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Wildfire Weather Increasing in California, West, Report Says

Wildfires have become more frequent in the Western United States over the past five decades, with some of the largest increases in California, according to a new report from Climate Central, a nonprofit media outlet that reports on the climate change.

The report examines three key weather conditions – heat, drought and wind – which, when combined, cause wildfires to spread quickly and grow in size, said Kaitlyn Trudeau, senior research associate at Climate Central.

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“We’re really talking about days where the stage is set for significant wildfire growth,” she said. “These three conditions work together to create truly dangerous weather conditions. »

The report is a reminder that the western United States has become hotter and drier, which tends to encourage more large wildfires, said Park Williams, a climatologist and professor in the UCLA department of geography. , who did not participate in the analysis.

“We’ve seen in a lot of academic research over the past decade that fires have increased in the United States – and most of that increase has been in the West – over the last half-century,” he said. he declared. “This report is therefore entirely consistent with this general conclusion. »

Burning fossil fuels has increased the risk of fires due to rising global temperatures, Trudeau said. Because warmer air can hold more water, the atmosphere has become thirstier, drawing more moisture from plants and soils and causing them to burn more easily, she said.

“Because we can attribute this warming to climate change, and because of the relationship between relative humidity and temperature, we know we can attribute some of it to climate change,” she said.

Researchers analyzed hourly observational data from 476 weather stations in the Lower 48 states to calculate the number of fire days recorded at each station, on average, for each of the past 51 years. They defined a fire weather day as a day when temperatures, relative humidity, and sustained wind speeds simultaneously reached certain thresholds for at least two hours out of 24.

The report finds that the average annual number of fire days is increasing across the majority of California, particularly in inland areas of the state. The southern interior, the part of the state in the southwest desert basin, saw the largest increase with an increase of 61 fire days per year, on average, between 1973 and last year.

Generally speaking, an increase in fires would tend to have the least effect on this already very dry and sparsely vegetated desert region, because the warming and drying of the atmosphere does not necessarily make it more likely. of fires, Williams said.

“Instead, giving the earth a little water in order to grow new fuels is the first thing you need to increase the fire,” he said.

But higher elevation desert areas that have received good doses of precipitation over the past couple of years may have enough connected vegetation to fuel a major fire. The basin region includes part of the eastern Mojave Desert, where the York Fire last year became California’s largest fire.

California’s San Joaquin and Sacramento watersheds have seen smaller, though notable, increases in average annual fire days of 14 and 13, respectively, since 1973, according to the Climate Central report.

Parts of New Mexico, Texas and Arizona have seen large increases in the average annual number of fire days. In contrast, parts of North and South Dakota, where the spring cooled slightly, saw a decline, according to the report.

There were some drawbacks to doing a national-scale analysis, Trudeau said. There are different ways to define fire weather depending on a region’s local climate, and it was difficult to come up with a range of criteria that could be used across the United States, she said .

“For this reason, parts of this analysis do not fully represent the story,” she said.

For example, the researchers decided to use region-specific relative humidity thresholds, based on criteria set by the National Oceanic and Atmospheric Administration and the National Weather Service’s Storm Prediction Center, that are less than or equal to 20 % for a large swath of the West, including the California coast, Trudeau said.

As a result, the report calculates relatively small increases in the average annual number of fire days for California’s regional coasts: four days for the Central Coast, three for the South Coast and one for the North.

“It’s not necessarily the full picture because we’re essentially asking a coastal area to meet this threshold which is very difficult to achieve when you’re right next to a large body of water,” Trudeau said.

Still, she said, the fact that there have been days when the California coast was so dry and that the frequency of those days is increasing, even modestly, is significant.

“In fact, we’re seeing these areas of California have very poor conditions, where the weather becomes extremely dry even when you’re right on the ocean,” she said.

Trudeau also noted that the report does not take into account the profound changes in vegetation that have occurred over the past 50 years, as parts of California’s forests have been altered by fire suppression, the cancellation of indigenous cultural fires and industrial logging. It also does not take into account the incidence of dry lightning, which sparked some of the state’s largest fires, or the extent to which development has spread into wild areas, putting more people and properties at risk, she said.

“Our analysis should certainly not be viewed as a blanket summary of what the fire risk is in California, or the weather risk, because there are many other variables,” she said. “But it gives you a piece of the puzzle.”

Trudeau took a personal interest in the research. She grew up in Placerville, a small town in the foothills of the Sierra Nevada between Sacramento and Lake Tahoe, and saw first-hand how warming temperatures have reshaped the landscape. “I saw the seasons change, I saw the fires coming, I saw we had a lot less snow, a lot less rain,” she said.

“Just being a Californian and having spent most of my life here, the results really follow what we’re seeing and follow my experience living in this great part of the world that carries, unfortunately, really immense and increasing risk.

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