Wildfire is a natural and common phenomenon in Southern California, and has been a part of life here for so long that our local ecosystems are adapted to it. Fire can be a source of renewal in the chaparral landscapes of our hills and the coniferous forests high in our mountains. Some species depend on fire for their very existence. Of course, wildfire becomes a major concern when it threatens people's lives, homes, and other structures.
In the past few decades, as development has encroached into wildfire zones, we've seen human and economic impacts of wildfires expand. And since hot weather is an important factor in wildfire risk, many planners and researchers worry that climate change will increase wildfire's toll even further.
To answer questions about the future of wildfire, scientists from UC Davis, UC Irvine, and UCLA, the US Forest Service, and NASA's Jet Propulsion Laboratory teamed up to quantify the drivers and impacts of wildfire in the current climate, and to project future impacts given climate change. Their findings are now published in a paper titled "Identification of Two Distinct Fire Regimes in Southern California: Implications for Economic Impact and Future Change" in the scientific journal Environmental Research Letters, published September 8.
Wildfire and Its Impacts in the Current Climate
Before considering the future of wildfire in Southern California, the researchers set out to understand wildfire in the current climate: what drives it, where it occurs, how much damage it does, and how much that damage costs. Concentrating on the period of 1990 to 2009, the researchers studied the wildfires that occurred in seven California counties: Santa Barbara, Ventura, Los Angeles, San Bernardino, Orange, Riverside, and San Diego. They classified the fires into two broad categories: those that were driven by Santa Ana winds, and those that were not. It was important to the researchers to consider these two types of fire separately because they have very different characteristics.
Santa Ana Fires: The Santa Ana winds typically blow between the months of October and April, when dry air from the Mojave Desert flows toward the coast. These winds become concentrated as they flow through mountain passes, and warmer as they descend in elevation. The result is powerful, hot, dry winds that happen to be very good at stoking and spreading fire. Wildfires driven by the Santa Ana winds are common in windy corridors and near the coast, and they tend to spread very quickly. A typical Santa Ana fire burns intensely; of all the area it ultimately burns, half is burned in just the first day. San Diego's Cedar Fire, as well as the dozen or so other fires that burned simultaneously throughout Southern California in late October 2003 as shown in the following photo, are examples of Santa Ana fires.
Non-Santa Ana Fires: These fires are more common in the months of June through September, when temperatures are high and any moisture coming from winter rains or spring snowmelt is long gone, leaving plants and trees dry and prime to serve as fuel for fire. Non-Santa Ana fires are more common in inland areas. They spread more slowly and are less intense than Santa Ana fires, but tend to burn longer. The 2009 Station Fire in the San Gabriel Mountains is an example of a non-Santa Ana fire.
Examining individual fires in both categories from 1990 to 2009, the researchers found that even though there were three times as many non-Santa Ana fires as Santa Ana fires, the two types were responsible for burning roughly the same amount of total area over the period as a whole.
The Santa Ana fires were far more destructive to human structures, though, damaging or destroying nearly five times as many structures as non-Santa Ana fires. This difference reflects the fact that Santa Ana fires tend to spread faster and tend to occur in more densely populated areas. Over the 1990-2009 period, Santa Ana fires racked up at least $20 billion in property losses.
Climate Change and Wildfire
To assess how climate change is likely to affect wildfire risk in the future, the researchers built models of wildfire risk based on their observations of the relationship between fire risk and climate variables like temperature, wind, and relative humidity in the present climate. Then they fed these models with data from five projections of future Southern California climate from 2041 to 2060. These projections were produced by taking output from five different global climate models and using techniques to bring it to a very high resolution that reflects the influence of Southern California's unique topography on its climate. (Global climate models are the tools scientists use to project future climate, and the model outputs selected were from the same set the Intergovernmental Panel on Climate Change used in its most recent climate change assessment.) The global climate model projections assumed that greenhouse gas concentrations, the main drivers of climate change, would proceed on a "business as usual" path, with no significant global action taken to curtail greenhouse gas emissions.
The researchers compared the wildfires projected for the 2041-2060 period with those occurring in a baseline period of 1981-2000. Here's what they found:
- The area burned by Santa Ana fires increases by 64 percent, mainly because four of the five global climate model projections showed more intense Santa Ana wind events.
- The area burned by non-Santa Ana fires increases by 77 percent, mainly because of an increase in temperatures.
- The number of structures destroyed by Santa Ana fires increases by 20 percent, and the number of structures destroyed by non-Santa Ana fires increases by 74 percent.
The following figure shows how the increase in the number of fires and area burned breaks down by month of the year:
Implications for Fire Management
The researchers' findings indicate that Southern California should be prepared to fight more and larger fires in the future. Prescribing the most effective approaches to controlling future wildfires was outside the scope of the study, but the findings do offer some clues. Santa Ana fires were mainly driven by wind and relative humidity, and gobbled up fuels in their path regardless of the age of tree and plant stands. By contrast, the age of trees and plant stands did affect the spread of non-Santa Ana fires; younger stands were less flammable and curtailed the spread of fire.
This suggests that efforts to reduce fuels, such as prescribed burns or other methods of thinning vegetation, might help to control non-Santa Ana fires. Finding effective strategies for curtailing the spread of non-Santa Ana fires could be very important, since these fires occur mainly in the summertime, when fire risk is high throughout the Western U.S. Other studies have found increased risk of fire in the future from climate change in other areas of the Western U.S., so an increase in summertime fires in Southern California could mean greater competition with other regions for fire-fighting resources.
Currently, about the same amount of money is spent to fight Santa Ana fires as to fight non-Santa Ana fires. That Santa Ana fires are so much more destructive to human structures suggests that fire managers may want to consider allocating more funds toward fighting them, but an assessment of the pros and cons of this approach was beyond the scope of the study.
What About Public Health and Ecosystem Impacts?
Summertime fires pose risks to public health by contributing to poor air quality. While the study did not focus on future public health impacts of fires, the researchers expect that increases in non-Santa Ana fires would worsen them.
While Santa Ana fires tend to occur in more urbanized areas near the coast, non-Santa Ana fires tend to occur in the wildlands further inland and in the mountains. Increased non-Santa Ana fires could damage mountain forests more severely, especially since a long history of suppressing fires renders these forests more vulnerable to fire. (The irony of constant fire suppression is that it lets fuels buildup and makes forests more flammable.)
This study was funded by grants from NASA and the Jenkins Family Foundation. The climate change projections used to analyze the future of wildfire were created by UCLA as part of the Climate Change in the Los Angeles Region project, organized by the Los Angeles Regional Collaborative on Climate Action and Sustainability and funded by the US Department of Energy, the City of Los Angeles, and the National Science Foundation.