There was a time when fire season in California started around May and went through September. Now, thanks to a drought that’s stretching into its fourth year, the state seems to have become a year-round tinderbox.

The long running drought has “created explosive fire conditions,” said Mike Mohler, a fire captain with the California Department of Forestry and Fire Protection (CAL FIRE). “Five years ago without a drought in California you would still get wildland fires. But the vegetation wouldn’t burn as quickly. Now there’s zero moisture and you get explosive fire growth.”

The drought has fed into a trend that’s been developing for over the past decade, said Daniel Berlant, chief of public information for CAL FIRE.

“Since 2000 we’ve been seeing larger and more damaging fires,” he said. “What we’re seeing now is that the rain is starting later and stopping much earlier. The fires are burning at explosive speed because the vegetation is so dry and that allows them to get much larger.”

It’s not just the low precipitation that’s the problem, Berlant says. It’s also the higher temperatures.

Those high temperatures are baking the moisture out of the soil and air, leaving vegetation especially dry and vulnerable to catching fire, said Park Williams, an assistant research professor at the Lamont Doherty Earth Observatory of Columbia University.

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Rising temperatures alone can boost the risk of wildfires, Williams said. “When you see wildfires in the Sierra Nevada and Northern Coast Ranges occurring early in the season it’s a testament to how warm conditions have become in spring,” he adds.

In a recent study published in the International Journal of Wildland Fire, Williams found that, in the Southwest, rising temperatures raised fire risk even in wetter years. That finding is likely to hold true for California, too, he said.

“Forests are excellent gauges of climate because they are not irrigated and they live for a very long time, allowing for tree-ring records and other aspects of forests to tell us how recent conditions fit into a long-term context,” Williams said. “As temperatures rise, the amount of water necessary to reduce a forest’s flammability goes up. In the absence of substantial increases in precipitation, warming causes increased flammability, and that very reliably translates to increased areas of forest burned.”

The hot and dry conditions seem to have boosted the likelihood of fires in California even in the early part of this year. In past years on average between January 1 and April 18, there would be 492 fires burning 1,300 acres. During the same time period in 2014, it was 862 fires burning 2,417 acres, and in 2015, 838 fires scorching 3,534 acres.

In just the past week, there were more than 170 new wildfires that popped up in California.

Although the ongoing drought has intensified the problem, there’s been a growing trend toward greater flammability in California.

“Over half of the top 20 largest wildfires have occurred since 2000,” CAL FIRE’s Berlant says. “It’s important to note that the drought hasn’t created the problem, it only magnifies it. Because the vegetation is so dry it burns at an explosive rate. And then the fire gets large before we can even get on the scene. In the King Fire last year we saw about 60,000 acres burned in a six-hour period.”

Mohler says the new conditions haven’t changed firefighting that much. “Our tactics remain the same, although we are aware of the types of brush conditions,” he says. “But because of the drought we are having to locate other water sources for our aerial program. Some of the holding ponds in central California are just not there anymore. So we have to plan prior to the fires where the helicopters can go to fill up.”

The state has recognized it has to beef up its troops in the face of a more ferocious fire season.

“This current fiscal year, under the Governor's State of Emergency, our drought augmentation gave us an increase in our fighting force at a cost of $113.7 million,” Berlant said. “This has resulted in several hundred additional firefighters hired specially due to drought-caused fire activity.”

Ultimately, though, the increased year-round flammability of the state has forced CAL FIRE to make a paradigm shift to emphasize prevention.

Homeowners are encouraged to create a 100-foot defensible zone around houses where brush and anything combustible is cleared out. Builders are encouraged to put up homes made of fire resistant materials.

“Before we were focusing heavily on suppression,” Berlant said. “Now we’re spending millions and millions on prevention work. We’re trying to make communities more sustainable against fires. And we’re making sure we’re clearing brush and making forests healthy.”