IStock Photo 5317435 © Michael Walker

A thick fog of smoke and steam drifts across the hillside, shrouding, then exposing the black silhouettes of skeletal trees. The still warm soil hisses and squeals as suffocating coals exhale their final puffs of steam and gas.

The inferno that lit up the night sky has been extinguished by a helpful rain and carefully dug firebreaks. The forest will recover over the next couple of years; indeed many plant species that live in fire-prone areas, like lodgepole pines, depend on the heat from periodic burning for their seeds to be able to germinate.

Fire is a natural part of the forest life-cycle, but for a given fire, which is more likely: that it is caused by nature or by humans?

Odds are the cause is human. The odds that a wildfire will be started by humans are 1 in 1.18 (85%). This means that about 85% of wildfires are started, intentionally or accidentally, by people. The most common causes are arson, campfires, discarded lit cigarettes, and debris burning.

Property and lives were lost in parts of Southern California caused by wildfires allegedly started by an arsonist’s match. On Oct. 20, 2009, a homeless man was charged with five counts of murder for allegedly setting a 91,000-acre fire that destroyed nearly 1,000 homes in San Bernardino in 2003. And more recently, two firefighters died in a wildfire north of Los Angeles in August. Arson is suspected in that blaze too.

The odds that a wildfire will be started by lightning, on the other hand, are 1 in 6.45, which translates to about 15% of all wildfires. Wildfires caused by other natural processes, such as heat from the sun, make up a tiny fraction.

Though a given fire is more likely to have been started by people than nature, the odds tell a different story when viewed from the perspective of the area burned. The odds a wildfire-burned acre was caused by humans are 1 in 2.72. The odds a wildfire-burned acre was caused by lightning are 1 in 1.58. In other words, humans account for about 37% of acres burned, while lightning claims the remaining 63%.

Humans may be more prolific fire-starters, but nature’s fires burn more acreage.