California Has Two Fire Seasons, and Climate Change Will Make Both Worse

A new study confirms what many Californians have suspected for some time: that climate change stands to make California wildfires more frequent and more destructive.

The study, published Tuesday in the journal Environmental Research Letters, found that destructive fires in California have increased in both number and severity over the last decades, a change the authors pin at least in part on the planet's warming climate.

The study's authors identify two distinct fire seasons in California. There's the well-known Santa Ana season, characterized by hot, dry winds starting in autumn that blow toward the coast from the interior, and an earlier summer season in which dry, sun-warmed vegetation falls prey to wildfire. Each season has its characteristic fire patterns... and climate change seems to be making each season worse.

[Related: 10 Things to Know About California Wildfires]

The study was conducted by scientists from the University of California at Irvine, UC Davis, and UCLA, as well as the U.S. Forest Service and the Jet Propulsion Laboratory. Researchers examined data on Southern California wildfires over a 50-year period from 1959 through 2009, obtained from the California Department of Forestry and Fire Protection and the U.S. Forest Service.

The results were striking, with sobering implications in a warming world. Santa Ana fires, which can occur from October through April, really are quite different from their summer season counterparts.

The Santa Ana fire season is better known for a reason: Santa Ana fires tend to strike more developed areas along the coast, and thus inflict more economic damage. Examples include the 2003 Cedar Fire in San Diego County and the devastating 1993 Laguna Beach Fire. Though the phrase "Santa Ana winds" is generally used only in Southern California, 1991's destructive Oakland Hills fire also occurred during the Santa Ana fire season, stoked by intense dry winds blowing westward from the interior of the state.

While Santa Ana fires tend to burn along the coast, summer fires can take place anywhere in the state, often in remote and wild areas where summer temperatures have dried out accumulated woody debris. (The 2009 Station Fire in the San Gabriel Mountains offers a familiar example of a recent summer fire.)

Summer fires tend to burn between June and September, according to the study, which means that California cycles between Summer Fire Season, Santa Ana Fire Season, and May.

Non-Santa Ana fires tend to burn more slowly, say the study's authors, while Santa Ana fires burn fast and dangerous, often doing half their total damage in the first 24 hours after they're sparked. Both summer and Santa Ana fires burned about the same land area over the period studied, but Santa Ana fires did 10 times as much economic damage as summer fires.

Both types of fire increased in frequency and severity over the study period, suggesting that climate change -- and a generally warming, drying climate in California -- might be creating conditions more conducive to fires. And as forecasters expect a warming climate will make Santa Ana winds more frequent and faster, that Santa Ana blowtorch is likely to do a lot more damage to the developed parts of the state.

Another important detail emerging from the data was that areas burned in summer fires tended to burn only once during the 50-year study period, suggesting that so-called "let-burn" policies might be a reasonable approach for remote summer wildfires that don't threaten public safety. Allowing a remote summer fire to burn out safely might prove a sensible fire management strategy. Santa Ana fires, on the other hand, often recurred in areas that had seen several previous Santa Ana fires.

In a year in which firefighters are spread thin across California and the rest of the western United States that might offer planners a bit of guidance in allocating scarce resources. Increasing heat and more frequent drought will likely make summer fires more common, increasing pressure on limited firefighting crews and equipment. Focusing on fighting more costly Santa Ana fires will likely make more and more sense, say the study's authors.

"This research is coming at the right time, considering that California and western areas of the United States are expected to face increased fire risk in the near term as the current multiyear drought is expected to continue and grow in intensity," said lead author Yufang Jin, an assistant professor in the Department of Land, Air & Water Resources at UC Davis.

"The traditional one-size-fits-all fuel management strategy will likely not be effective in reducing fire risks and preventing large fires," said Jin.

"The large economic and human impacts of Santa Ana fires raises the question of whether more resources during fall could be marshaled for suppressing these fires," said James Randerson, Chancellor's Professor of earth system science at UC Irvine, and senior author on the paper.