People cause the vast majority of forest fires. In the United States, human-caused fires account for 88 percent of all wildfires, based on the most recent ten-year national average. Lightning and other natural causes make up the remaining 12 to 13 percent. Globally, the pattern holds: most forest fires trace back to human activity, whether accidental or deliberate.
How Humans Start Most Forest Fires
The specific human activities behind wildfires range from carelessness to criminal intent. The most common triggers include debris burning that escapes control, campfires left unattended, sparks from equipment like chainsaws and mowers, negligently discarded cigarettes, and intentional arson. Of these, arson alone may account for over 20 percent of all human-caused wildland fires, and in some regions that figure reaches 70 percent or higher.
Power line failures are another significant source. Electrical equipment faults may seem like a small share of total ignitions, but the fires they start tend to be devastating. In San Diego County, for instance, power line fires accounted for just 5 percent of all ignitions but burned 25 percent of the total acreage. That’s because power line failures often happen during high-wind events, which drive fires across dry landscapes before firefighters can respond.
Lightning: The Leading Natural Cause
Lightning is responsible for nearly all naturally caused forest fires. The type that matters most is “dry lightning,” which occurs when thunderstorms produce electrical strikes but little or no rain reaches the ground. Without rainfall to wet the vegetation, a single bolt can smolder in dry brush or forest litter for hours or even days before erupting into a visible fire.
Although lightning fires make up only about 13 percent of wildfires by number, they can be disproportionately destructive. Canada’s record-breaking 2023 fire season, for example, was driven largely by lightning ignitions in remote boreal forests where suppression crews couldn’t reach the fires quickly. The likelihood that any given lightning strike develops into a fire depends heavily on how dry the air and vegetation are at the time of the strike.
Why Some Sparks Become Infernos
Not every cigarette butt or lightning bolt starts a wildfire. Whether an ignition source actually produces a fire depends on three things: fuel, heat, and oxygen. Firefighters call this the “fire triangle,” and the most variable factor is fuel moisture. Dead vegetation like fallen leaves and twigs becomes flammable when its moisture content drops below about 30 percent. Green grass won’t burn, but once it cures and dries to 30 or 40 percent moisture, it ignites readily on a hot, windy day.
The dryness of the air itself plays a critical role. When the atmosphere pulls moisture out of plants and soil faster than it can be replaced, vegetation essentially becomes kindling. Research in boreal forests has found a direct relationship between this atmospheric dryness and the likelihood that a lightning strike develops into a sustained fire. This is why fire seasons peak in late summer and early fall, when months of heat have drawn moisture out of the landscape, and why prolonged drought makes any ignition source far more dangerous.
Tropical Forests Face Different Threats
In tropical rainforests like the Amazon, the fire equation looks different. These forests evolved in wet conditions and don’t naturally burn the way temperate or boreal forests do. Very little lightning activity has been recorded in the Amazon fire zones, and the overwhelming majority of fires there are believed to be human-set. The primary driver is slash-and-burn agriculture, where farmers and ranchers clear forest by cutting and burning it to create cropland or pasture.
Small-scale farmers and Indigenous communities have used fire for land preparation for centuries, and in limited doses it was manageable. But decades of accelerating deforestation for mining and large-scale agribusiness have transformed the practice into a major threat. As more forest is cleared, the remaining landscape dries out, fire escapes its intended boundaries more easily, and blazes reach into previously untouched primary forest. The result is a feedback loop: deforestation makes the region more fire-prone, and fires drive further deforestation.
Climate and Fire Risk Are Linked
Climate conditions don’t directly ignite fires, but they control how easily fires start and how aggressively they spread. Hotter temperatures, lower humidity, and stronger winds all reduce fuel moisture and create conditions where a small spark can become a catastrophic blaze. A campfire that might safely burn out on a cool, humid evening can jump its ring and race across a hillside when the air is hot and dry.
This matters because the conditions that allow fires to grow are becoming more common in many parts of the world. Longer dry seasons, more intense heat waves, and extended drought periods all push fuel moisture lower for longer stretches of the year. The ignition source, whether a person or a lightning bolt, is just the match. The landscape determines whether that match fizzles or becomes a disaster.