The devastating blazes that swept through Southern California these last few days were largely unrelated to changing weather patterns due to global warming, as some newscasters and pundits have stated. I have been a believer in the danger of global warming for more than two decades, but I don't think it's the culprit here. Rather, the blame for the conflagrations should be placed on chaparral, a growing population, and inadequate suburban planning. Fire in the hills behind Malibu and San Diego is inevitable, and at the moment at least is not connected to our pumping of greenhouse gases into the atmosphere.
It is true that droughts in the West triggered by global warming and/or natural cycles indeed lay conditions for devastating forest fires in higher elevations. This holds true for the conifer forests above 6,000 feet or so in the San Gabriel, San Bernardino and other local ranges. It may have played a role in the Lake Arrowhead blazes. But for most of the Southland, it's a different scenario.
The Santa Monica mountains and many other hills and ranges in Southern California are primarily covered by the dense prickly shrubs and small trees known collectively as chaparral. This native vegetation blankets our mountain slopes up to the point at which conifers take over. Manzanita, scrub oak, chamise, laurel sumac, California lilac, and buckwheat are among the predominant species of chaparral. The toyon shrub, also called "Christmas berry" or "California holly," reputedly put the "holly" in Hollywood. Chaparral thrives in Southern California's Mediterranean climate of mild winters with moderate precipitation and hot, dry summers.
Chaparral adapted over the millennia before man's arrival to natural fires caused by lightning strikes. Most chaparral species can't reproduce without brushfires. They have hard seeds that will not germinate without fire; they lie dormant in the earth for decades until the next blaze comes along. Many chaparral species also sprout back vegetatively from root crowns after a fire or other disturbance. "Fire ecology" studies such interrelationships. Where there's chaparral, there are going to be wildfires.
Over the long run, high rainfall during wetter years can actually set the stage for massive conflagrations during the dry season. When the rainy season (usually November through April) is wetter than usual, the chaparral grows abundantly, laying in extra fuel for the next big blaze. The most flammable areas of chaparral are those with the most biomass, typically patches at least thirty to forty years old. The older the chaparral, the more potential there is for a catastrophic wildfire.
Global warming could play an exacerbating role if it causes Southern California rainfall to increase. But the numbers don't show that happening in L.A., for example, nor do they show annual precipitation to be varying more than usual there. Los Angeles has rainfall records stretching back to 1877. Downtown L.A. averaged 15.11 inches from 1877-2005, according to the NOAA (National Oceanic and Atmospheric Administration). While Los Angeles had only 3.21 inches in 2006-07, its all-time low, and 37.25 inches in '04-05, close to its record high, those numbers are not unexpected. L.A. has had seven seasons of rain above 30 inches since 1877, and seven years in the four to six inch range. It's had 35 seasons below 10 inches, and 25 above 20 inches. Annual rainfall fluctuates greatly now, just as it did a century ago. The last ten years have averaged 15.3 inches, close to the norm. Global warming could cause the swings to get more extreme, but that hasn't happened yet. Just check the NOAA graphs.
The extremely dry winter of 2006-07 created prime conditions for this year's fires, but the potential for wildfires is always present in chaparral areas. The Southland's long hot dry season (usually May through October) and the fall's Santa Ana winds inevitably create dangerously flammable conditions. And a 50mph Santa Ana on a 90-degree day will dry out brush by itself and threaten to take any fire to a catastrophic level.
Chaparral will burn with regularity over the long term, and anyone who chooses to live near it will eventually be threatened by a brush fire. As California's population swells, bringing with it an increase in accidental and intentional fires, and as developers continue to build new homes without appropriate planning in foothills and former wilderness areas, we can expect the fire danger to grow. For the near future, at least, these are the major factors in our wildfires, not global warming.