Forest fires are caused by lightning and by human activity such as campfires, cigarettes, and power lines, and spread in dry, windy conditions.
Natural causes
Lightning is the main natural cause. Heat and dry conditions help fires spread.
Human causes
The US Forest Service says humans cause almost 85 percent of wildfires. Common sources:
- Unattended campfires.
- Discarded cigarettes.
- Equipment sparks and vehicles.
- Power lines.
- Arson and debris burning.
Spread
Fires spread fastest uphill, in wind and dry fuel.
Climate
Hotter, drier weather lengthens the fire season. The US has seen the annual area burned rise since the 1980s.
Prevention
Fully extinguish campfires, follow burn bans and clear defensible space around homes.
Report
Call 911 if you see a fire.
Natural causes
Lightning is the main natural cause of wildfires. The National Interagency Fire Center reports that lightning starts about 10% to 15% of US wildfires, but those fires burn a large share of land area because they often occur in remote areas. Other natural causes are volcanic eruptions, spontaneous combustion of dry vegetation and sparks from rock falls.
Human causes
Human activity causes about 85% to 90% of wildfires in the US, according to the National Park Service. Common causes are:
- Unattended or poorly extinguished campfires.
- Discarded cigarettes.
- Debris burning, such as yard waste.
- Equipment, like sparks from chainsaws, mowers, vehicles and power lines.
- Arson.
- Fireworks and gender reveal devices.
- Railroads and downed power lines. Power lines caused the 2018 Camp Fire in California, which killed 85 people and destroyed about 18,000 structures.
How fires spread
Fire needs three things: fuel, oxygen and heat. Dry grass, brush, dead trees and dense forests supply fuel. Wind carries embers far ahead of the front, sometimes more than a mile, starting spot fires. Fires move uphill faster, because heat rises and preheats the fuel. Low humidity and high temperatures make vegetation easier to ignite.
Climate
Research shows that climate change has increased wildfire risk. Warmer temperatures, earlier snowmelt and longer droughts dry out forests. A 2016 study estimated that climate change doubled the area burned in western US forests from 1984 to 2015. The fire season in the West is about 78 days longer than in the 1970s. Insect infestations and a century of fire suppression also leave a heavy build up of fuel.
Prevention
- Check fire restrictions before burning, and never leave a campfire unattended. Drown it with water, stir and feel the ashes until cold.
- Properly dispose of cigarettes.
- Keep vehicles off dry grass, as catalytic converters and chains can spark.
- Create defensible space of at least 30 feet around buildings and clear leaves from roofs and gutters.
- Use fire resistant roofing and vent screens.
- Prescribed burns and forest thinning reduce fuel.
Report
Call 911 immediately if you see an unattended fire or smoke. Give your location, and do not try to fight a large fire. Evacuate when ordered.
Numbers on wildfire frequency and damage
In the United States, humans cause about 85 percent of wildfires, according to the National Park Service, while lightning causes most of the rest and the largest burned acreage in remote western areas. The National Interagency Fire Center records roughly 60,000 wildfires and 7 million acres burned in an average year over the last decade, with peak years above 10 million acres, such as 2015, 2017, 2020 and 2021. Worldwide, satellite data shows around 3 to 4 million square miles burn every year, mostly savanna and grassland in Africa. Wildfire smoke contains fine particles (PM2.5) that travel thousands of miles and are linked to asthma, heart attacks and premature death. The 2018 Camp Fire in California killed 85 people and destroyed about 18,800 structures, making it the deadliest and most destructive wildfire in the state’s history.
Based on data from the National Interagency Fire Center and the National Park Service
