California wildfire season is changing, with an increase in fires following traditional high-risk periods, study finds

Climate change is shifting wildfire seasons in North America, but the direction of those changes depends on local ecosystems, a new study shows.

On average, fire seasons are moving forward in the northern boreal forests of Alaska and Canada. Almost no changes can be seen in the grassland area. And the dry West and California’s fire season has been extended further into late fall and winter. The research results were published in the journal Geophysical Research Letters on February 24th.

“This is very region-specific,” said Hongliang Zhang, an environmental scientist at China’s Fudan University. Understanding these specific changes is important for managing wildfires and preparing resources for the worst parts of the wildfire season, Zhang told Live Science. Seasonal patterns are also important in predicting the health effects of wildfire smoke, which can be severe.

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Zhang and colleagues used data on burn areas in North America from 2001 to 2020 obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments aboard NASA’s Aqua and Terra satellites. They also collected data on meteorological variables, vegetation, likelihood of lightning strikes, and other environmental factors at the time of the fire.

They found that fires were occurring earlier in the boreal forest, or taiga, of Canada, Alaska, and the Great Lakes region. This is because the snow melts faster and the fuel dries faster. Canada experienced its worst wildfire season in 2023, followed just two years later by its second-worst season.

In the temperate deserts of the Southwest and California’s Mediterranean-like climates, fire seasons became longer, with more fires occurring after the traditional danger period had passed.

Grasslands and grasslands experienced small changes in fire season intensity and minimal changes in seasonal timing. The forests of the Appalachians and Southeast also showed little seasonal change.

The researchers also modeled future scenarios. Under high-emissions climate change scenarios, the boreal forest’s fire season could be brought forward by about a week, while California’s annual fire season could be delayed by more than a month from its current June-to-October season, researchers found. A similar fire season could continue in the desert Southwest, the researchers wrote.

The model will be useful for more detailed studies, Zhang said. He and his team plan to use it to study the effects of other factors, such as changes in vegetation and human activity. (According to the National Park Service, 85% of wildland fires in the U.S. are caused by human acts, such as arson or failure to properly extinguish campfires.) The model can also help predict pollution and carbon emissions from these fires, Zhang said.

“This model is very good at predicting wildfires,” he said. “So now we want to predict the emissions into the atmosphere from wildfires.”