What is causing California's wildfires?

California's Rocky Fire has caused mass damage and displacement. Most wildfires are started by humans, but hotter, drier weather is said to be behind the recent outbreak.

California’s Rocky Fire was sparked Wednesday, exploded Friday, and by Sunday had burned through 54,000 acres of land over three counties, NBC reported. As of Monday, the fire still rages and torched acreage has increased to 60,000.

There were 21 active wildfires across the state of California as of Monday morning, according to data from the California Department of Forestry and Fire Protection (CAL FIRE), but the region west of Sacramento where the Rocky Fire is burning has “little to no fire history,” according to the agency. The “very dangerous fire” has led to the evacuations of more than 12,000 people so far, CAL FIRE spokesman Daniel Berlant told NBC, but the cause is still under investigation.

The Rocky Fire's origin may be unknown, but the surge in wildfires that have burned through California in recent years has been widely attributed to the state’s severe drought.

“California’s drought continues to cause wildfire activity to remain much higher than normal,” CAL FIRE reported. “In 2014, CAL FIRE responded to 1,000 more wildfires than in an average year and sadly that trend has continued well into 2015.”

California Gov. Jerry Brown ordered a state of emergency Friday night, according to NBC. "California's severe drought and extreme weather have turned much of the state into a tinderbox," he said.

According to the National Park Service, 90 percent of wildland fires are caused by humans. Dry conditions make it particularly dangerous to burn landscape debris or build campfires, or even to use everyday equipment like lawnmowers.

The other 10 percent of fires are the result of natural causes, like lightning and lava, and if no humans are threatened, these fires will often be allowed to run their course and burn out naturally to benefit the ecosystem.

Whether or not the Rocky Fire started because of human activity, it continues in spite of it. Officials told Capital Public Radio Friday’s east winds exacerbated the fire and made it hard to control in the west. CAL FIRE said more than 2,700 fire personnel were combating the blaze Sunday, but the region’s “steep and rugged “ terrain allowed for “limited access.” As a result, the fire was only 12 percent contained as of Monday.

Climate change activists expect wildfires to become an even greater threat as temperatures rise. The National Wildlife Federation (NWF) says warmer, drier climates will lead to longer fire seasons since snowpack will continue melting earlier in the year. They will also cause more intense fires because insects that feed on trees will thrive, leaving more wood dead and combustible. An increase in naturally occurring fires can also be expected, since more severe thunderstorms may bring more lightning – an estimated increase of 12 to 30 percent in the western United States by mid-century.

“The overall area burned is projected to double by late this century across 11 western states if the average summertime temperature increases 2.9 degrees Fahrenheit,” with the Southwest becoming the hardest-hit region, the NWF says.

Matt Jolly, a US Forest Service ecologist at the Fire Science Laboratory, told the Christian Science Monitor's Pete Spotts a global analysis of daily fire-weather conditions between 1979 and 2013 indicates that climate change is causing more regions of the country to experience longer wildfire seasons. Of the three factors that contribute to wildfires – weather conditions, fuel, and sources of ignition – “weather is the thing that we’re not going to have control over. So we have to focus on the other two aspects if we’re going to adapt,” Dr. Jolly said.

The study, which Jolly led and which appears in the current issue of Nature Communications, found that over the 35-year period fire seasons extended by an average of 18.4 percent, or four days – growing by as much as nine days in North American conifer forests and 33 days in South American tropical and subtropical forests, grasslands, and savannahs.

Additionally, "averaged around the world, the area of vegetated landscapes affected by longer wildfire seasons grew by 108 percent."

The Rocky Fire has not resulted in any reported injuries, Mr. Berlant told NBC Sunday, but a firefighter died Saturday fighting the Frog Fire about 100 miles south of the Oregon border.

Governor Brown said he and his wife were “saddened to learn of the tragic death of US Forest Service Firefighter Dave Ruhl, who left his home state to help protect one of California's majestic forests."