A wildfire is defined as any unplanned, unwanted, and uncontrolled fire burning in a natural area, such as a forest, grassland, or shrubland. These blazes begin when an ignition source interacts with the surrounding environment under receptive conditions. While natural events like lightning have always been part of the fire cycle, the vast majority of ignitions globally are now linked to human activities. This article details the sources that cause these fires, from the necessary environmental ingredients to natural and human actions.
The Essential Ingredients for Wildfire Ignition
For any wildfire to ignite, three components must be present simultaneously, a concept known as the Fire Triangle: heat, fuel, and oxygen. The initial spark or flame provides the necessary heat, which must be intense enough to raise the surrounding fuel to its ignition temperature. This heat source can be as simple as a discarded match or as powerful as a lightning strike.
The fuel consists of any flammable material in the wildland environment, including grasses, dead leaves, shrubs, and trees. The moisture content of this fuel is a factor, as dry vegetation ignites far more easily than wet material. Oxygen, naturally abundant in the atmosphere, sustains the chemical reaction of combustion once the fire has begun. Removing any one of these three elements is the principle used in fire prevention and suppression.
Natural Ignition Sources
Fires that start without human involvement are primarily caused by atmospheric electrical discharges. Lightning is the most common natural source of ignition, especially dry lightning, which occurs during thunderstorms where precipitation evaporates before reaching the ground. The absence of rain means there is no moisture to dampen the ground fuels or extinguish the initial ember.
Lightning strikes that cause fires are often characterized as “hot lightning,” which carries lower voltage but possesses a longer duration of electrical current, transferring enough heat to ignite fuels like tree bark or dry wood. Other natural causes, such as volcanic activity or rockfalls that generate friction sparks, are geographically limited and rare. While lightning starts many remote wildfires, its occurrence is unpredictable and impossible to prevent.
Accidental Human Ignition
Despite the nature of lightning, human activity accounts for nearly 85% of all wildfires in the United States. Most starts are accidental, resulting from negligence, error, or unintended consequences. Debris burning, which involves burning yard waste or trash, is one of the largest sources of human-caused wildfires, often escaping control due to high winds or being left unattended.
Improperly extinguished campfires and outdoor cooking fires represent another cause, as embers can retain heat for many hours after the visible flames have disappeared. If the ashes are not thoroughly doused and stirred, residual heat can easily ignite surrounding dry grasses or duff. Equipment use also contributes, with sparks from internal combustion engines, chainsaws, and construction tools igniting fires. Hot exhaust systems on vehicles can ignite dry grass when parked over it, or metal-on-metal contact can produce a spark.
Electrical infrastructure is a growing source of accidental ignition, particularly during high-wind events. Power line failures, where damaged lines fall or arc, can send electrical material onto dry vegetation below. Even the negligent disposal of smoking materials, such as a discarded cigarette butt, can smolder and ignite a wildfire if it lands in receptive, dry fuel. These accidental ignitions often occur close to human populations, making them destructive to property and infrastructure.
Intentional Human Ignition
Fires started through deliberate actions, whether malicious or reckless, contribute to the annual wildfire toll. Arson is the most recognized form of intentional ignition, involving the malicious setting of a fire with criminal intent. Arson-related fires account for a notable percentage of human-caused starts.
Other intentional acts involve the misuse of fire that escapes control due to poor judgment. This includes the use of fireworks outside designated safety zones, where hot embers easily ignite dry wildland fuels. Intentional burning for agricultural purposes or attempts at controlled burns can sometimes escape their boundaries due to unexpected weather shifts or inadequate preparation, leading to an uncontrolled wildfire.