This story was originally published by Wired and is reproduced here as part of the Climate Desk collaboration.

California is famous for its beach weather, but it’s also becoming infamous for its “fire weather,” where high temperatures, strong winds, and low humidity make the landscape burn. It is no coincidence that we have heard so much about forest fires in recent years: thanks to climate change, fire weather is increasing, a new analysis shows.

“It’s not just hot. It’s not just dry. All of these conditions happen at the same time, ”said Kaitlyn Weber, data analyst at Climate Central, a nonprofit news group that published the analysis. “There is very clearly an increase in these fire weather days that has been occurring in much of the western United States since the early 1970s.”

Weber analyzed data from 225 weather stations in 17 western states since 1973 and examined temperature, humidity and wind speed, the three main variables that cause catastrophic fires. High temperatures and low humidity suck moisture from vegetation to create dry fuel so a spark can easily ignite wildfire, which can then be driven across a landscape at incredible speeds by fast winds. The 2018 bonfire, for example, moved so fast that it overwhelmed the town of Paradise, killing 86 people, many in their cars, trying to leave town.

FIGURE: LOCAL CLIMATOLOGICAL DATA FROM NOAA / NCEI

In the maps above, we can see the percentage change in the anniversaries on which these three variables exceeded the thresholds Weber used for their analysis. (Blueer colors mean fewer days, redder colors mean more days.) With wind, for example, this means speeds of over 24 km / h, and for the temperature it is 45 to 55 degrees Fahrenheit, depending on the season.

You’ll find that the southwest in particular has gotten a lot hotter and drier – perhaps no surprise. At the same time, there are many more windy days in the region when an ignition can quickly turn into an intense fire.

FIGURE: LOCAL CLIMATOLOGICAL DATA FROM NOAA / NCEI

The above map visualizes when these three variables – temperature, humidity, and wind – were combined to produce fire weather days, plotted as a percentage change since 1973. All parts of Colorado have seen at least 100 percent more fire weather days. Texas also looks gnarled, with a 284 percent increase at the southern tip of the state. And central California is similarly troubled, with fire weather days up 269 percent. “The southwest has really caught on,” says Weber. “We even see some parts of Oklahoma and Kansas, some of those places where we traditionally don’t think of fire.”

But if you’re wondering why we don’t hear too often about catastrophic fires in the Prairie States, as we do in California, Oregon, and Colorado, it’s because “fire weather” just means the conditions are right for a fire – it’s not. It doesn’t mean that they necessarily happen. “We’re not talking about igniting fires here,” says Weber. “We’re talking about the number of days in the year that the elements of the weather have prepared the landscape for these high-risk fires that are really more dangerous and really harder to fight.”

Atmospheric conditions aren’t the only variables that increase the likelihood of forest fires. Land management decisions in California and Oregon, for example, play a role. These coastal regions are covered in forests that once burned regularly in a healthy way: lightning would ignite a relatively small fire that gnawed through the bushes, making room for new growth, but left many old trees alive. Historically, Native Americans also deliberately set fires to strategically rebuild ecosystems. The landscape burned a lot, but that also meant it burned less intensely, as combustible shrubbery could not pile up between burns.

But for the past century or so, land managers have taken the opposite approach: extinguishing fire or immediately extinguishing anything that could enter residential areas. This enables the build-up of dry vegetation – more fuel. And as more and more human communities live in the “wildland-urban interface” where the forest meets the cities, people also start more accidental fires, be it from a cigarette butt thrown out of the window or from a malfunction of the electrical infrastructure .

This is one of the reasons California fires are so much more catastrophic than Kansas or Oklahoma: there is simply a lot more forest with a lot more accumulated fuel and a lot more people at risk. To adapt, land managers in the western states need to conduct more controlled burns, which do the brush-clearing work that used to do common, smaller-scale forest fires.

Climate change has also forced some seemingly contradicting seasonal changes. Since a warmer atmosphere contains more water, the amount of precipitation may even increase in the future, while the duration of the rainy season becomes shorter. In California, rain usually comes in October and lasts through March. Now they come later in the year. “The dry season will expand into the normal rainy season,” says climate researcher Ruby Leung of the Pacific Northwest National Laboratory. “If we look at climate models projecting into the future, the fire season will be longer.”

The fire brigade already sees this. California had its biggest flames in autumn, just before the seasonal rains set in, when the landscape dried out after half a year without water. This coincided with violent seasonal winds that would start huge forest fires. But now, because the rainy season is so short and the landscape has more time to dry out, the fire season comes even earlier. “What we are seeing more and more regularly is the fact that these fires are getting bigger and bigger, earlier than usual in the past,” said Issac Sanchez, communications battalion chief for the California Department of Forestry and Fire Protection, Wired said earlier this month. “So when August pulls by, by the end of July, we’ll see these arid conditions that are absolutely due to climate change.”

Oregon has also seen an increasing number of catastrophic forest fires recently, fueled by the inexorable increase in fire weather days. And Weber believes things will only get worse until we slow global warming. “I think we can definitely expect the days of fire weather to increase as the climate continues to warm,” she says. “No matter what we do, there is no easy way out. We should just call it what it is: there is no substitute for reducing our emissions and that is really the name of the game. “

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