IStock Photo 5317435 © Michael Walker

Exactly 100 years ago, on August 20, 1910, storm winds whipped up small blazes into the largest wildfire in US history. Three million acres burned across Montana, Idaho, and Washington State. Towns were wiped out. An entire crew of firefighters lost their lives.

Only rain eventually put out the Great Fire of 1910.

Today the US Forest Service is better prepared. But when wildfires strike, damage and death still result—to trees, to buildings, and sometimes to humans as well.

A wildfire leaves an eerie landscape behind. 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 germinate. The Forest Service’s longstanding “Smokey Bear” campaign, while credited with reducing the number of acres burned by wildfires, has come under criticism for succeeding too well in preventing the small fires that periodically clear fuel, thus leaving over-fueled forests ripe for the huge wildfires of recent years.

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

Odds are the cause is human. The odds that a wildfire will be started by humans, whether intentionally or accidentally, are 1 in 1.18 (85%). The most common causes are arson, campfires, discarded lit cigarettes, and debris burning. To prove the point, new restrictions on open brush-burning in New York State have led to a 33% reduction in the number of wildfires there since last year.

A homeless man was charged in October of 2009 with five counts of murder for allegedly setting a 91,000-acre fire that had destroyed nearly 1,000 homes in San Bernardino back in 2003. Two firefighters died in a wildfire north of Los Angeles in August, 2009. Arson is suspected in that blaze too. Down Under, a man is currently on trial charged with starting one of last year’s deadly Australian wildfires.

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 by nature, the odds tell a different story when viewed in terms of area burned. The odds a wildfire-burned acre was burned due to humans are 1 in 2.72. The odds a wildfire-burned acre was burned due to lightning are 1 in 1.58 (63.3%). 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.