Twin Northern California blazes fueled by dry vegetation and hot, windy weather grew Monday to become the largest wildfire in state history, becoming the norm as climate change makes the fire season longer and more severe.

The two fires burning a few miles apart and known as the Mendocino Complex are being treated as one incident. It has scorched 283,800 acres (443.4 square miles or 1,148.4 square kilometers), fire officials said Monday.

The fires, north of San Francisco, have burned 75 homes and is only 30 per cent contained.

The size of the fires surpasses a blaze last December in Southern California that burned 281,893 acres . It killed two people, including a firefighter, and destroyed more than 1,000 buildings before being fully contained on January 12.

Hotter weather attributed to climate change is drying out vegetation, creating more intense fires that spread quickly from rural areas to city subdivisions, climate and fire experts say. But they also blame cities and towns that are expanding housing into previously undeveloped areas.

More than 14,000 firefighters are battling over a dozen major blazes throughout California, state Department of Forestry and Fire Protection spokesman Scott McLean said.

“I can remember a couple of years ago when we saw 10 to 12,000 firefighters in the states of California, Oregon and Washington and never the 14,000 we see now,” he said.

The complex of fire has been less destructive to property than some of the other wildfires in the state because it is mostly raging in remote areas. But officials say the twin fires threaten 11,300 buildings and some new evacuations were ordered over the weekend as the flames spread.

Farther north, crews gained ground against a deadly blaze that has destroyed more than 1,000 homes in and around Redding. It was nearly halfway contained, Cal Fire said.

A hillside smolders after flames passed through during the Ranch Fire in Clearlake Oaks, California, on Sunday

The wildfire about 225 miles (360 kilometers) north of San Francisco started more than two weeks ago by sparks from the steel wheel of a towed-trailer’s flat tire. It killed two firefighters and four residents and displaced more than 38,000 people.

Officials began allowing some residents to return to their neighborhoods. But tens of thousands of others were still evacuated.

The fires in Northern California have created such a haze of smoke in the Central Valley that Sacramento County health officials advised residents to avoid outdoor activities for the entire week.