014 and SSP2-4.5 forcing 2015 to 2050). Model results are shown as 21-y moving averages to emphasize gradual changes rather than year-to-year fluctuations. Red bars are observations. (B) Time series of trailing 21-y moving average considering both static and dynamic models and (C) mean burned area in the period 2031 to 2050. The median is shown as a solid line; the box indicates the 25 to 75 percentile range, while the whiskers show the 2.5 to 97.5 percentile range. The BA simulations span 10,000 different predictions × 24 GCMs. Graphic: Turcu, et al., 2023 / PNAS — Forest fire area projections in California using static and dynamic models. (A) Burned areas simulated from CMIP6 climate models and the static climate-fire model (historical forcing until 2014 and SSP2-4.5 forcing 2015 to 2050). Model results are shown as 21-y moving averages to emphasize gradual changes rather than year-to-year fluctuations. Red bars are observations. (B) Time series of trailing 21-y moving average considering both static and dynamic models and (C) mean burned area in the period 2031 to 2050. The median is shown as a solid line; the box indicates the 25 to 75 percentile range, while the whiskers show the 2.5 to 97.5 percentile range. The BA simulations span 10,000 different predictions × 24 GCMs. Graphic: Turcu, *et al*., 2023 / *PNAS*

Over the past 50 years, the area burned by summer wildfires in California has been increasing. The 10 largest California wildfires all happened in the last 20 years, five of which occurred in 2020 and eight after 2017. Besides their immense environmental impacts, these fires also have had widespread negative impacts on human health and mortality and numerous socio-economic consequences.

Non-climatic factors that have been implicated in changing wildfire characteristics include land management that has facilitated fuel buildup, which favors increased burn severity, and the increased susceptibility of California’s aging power grid to extreme weather and increased development in fire-prone areas that changes ignition patterns and fire management.

“Beneath these ‘external’ factors, natural climate variability also influences the occurrence and severity of forest wildfires, creating a noise that can mask the signal of human-caused impacts on wildfire changes,” Lucas said.

Using the latest simulations for climate-change attribution and detection studies and accounting for the uncertainties arising from the data-driven climate-fire model and climate models, the team quantified the influence of human-caused climate change on the burned area in recent years.

“We show that nearly all of the observed increase in burned area in California over the past half-century is attributable to human-caused climate change,” Lucas said.

Other institutions involved in the study include the University of Murcia, Spain; University of California, Merced; University of Cantabria, Spain; University of California, Los Angeles; University of California, Irvine; and the Barcelona Institute for Global Health.

The LLNL work was funded by a UC-National Lab Collaborative Research and Training Award on Mitigating and Managing Extreme Wildfire Risk in California.

Contact

Anne M. Stark, stark8@llnl.gov, (925) 422-9799

Human-caused climate change at the center of recent California wildfires

Anthropogenic climate change impacts exacerbate summer forest fires in California

ABSTRACT: Record-breaking summer forest fires have become a regular occurrence in California. Observations indicate a fivefold increase in summer burned area (BA) in forests in northern and central California during 1996 to 2021 relative to 1971 to 1995. While the higher temperature and increased dryness have been suggested to be the leading causes of increased BA, the extent to which BA changes are due to natural variability or anthropogenic climate change remains unresolved. Here, we develop a climate-driven model of summer BA evolution in California and combine it with natural-only and historical climate simulations to assess the importance of anthropogenic climate change on increased BA. Our results indicate that nearly all the observed increase in BA is due to anthropogenic climate change as historical model simulations accounting for anthropogenic forcing yield 172% (range 84 to 310%) more area burned than simulations with natural forcing only. We detect the signal of combined historical forcing on the observed BA emerging in 2001 with no detectable influence of the natural forcing alone. In addition, even when considering fuel limitations from fire-fuel feedbacks, a 3 to 52% increase in BA relative to the last decades is expected in the next decades (2031 to 2050), highlighting the need for proactive adaptations.

SIGNIFICANCE: The ongoing intensification in forest fire activity in California has had a dramatic impact on human activities and ecosystems alike. While an increase in temperatures and dryness has been identified to be one of the major drivers of increased summer forest burned area (BA), the extent to which such changes are due to natural variability or anthropogenic climate change remains unresolved. Using the latest simulations for climate change attribution and detection studies and accounting for the uncertainties arising from the data-driven climate-fire model and climate models, we quantify the influence of anthropogenic climate change on recent changes in BA. We show that nearly all of the observed increase in BA over the past half-century is attributable to anthropogenic climate change.

Anthropogenic climate change impacts exacerbate summer forest fires in California