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Wildfires are uncontrolled fires that begin in forests or other wilderness areas. Wildfires often spread to neighboring urbanized areas or to agricultural land. They can be caused by a lightning strike, surface contact of flaming embers from an existing fire set for another purpose such as the burning of trash, by volcanoes, and by human involvement that can be accidental or deliberate (arson).

An example of an accidental wildfire occurred in 1965, when a malfunction caused a truck to ignite that was being driven through a forested area, triggering a wildfire that consumed nearly 510 acres in California’s Los Padres National Forest. The driver, singer Johnny Cash (1932–2003), was sued for damage by the federal government and eventually paid approximately $80,000 in an out of court settlement.

Wildfires occur worldwide in regions that are forested and can experience dry weather. For example, areas of Europe, Canada, and the United States that are cool (or cold) and moist during parts of the year can have extended hot and dry periods during the summer and fall. The dried forest floor and falling of dead branches provides the fuel for the ignition of a fire.

The increased warming of the atmosphere related to human activities that has occurred since the mid-nineteenth century and that has been accelerated since the 1950s has been suggested as being influential in the increased prevalence and intensity of wildfires, particularly in southern California and other areas of the western United States. However, as of 2008, a definite link between global warming and wildfires has yet to be established, although the increased frequency and severity of wildfires is consistent with some climate models that have predicted a warmer and drier climate in hard-hit areas such as southern California.

Historical Background and Scientific Foundations

Regions of the world including portions of the United States, Canada, Europe, Australia, and South Africa are prone to wildfires. All experience the protracted periods of hot and dry weather that parch the forest and forest floor or the grassland. Wind also plays an important role in the spread of a wildfire. In California, for example, wildfires that occur during later summer and fall can be fanned by Santa Ana winds—a hot and dry wind that blows westward from the California desert to the coast.

As devastating as wildfires can be to urban areas that border on wilderness regions, and even though they can pose great danger to those caught in a rapidly advancing blaze, wildfires are a natural part of the ecosystem. Vegetation that is aging or dead will be readily burned, providing more space for the growth of a new generation of plants. Indeed, some plants have adapted to be more fire tolerant and capable of rapid regeneration of shoots following a fire, or which produce seeds that are less affected by heat. Research has shown that as some plants burn in a wildfire, the smoke produced is a signal for the subsequent germination of other plants after the fire has passed. This strategy allows the plants to gain a competitive advantage in the charred aftermath of a wildfire and immediately begins the rehabilitation of the area.

The giant redwood trees that grow in regions of northern California have been able to attain their tremendous height because of the periodic lack of competition for nutrients in the aftermath of forest fires.

However, the increased frequency of wildfires that has occurred in southern California through the 1990s and to 2008 has altered such a natural balance of the ecosystem. The repeated fires have eliminated some native plant species and the successors have proven to be more prone to catching fire.


CLIMATE MODEL: A quantitative method of simulating the interactions of the atmosphere, oceans, land surface, and ice. Models can range from relatively simple to quite comprehensive.

HABITAT: The natural location of an organism or a population.

SANTA ANA WIND: A warm and dry wind that blows coastward from the desert in southern California.

One reason that a dry forest, grassland, or shrubland is more flammable is the altered chemistry of the vegetation that occurs during hot and dry periods. As many plants dry, they release a flammable gas called ethylene that gathers in the air above the vegetation, increasing the likelihood of a blaze.

The amount of material that is in the vicinity of a fire (the fuel load, typically measured as tons per acre) determines how fiercely the fire will burn. A smaller fuel load will allow a smaller fire than will a high fuel load. Also, a high fuel load can create a hotter fire, which can cause a quicker combustion of wood because the wood releases hydrocarbons that mix with the oxygen in the air to cause an explosive ignition.

A wildfire is no different from a campfire in that the air emerging from the flames is hotter than the surrounding air and so tends to rise. This rising air can contain embers—small bits of still burning material—that can be blown by the current of rising air and any wind present. As the particles settle some distance away, they can ignite new fires, and this cycle can be repeated. As a result, wildfires can jump from location to location, which can make them hard to extinguish.

Santa Ana winds can be influential in spreading a wildfire. Southern California wildfires have covered up to 40 mi (64 km) in a single day during times when this wind is blowing. The advancing fire throws up embers that are spread ahead of the flames by the wind. Some of these wildfires have burned over 1,000 acres per hour.

