Increasing wildfire risk linked to multiple conditions

Experts at the U.S. Department of Energy’s Pacific Northwest National Laboratory are working to understand which conditions are the most conducive to large wildfires. Using machine learning to group past fires with the conditions that preceded them, the researchers found an increased risk of almost every type of wildfire. 

Study co-authors Ruby Leung and Xiaodong Chen combined decades-long wildfire records and new simulations of past climate conditions to identify variables that lead to wildfires. 

The team was surprised to find that having humidity in the air, but not quite enough to lead to precipitation, can boost the likelihood of dry grasslands or trees being ignited by lightning. For example, the CZU Lightning Complex fires in California were triggered by lightning on August 16, 2020, and burned nearly 1,500 structures.

According to Leung, scientists have known the importance of such hydro-meteorological conditions, but generating enough data to tease out lengthy soil moisture or humidity trends and thoroughly representing their influence is only recently possible through computational advances in modeling.

The researchers used this technology to group wildfires into various types, such as those that strike when soil is damp or during cloudy days. 

The most rapidly increasing fires are those that spark on sunny days that are hot and dry. These “compound case” wildfires, named for their multiple contributing factors, strike more frequently than any other type. A warming climate is likely to exacerbate the trend, said Leung.

“Based on the historical trends we see over the past 35 years, it is very likely that trend will continue. That is partly driven by rising temperature and partly driven by reduced soil moisture as snowmelt starts earlier in spring, reducing soil moisture in summer and fall.”

The study represents progress toward building a more comprehensive understanding of how wildfires evolve. “This allows us to draw a very complete picture of how wildfire is triggered across the whole Western United States,” said Chen.

The researchers found that nearly all types of wildfire, including cloudy day fires, are happening more often. “Wet case” fires, which occur when soil moisture levels are higher, are the only exception. Their decline coincides with an overall drying trend in the Western United States. Leung pointed out that California’s wet season window is also narrowing, adding another challenge to an already fire-ravaged state.

The research was presented at the American Geophysical Union’s 2020 fall meeting.

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By Chrissy Sexton, Earth.com Staff Writer