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Atmospheric Sciences & Global Change
Research Highlights

November 2015

California: A Coming Century of Fire?

Researchers use risk analysis and modeled climate change to assess fire risk in The Golden State's future

California fire national forest
A 2013 fire in California’s San Bernardino National Forest consumed 27,531 acres before it was extinguished. Fires in California not only destroy national lands and forests, they threaten communities, livestock, and the livelihoods of many, from ranchers to winemakers. Understanding the forces that impact fire risk is important for these communities as they prepare for the future. Photo courtesy of the U.S. Department of Agriculture. zoom Enlarge Image.

Results: The number of days of extreme fire risk in California might be six times the current number by the end of the century, according to a study led by scientists at Pacific Northwest National Laboratory. The research, published in the Bulletin of the American Meteorological Society, likely underestimates the increase, compared to the actual fire risk over the past few decades. The model, however, assumes the worst-case scenario of greenhouse gas emissions, offering hope that if emissions are curbed, the impact would be reduced. The research modeled fire risk, not individual fires.

"From the climate side of things, can we model individual fires? Not yet," said a PNNL atmospheric scientist Dr. Jin-Ho Yoon, lead author of the study. "We aren't considering ignition factors or the way people behave. But can we model the risk? Yes."

Why it Matters: Though California is suffering through the longest, most severe drought in its history, some science studies predict California will generally be wetter in the late 21st century. But scientists have also wondered whether this would lessen the risk of fire despite climbing global temperatures, because hotter weather would be expected to increase the fire risk. To analyze these seemingly competing outcomes, the researchers turned to an often-used fire risk index and a global climate model called the Community Earth System Model (CESM1). They used these tools to explore the complex forces controlling California's climate, and to help provide a handle on what the future might be like in a warmer state.

Methods: To explore the future of fire seasons in California, Yoon and colleagues from PNNL and the Climate Center at Utah State University compared fire risk data observed between the 1980s through present day using an index called the KBDI (named after the two researchers who developed it) and satellite data showing burned areas.

The team then used the CESM1 to simulate precipitation and air temperatures and to predict wildfire probabilities based on soil moisture and vegetation amount. They simulated two time periods: 1920 to 2005; and 2006 to 2080, assuming a worst-case greenhouse gas scenario for the latter time period. They compared these modern simulations to fire risk in pre-industrial times. The model simulations showed an in­crease in fire risk in the future despite the overall lack of change in annual precipitation for the next 50 years. The model study suggests that the cause is global warming by increased greenhouse gases.

The higher risk results in the future were somewhat surprising to the team. They attribute it to larger amounts of carbon dioxide in the air, which causes plants to grow more efficiently.

"Even though California might be somewhat wetter by the end of the century, it may come down to more vegetation—fuel for the fires—and increasing temperature." said Yoon.

What's Next? CESM1, the state-of-the-art climate model used in this study, has improved performance in the last few decades. Yet, challenges remain, such as spatial resolution and its ability to simulate vegetation, or burned area. Improvements in these areas look promising in a DOE-sponsored, next-generation climate model being developed by scientists at PNNL and other collaborative research institutions.


Sponsor: The research was supported by the U.S. Department of Energy (DOE) Office of Science Biological and Environmental Research for the Earth System Modeling program.

Facilities: The research used computing facilities located at the National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley National Laboratory, and EMSL, a DOE national user facility located at PNNL.

Research Team: Jin-Ho Yoon, Ben Kravitz, and Philip J. Rasch, PNNL; S-Y Simon Wang, Robert R. Gillies, and Lawrence Hipps, Utah Climate Center at Utah State University.

Research Area: Climate & Earth System Science

Reference: Yoon JH, SY Wang, RR Gillies, B Kravitz, L Hipps, and PJ Rasch. 2015. "Extreme Fire Season in California: A Glimpse into the Future?" Bulletin of the American Meteorological Society 96(12):S5-S9. DOI: 10.1175/BAMS-D-15-00141.1

November 8, 2015

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In one sentence: Researchers at Pacific Northwest National Laboratory found that the number of days of extreme fire risk in California might be six times the current number by the end of the 21st century.

In 100 characters: California extreme fire risk days up to 6 times higher by century's end if greenhouse gases aren't cut