Nobody who lived through the 2018 California wildfire known as the Camp Fire is ever likely to forget it. The blaze, set off by a faulty power line in Butte County, in the northern part of the state, raged for 17 days, from Nov. 8 to Nov. 25, incinerating 240 sq. mi. of land, destroying more than 18,000 homes, and claiming 85 lives. By any measure, the Camp Fire was a traumatic event for those who experienced it. Now, a new paper published in PLOS Climate, has determined exactly how traumatic it was for the survivors, offering fresh insight into the long-term psychological cost of extreme climate events.

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The study, led by a team of researchers at the University of California, San Diego (UCSD), is based on surveys of 75 adults conducted in 2019 and 2020—six to 12 months after the Camp Fire occurred. Forty-eight of the subjects lived in the northern California region in or around Butte; another 27, chosen as a control group, live in the San Diego area. Of the 48 from Butte County, 27 were directly exposed to the fire—with their land or home damaged or destroyed by the flames; the other 21 were indirectly exposed—reporting that they knew of a friend or family member who suffered home or property loss. The 27 members of the control group were entirely unexposed.

The researchers found that exposure—even indirect exposure—to a climate trauma had a long term impact on mental health, in the form of both depression and anxiety. What’s more, the ability to focus on and perform cognitive tasks was adversely affected; both sets of results add one more item to the growing cost of climate change on the health and well-being of the population of the planet.

The researchers began their work by having all 75 subjects answer a standard screening question for post-traumatic stress disorder (PTSD): “Were you recently bothered by a past experience that caused you to believe you would be injured or killed.” The options for answers were “not bothered at all,” “bothered a little,” and “bothered a lot.” Of those directly exposed to the fire, 67% responded that they were either bothered a little or bothered a lot, compared to 14% of the indirectly exposed group and 0% of those unexposed.

The same subjects then filled in two more-comprehensive questionnaires measuring depression and anxiety. The depression survey asked them 10 questions including if, over the past two weeks, they had little interest or pleasure in doing things; were feeling down or hopeless; had trouble concentrating; and were chronically tired or had little energy. The anxiety survey asked such questions as whether the subjects were feeling anxious, nervous, or on edge; unable to stop or control worrying; and were so restless it was hard to sit still. On both surveys, the four possible responses were “not at all,” “several days,” “more than half the days,” and “nearly every day.” The tests were then scored on a one to 27 basis, with a score of one to four indicating minimal depression or anxiety; five to nine signifying mild cases of the conditions; 10 to 15 indicating moderate cases; and 15 or more qualifying as severe.

The results were striking. Those directly exposed to the fire scored an average of 10.1 on anxiety and 8.9 on depression, compared to 9.7 and 11.8 for those indirectly exposed, and just 3.2 and 2.6 for those not at all exposed. The results were especially noteworthy since both the directly and indirectly exposed people scored more or less equally on both the depression and anxiety scales—with the indirectly exposed individuals actually ranking higher on depression, suggesting that second-hand exposure to climate catastrophes can be as bad as or worse than first-hand.

“On the whole,” says Jyoti Mishra, a UCSD neuroscientist and a co-author of the paper, “depression and anxiety were one and a half to three times more prevalent in the directly and indirectly exposed group compared to the unexposed.”

The findings of the new study add to a growing body of work showing the psychic impact of extreme climate events. Previous studies in the journals Lancet Psychiatry and Psychiatry Services showed adverse mental health effects on hurricane survivors. A 2021 study by Mishra and others also showed higher rates of PTSD among 725 survivors of the Camp Fire.

By themselves, the depression and anxiety findings in the new study were troubling. But the researchers then went further, studying the brain function of the three groups. The subjects were fitted with electroencephalogram (EEG) arrays, while they played a series of four on-screen games designed to measure their memory as well as their ability to pay selective attention, filter out distractions, process emotions, and more. The subjects performed well on all of the tests except one—the one designed to measure the ability to filter out distractions.

That game, called Middle Fish, involved flashing the subjects a picture of a school of fish with one clearly in the center. The central fish was facing either left or right while the flanking fish were arrayed with some facing one way and some facing the other. The subjects had one second to click on the direction the middle fish was facing while ignoring the flanking fish—a harder task than it sounds given the deliberate distractors and the brief time frame. Here there was a marked difference among the groups. For purposes of comparison, the score of the unexposed control group was recorded as 1.0; the indirectly exposed group underperformed with a score of 0.8, while the directly exposed group weighed in at just 0.6.

“The flanker fish interfere with your processing,” says Mishra. “The directly and indirectly exposed groups were more sensitive to those distractions.”

The EEG readings revealed another dimension of the test results. In general, the lower the directly exposed subjects scored, the greater the activity was in the frontal and parietal regions of the brain, indicating that they were making more of an effort to get the game right, but were nonetheless performing worse on it than the unexposed group.

“The directly exposed group especially was putting in about 20% more effort than the other two groups,” Mishra says.

As the authors of the paper added in a statement accompanying its release: “Our study shows that climate trauma may affect cognitive and brain functions especially with regard to processing of distractions.” Though the researchers didn’t measure the knock-on effects of that finding, a reduced ability to filter out distractions could have a negative impact on work performance, child-rearing tasks, and other activities that require close focus, including driving or operating machinery.

Performance on video games hardly matters by itself when it comes to the suffering experienced by people in the path of wildfires and other extreme events like hurricanes and flooding. But the study does show that the post-traumatic effects of climate change are real, and should become part of the thinking when it comes to regulating climate-changing activities and providing mental health services to survivors of climate-related disasters.

“Our study is a first step toward quantifying these effects,” says Mishra. “We need to keep this in mind when we’re thinking about the solutions we’re going to create for our communities and the impact of these events on the people living in the areas affected.”