Wildfire patterns change with time, but never in the past century have they been larger and more devestating in Southern California, research shows.

"Fires are clearly larger," said UC Riverside Professor of Earth Sciences Richard Minnich, who has specialized in the ecology and meteorology of wildfires.

Analyzing wildfire data as far back as the late 1800's, Minnich sees a shift from smaller summer fires to larger fall fires more likely to occur during extreme weather events such as Santa Ana winds and heat waves that amplify fire growth.

Minnich's conclusion that much of the change is due to fire supression has drawn controversy, but he contends the data clearly show that patterns began changing after the onset of efforts more than a century ago to extinguish wildfires burning on federal forest land and near growing communities.

The data he analyzed show that fires in the late 1800's, often ignited by lightning, occured most often during summer, rarely during windstorms, and seldom burned more than 10,000 acres. Analyzing fires by decade, he found larger fires began appearing in the early 20th century, and that a signifcant inflection point occurred in 1970, when Southern California was hit by three massive wildfires.

Minnich credits William Mulholland, Los Angeles' chief water infrastructure engineer, with realizing as early as the 1920's that extinguishing small fires enabled a build-up of vegetation.

Ultimately, however, the fuel is ignited during an extreme weather event, and with more energy from the added fuel, the fire burns for an extended period of time before it can be extinguished.

"We've robbed Peter to pay Paul," Minnich said. "Instead of slow burning fires during summer, we have fast burning fires in fall."

The focus on fire origin, Minnich, believes is less significant than the conditions that empower a fire to grow quickly and expand uncontrollably until circumstances change.

"Now that we're in suppression we're preventing all but the fires in the most exteme weather states, and that tends to syncrhonize fires at once," Minnich said.

Minnich's research focused on the impact of the buildup of heavy brush and trees in forests, and he acknowledges that other factors are having an effect on the major firestorms this week in the Anaheim Hills and, even worse, in Northern California's wine country, where multiple fires in four counties have claimed at least 23 lives and continue to burn out of control.

The wetter than usual winter enabled growth of grasses that dried out during summer, becoming a major source of fuel for the wildfires at both ends of the state.

Unlike trees and brush that accumulate over decades, grasses are annuals that sprout anew each year and are largely independent of an area's fire history. Fire burns more quickly through dry grass than most other vegetation, and so it can contribute to fast moving blazes. Minnich suggests it would be worthwhile to consider cattle grazing to remove dry grass from fields and slopes within striking distance of homes and communities.

Minnich believes there are locations deep in wilderness areas adjacent to recent burns where it is possible to allow fires to burn out safely, but not of course where homes or communities are nearby.

Given the situation that has evolved over the past century, with fuel buildup near developed areas, Minnich stresses the importance of making homes there as fire resistant as possible, without any open wood eves or exposed wood siding or decks. He believes the tile roofs, stucco siding and other precautions built into homes in Anaheim Hills, along with buffers of brush clearance, reduced the number of structures burned there during the second Canyon Fire. Minnich said it appears that some houses burned from the inside out, likely caused by airborne embers entering attic space through a vent, and suggested building codes require a means to seal vents when fire approaches.