The four major natural causes of wildfire ignitions are lightning, volcanic eruption, sparks from rockfalls, and spontaneous combustion. The thousands of coal seam fires that are burning around the world, such as those in Centralia, Burning Mountain, and several coal-sustained fires in China, can also flare up and ignite nearby flammable material. However, many wildfires are attributed to human sources such as arson, discarded cigarettes, sparks from equipment, and power line arcs (as detected by arc mapping). Despite public belief discarded glass has no effect on starting a fire. In societies experiencing shifting cultivation where land is cleared quickly and farmed until the soil loses fertility, slash and burn clearing is often considered the least expensive way to prepare land for future use. Forested areas cleared by logging encourage the dominance of flammable grasses, and abandoned logging roads overgrown by vegetation may act as fire corridors. Annual grassland fires in southern Vietnam can be attributed in part to the destruction of forested areas by US military herbicides, explosives, and mechanical land clearing and burning operations during the Vietnam War.
The most common cause of wildfires varies throughout the world. In the Canada and northwest China, for example, lightning is the major source of ignition. In other parts of the world, human involvement is a major contributor. In Mexico, Central America, South America, Africa, Southeast Asia, Fiji, and New Zealand, wildfires can be attributed to human activities such as animal husbandry, agriculture, and land-conversion burning. Human carelessness is a major cause of wildfires in China and in the Mediterranean Basin. In the United States and Australia, the source of wildfires can be traced to both lightning strikes and human activities such as machinery sparks and cast-away cigarette butts.”
On a yearly basis in the United States, typically more than six times the number of wildfires are caused by human means such as campfires and controlled agricultural burns than by natural means. However, in any given year there could be far more acres burned by wildfires that are started by natural means than by human means as well as vice-versa. For example, in 2010, almost 1.4 million acres were burned by human-caused wildfires, and over 2 million acres were burned by naturally-caused wildfires. However, far more acres were burned by human-caused fires in 2011, when almost 5.4 million acres were burned by human-caused wildfires, and only about 3.4 million acres were caused by naturally-derived wildfires.