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Climate change caused the increase in size of wildfires occurring across California in the last 50 years, according to a new study published in this week’s journal Earth’s Future.

Since the early 1970s, California wildfires have increased in size by eight times, the study says, and the annual burned area has grown by nearly 500%.

“Human-caused warming has already significantly enhanced wildfire activity in California, particularly in the forests of the Sierra Nevada and North Coast, and will likely continue to do so in the coming decades,” the authors of the paper wrote.

Last year, for example, the Camp Fire claimed 85 lives in California, making it the deadliest in state history.

Summertime forest fires becoming more of a reality

Park Williams, the study’s lead author and a professor at the Lamont-Doherty Earth Observatory at Columbia University, told CNN that California citizens need to be prepared for continued increases in wildfire activity, and the risks to property and health that come with it.

Williams said the assumption that wildfires would not affect modern society is proving to be wrong in many parts of the western United States.

“Increases in state-wide burned area over the last several decades were dramatically punctuated in 2017 and 2018 by particularly extreme wildfire activity with substantial loss of life and property,” Williams said.

In 2017, almost 1.25 million acres were burned in California. That year, the Tubbs fire alone destroyed more structures than any previous wildfire in the state’s history.

One year later, the Mendocino Complex burned 459,000 acres, making it the state’s largest wildfire in acreage. The cause of that fire is still under investigation.

“In these two years, the state spent over $1.5 billion, more than any previous two year period,” said the study. The California Department of Forestry and Fire Protection says 13 of the 20 largest fires occurred during the summer. All but one happened in the last five decades.

The connection to the climate crisis

The cause of the increase is simple. Hotter temperatures cause drier land, which causes a parched atmosphere.

“The clearest link between California wildfire and anthropogenic climate change thus far has been via warming-driven increases in atmospheric aridity, which works to dry fuels and promote summer forest fire,” said the report.

“It is well established that warming promotes wildfire throughout the western US, particularly in forested regions, by enhancing atmospheric moisture demand and reducing summer soil moisture as snowpack declines.”

Williams told CNN that human-caused warming of the planet has caused the vapor pressure deficit to increase by 10% since the late 1800s, meaning that more evaporation is occurring. By 2060, he expects this effect to double.

“This is important because we have already seen a large change in California wildfire activity from the first 10%. Increasing the evaporation has exponential effects on wildfires, so the next 10% increase is likely to have even more potent effects,” he said.

The threat continues to increase for fall fires

Wildfires are not only common in the summer. In the fall, many California fires occur in coastal shrublands and are driven by extreme wind events, such as the Santa Ana and Diablo winds.

These strong offshore winds have very low humidity, which quickly dry the vegetation on the ground and spread wildfires when they occur prior to the onset of the winter precipitation.

For example, the Thomas fire in December 2017 was the state’s second largest wildfire in history.

Fires in the fall are often driven by these strong winds, and while this study only focused on the increases in summer fires from increasing temperatures, climate models indicate their coverage will also become increasingly large in the decades to come.