Lightning from thunderstorms rolling through Central California on Sept. 2 ignited numerous wildfires, including several large fires in the Sierra Nevada foothills that had burned more than 19 square miles by Wednesday morning. The California Governor’s Office of Emergency Services recorded more than 9,000 lightning strikes in a single day.
Lightning is a major source of wildfire ignition in the western United States every summer.
In August 2020, more than 15,000 lightning strikes were recorded in Central and Northern California over a few days, igniting more than 600 fires and burning more than 2 million acres. More than 20 people died in the fires. Wildfires ignited by cloud-to-ground lightning during the summer are responsible for more than two-thirds of the total acreage burned yearly across the West.
UC Merced researchers warn that climate change is likely to bring an increase in both cloud-to-ground lightning and the risk of lightning-caused wildfires.
In a paper published last week in Earth’s Future, lead author Dmitri Kalashnikov, a National Science Foundation postdoctoral fellow affiliated with the university’s Sierra Nevada Research Institute, said his projections show an increase in cloud-to-ground lightning over the next three decades or so, particularly in the interior northwestern United States and northern Rocky Mountains.
Kalashnikov led the project, which he began while completing his Ph.D. at Washington State University and finished while a postdoctoral fellow at UC Merced. Co-authors include researchers from UC Merced, Colorado State University, Portland State University and other institutions.
“Lightning is a big wild card when it comes to the outcome of fire seasons here in California and other parts of the Western U.S.,” said co-author John Abatzoglou, a climatologist and management of complex systems professor at UC Merced. “Some of the biggest fire seasons — like 2020 — really got going due to widespread lightning outbreaks in summer.”
Dry lightning, or lightning that strikes outside of rainstorms, is particularly dangerous.
“The Northwest is emerging, in this study as well as in others, as the region where fire- and fire-related hazards are likely to increase substantially more than in other parts of the western U.S.,” said Deepti Singh, an associate professor in the School of the Environment at WSU Vancouver and co-author of the paper.
The study underscores the importance of managing forests to mitigate wildfire risk and to prepare at-risk communities for fires as the planet continues to warm and wildfires grow in size and severity, the researchers said. Current global climate models are unable to directly simulate future lightning because they rely on geographic resolutions too coarse to capture the conditions that create it.
The study used a machine-learning technique called “convolutional neural networks" to project future lightning over the western U.S. The technique is used to predict lightning based on the meteorological conditions that are simulated by climate models, allowing researchers to project the number of cloud-to-ground lightning days in the future.
The machine-learning models developed in this study zoom in to create the most detailed picture yet of future lightning patterns and lightning-caused fire risk across the West.
“There are already a lot of studies that say future wildfire activity will increase in the Western U.S. and that’s without even considering the potential of increased lightning, which we’re showing is going to happen in many areas,” Kalashnikov said. “Our study makes projections of increased lightning and fire risk for the relatively near future — 2031 to 2060 — whereas other studies tend to make projections for later in the 21st century. This should carry additional relevance for near-term planning and policy decisions.”
Researchers also project an increased likelihood of cloud-to-ground lightning occurring on days with meteorological conditions favorable for wildfires, increasing the risk of lightning-ignited wildfires. These findings are important for understanding changes in lightning-ignited wildfire risk, and for planning wildland fire management and suppression needs in a warming climate.
“There has been very little research devoted to how lightning may change in future climates,” Abatzoglou said. “A few lightning studies have generally pointed to increases in lightning, but these approaches have not been well codified for the western United States, where we get a lot of dry lightning. This study fills a void in this literature.”
An increase in lightning days does not result in a 1-to-1 increase in fire risk, however, because fire risk depends on other variables, such as temperature, rainfall, wind and vegetation dryness. Across the Rockies, for example, the number of days with a high likelihood of lightning-caused fires is expected to grow by three or more by the mid-21st century, though the overall increase in lightning days is larger.
On the other hand, parts of Utah and Arizona showed a reduction in lightning days — but an increase in days of potential lightning-caused fires, due to higher wildfire risk in general.
The Southwest overall showed fewer projected increases in lightning days — and even declines in some areas — but the region is still expected to see a rise in days with a likelihood of wildfires ignited by lightning.
Public Information Officer
