![]() Outside of the forested areas of California, wildfire area has not significantly increased since 1972. That means fuels are drier today than they were a century ago.” It is also warmer by about 1.5☌ than it was century ago. “When you get into fall, that's when vegetation is driest because it has not rained since spring. “Even though rain and the winds haven't changed by our calculations, the fall fire risk has actually increased,” said Williams. ![]() But if the winds are strong and rains are late, then the extra warming helps create destructive fires. (The Camp Fire is a prime example.) Unlike summertime forest fires, the severity of fall fire season is influenced by the timing (regular or late arrival) of winter rains and by strong Santa Ana wind events (to fan and spread flames), neither of which showed a significant increase. Although fewer fires occur in that season, they tend to be more dramatic now. Both plots show the future expected temperature trends for the region based on the climate model, Coupled Model Intercomparison Project (CMIP5).įall wildfires also evolved. Over the same period, the maximum temperature for October to November increased by 1.67☌, which corresponded to an increase of vapor pressure deficit of 14 percent. These observed trends match expected trends from climate models responding to anthropogenic (human-caused) emissions. From 1896 to 2018, the maximum temperature for March to October increased by 1.81☌, which corresponded to a 13 percent increase in vapor pressure deficit (the amount of water missing from the atmosphere). The plots above show the maximum temperatures from March to October and October to November, averaged across the four California study regions. “By fighting fires and living in forests, humans have increased the vegetation available to burn and the sparks needed to start fires,” said Williams, “so now more than ever the table is set for big fire as soon as hot and dry conditions arrive.” In fact, human activities have probably strengthened the effect of warming in some areas. That is not to say other non-climate factors are not important, said Williams. Specifically, the team found the strongest relationship between summer forest fires and the aridity of the atmosphere-a combination of heat and the absence of water vapor. “Essentially all of the increase we’ve seen in annual wildfire area in California over the last 50 years is driven by warming in the atmosphere during summertime,” said Williams. Overall, burned area from summertime forest fires increased by eightfold over the past five decades. The scientists found that the greatest increases in burned area occurred during the summertime (May-September), and particularly in the forests of the North Coast and Sierra Nevada regions. ![]() (Much of this data was derived from NOAA and NASA observations). Using climate models, the team then correlated the burned area with data on monthly precipitation, temperatures, humidity, wind speed, and solar radiation. The burned area data came from CalFire Fire Resource and Assessment Program and the Monitoring Trends in Burn Severity dataset, which includes Landsat observations. Overall, the annual burned area increased by 405 percent during that period. The graphs above show the seasonal and annual burned area in California for 1972-2018. Williams and colleagues separated California into four different regions and examined fire activity during different seasons. ![]() “But in California, the story is bound to be really complicated because there are a lot of different climate types, vegetation types, and different ways humans are interacting with the landscape.” “Warming is affecting forest fire,” said Park Williams, lead author and a researcher from Columbia University. According to the researchers, the annual burned area across California has increased fivefold over the past five decades, and the main driver is higher temperatures. They fit with a trend of more frequent and bigger fires. In 2018, California also suffered its most destructive fire ever and set a new record for burned area in one year.Ī study published in July 2019 shows these remarkable fire years are no longer freakish anomalies. One year later, the Mendocino Fire Complex took its place. In 2017, California endured the Thomas Fire, the state’s largest fire (by area) at that time. If it seems like California has been getting hit by more and larger fires lately, that's because it has. ![]()
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