Complicating the fight to extinguish a wildfire can be a phenomenon called smoldering. This occurs when underbrush is slowly combusted without bursting into flame. An example of smoldering can be seen when a campfire or fire in a wood stove is in the process of being lit. A smoking piece of flammable material can be encouraged to burst into flame when air is supplied by blowing on the material. Similarly, debris in the aftermath of an active wildfire may smolder for days before igniting. In the frenzy of battling an ongoing blaze, these dangerous remnants may be overlooked and can be the cause of the re-emergence of a wildfire in an area that has previously been affected.

Topography also influences the spread of wildfires. A fire can advance more easily uphill, as the flames carry embers upward. Once a fire has reached the peak of a hill, it can become more difficult for it to spread farther, unless embers are dispersed by wind. This behavior can be helpful in fighting a fire; if a fire is contained to the base of a hill, then it is encouraged to travel uphill and so can become easier to extinguish.

Wildfires are battled on the ground and from the air. Fire-fighting crews popularly known as hotshots can remove surface vegetation ahead of the fire in an effort to reduce the amount of available fuel. This is sometimes done for very large wildfires that cannot be safely battled up close by deliberately setting and, hopefully, controlling a fire to consume the surface vegetation. In smaller fires, the ground crews can also carry portable pumps and canisters of water to douse small fires or smoldering areas. In some cases, specially trained fire-fighting personnel dubbed smokejumpers will parachute down near a fire located far from roads.

Often, the effort of the ground personnel is accompanied by the dropping of water or fire retarding chemicals onto the blaze from overhead planes or helicopters (although the latter have to be carefully used, since their blades may fan the fire).

Battling a wildfire is a coordinated process. The locations and activities of the ground and airborne personnel must be known at all times, as any changes in weather such as wind direction or the appearance of a thunderstorm can alter where and how a fire will be dealt with. Rain usually helps firefighters control even the largest wildfire.

Impacts and Issues

On average, about five million acres of forest, grassland, and shrubland are burned each year in the United States. Controlled wildfires, which are also called controlled burns, can be helpful in eliminating underbrush that could be a potential source for more disastrous fires in the future, and can create conditions that are more favorable for the growth of the trees in the area.

In areas where urban settlement is near wilderness, controlled burns are a wise preventative strategy. Ironically, one reason for the preventative approach is the influence of urban growth on the increased frequency of wildfires. This has been particularly true in southern California and in Colorado. As people seek to live in a natural setting and homes are constructed in fire-prone environments and are not as fire-retardant as they could be, the homes become an excellent fuel that spurs the spread of a fire.

In southern California, some wildfires that have occurred since 2000 have been fought using a newly designed fire retardant chemical. The gel-like material is saturated with water, which resides in tiny bubbles dispersed throughout the material. This in effect creates layers of fire retardant in the gel that is sprayed onto a fire or, more commonly, on buildings that are under threat of fire. The layers of the gel burn over time, which increases the time that the material can protect a structure. Although it does not curb the spread of a wildfire, the gel has been effective in reducing property damage and saving homes.

Aside from direct danger and damage, the soot released by large wildfires can alter local climate. Airborne soot from some fires has been tracked in the atmosphere for over 3,100 mi (5,000 km). Atmospheric models have shown that when aloft, the soot can reduce incoming sunlight by up to 15%. Another effect of a wildfire is the sudden release of the carbon dioxide that had been sequestered in the vegetation. A study of an Indonesia forest fire that occurred during 1997 and 1998 demonstrated a release of over 2.5 gigatonnes of carbon dioxide into the atmosphere. Another study conducted at the U.S. National Center for Atmospheric Research has shown that the carbon dioxide released by a large wildfire that burns for several weeks is equal to that released over an entire year by the millions of vehicles in California. As carbon dioxide is a potent greenhouse gas, wildfires likely are a contributor to the warming of the atmosphere that has been accelerating since the 1950s.

See Also Ecodisasters; Reforestation; Wildfire Control



Halsey, Richard. Fire, Chaparral, and Survival in Southern California. San Diego, CA: Sunbelt Publications, 2008.

Reinhart, Kare. Yellowstone’s Rebirth by Fire: Rising from the Ashes of the 1988 Wildfires. Helena, MT: Farcountry Press, 2008.

Wuerthner, George. Wildfire Reader: A Century of Failed Forest Policy. Washington: Island Press, 2006.

Brian D. Hoyle