Wildfire

A wildfire, forest fire, brushfire or a bushfire is a fire that starts without being planned and spreads without control through an area of combustible vegetation. Some natural forests depend on wildfire to stay healthy. Today, forest management teams often use planned, controlled burns to lower the risk of wildfires and support the natural life cycle of forests. Sometimes a controlled burn gets out of hand and becomes a wildfire.

Wildfires can be grouped by what started them, how they behave, what kind of material is burning, and how the weather affects them. How bad a wildfire becomes depends on how much fuel is available, the land around it, and the weather. Wet periods that help plants grow a lot, followed by dry and hot weather, often come before severe wildfires. Climate change has made these cycles stronger. Less money or equipment for prevention, or a very large fire, can also make wildfires worse.

Wildfires are a common disaster in many parts of the world. They happen in Russia (Siberia), the United States (for example in California, Washington, Oregon, Texas, Florida), Canada and Australia. Wildfires can cause serious problems for people and their towns. The smoke and fire can make people sick, they can destroy homes and buildings (especially where wildland–urban interfaces are), and they can cause money losses. They can also dirty water and soil.

People have made the effects of wildfires worse around the world. The amount of land burned by wildfires is now much larger than it would be naturally. People affect wildfires through climate change (for example, stronger heat waves and droughts), by land-use change, and by wildfire suppression. The carbon released from wildfires adds to carbon dioxide concentrations in the atmosphere and helps the greenhouse effect. This creates a climate change feedback.

Wildfires that happen naturally can be helpful to ecosystems that have grown up with fire. Many plants need fire to help them grow and make new seeds.

Ignition

Wildfires can start from natural causes or from human activity. Some are started on purpose, and some are accidents. In the United States, over 85% of wildfires are started by people. Natural causes include lightning, volcanic eruptions, sparks from falling rocks, and spontaneous combustions.

Fires caused by people may come from arson, accidents, or fires used to clear land for farming, such as slash-and-burn farming. In the tropics, farmers often clear fields this way during the dry season. In middle latitudes, the most common causes are tools that make sparks, such as chainsaws and mowers, overhead power lines, and arson. Dry weather makes these fires more likely and more severe. Some online bots and trolls wrongly made arson seem like a bigger cause than it is during the 2019–20 Australian bushfire season. False claims about arson also spread during the 2023 Canadian wildfires. Arson is usually not the main cause in Canada. In California, about 6–10% of wildfires each year are caused by arson. Coal seam fires burn in the thousands around the world, such as at Burning Mountain in New South Wales, Centralia in Pennsylvania, and several coal-sustained fires in China. These fires can flare up without warning and set fire to nearby things that can burn.

Spread

How a wildfire spreads depends on what can burn, how the materials are arranged, how wet they are, and the weather. The land's shape, or topography, also matters because it changes how much sunlight and water plants get. Ground fires burn roots, dead leaves on the forest floor, and other buried organic matter. They often smolder slowly and can last for days or months. For example, peat fires in Kalimantan and eastern Sumatra, Indonesia, started after a riceland creation project accidentally drained and dried the peat. Crawling, or surface, fires move through low plants like leaves, fallen wood, grass, and bushes. They usually burn cooler than crown fires, but steep slopes and wind can make them spread faster. These fires can easily start new fires through spotting (see below).

Ladder fires burn material between low plants and treetops, such as small trees, fallen logs, and vines. Kudzu, Old World climbing fern, and other invasive plants that climb trees can also help fire spread. Crown, canopy, or aerial fires burn material high up, such as tall trees, vines, and mosses. Whether a crown fire starts depends on how thick the high plants are, how tall and connected the treetops are, how wet the plants are, and the weather. Fires started by people can spread into the Amazon rain forest, where plants are not well suited to heat or dry conditions and can be harmed.

Physical properties

See also: Combustion, Fire control, Heat wave, and Firestorm

A wildfire starts when a heat source reaches something that can burn, such as vegetation. There also needs to be enough heat and oxygen from the air. Wet material does not catch fire easily, because extra heat is needed to dry out the water and reach the fire point. Dense forests are shady and cool, with more humidity, so they are less likely to burn. Grasses and leaves catch fire more easily than branches and trunks because they hold less water. Plants lose water through evapotranspiration. When they do not get enough water back, often because of droughts, they dry out and burn more easily. As the fire gets closer, heat from convection and thermal radiation first dries the wood. Then it releases flammable gases through pyrolysis at 230 °C (450 °F). The wood can smolder at 380 °C (720 °F) or catch fire at 590 °C (1,000 °F). Heat from the fire can warm the air to 800 °C (1,500 °F) through heat transfer before the flames arrive, which helps the fire spread faster. Long, hot fires may cause flashover or torching. This happens when tree canopies dry out and catch fire from below.

Wildfires can move quickly. They can reach up to 10.8 kilometres per hour (6.7 mph) in forests and 22 kilometres per hour (14 mph) in grasslands. They can spread sideways or even backward. Wind can carry hot embers through the air, so fires can jump over roads and rivers, which may act as firebreaks. In Australian bushfires, new small fires have started as far as 20 kilometres (12 mi) from the main fire. Very large fires can make strong rising air that pulls in cooler air. This happens through the stack effect, updrafts, and thermal columns that draw in new air. These conditions can form pyrocumulus clouds and fire whirls. Fire whirls can spin with the force of tornadoes at speeds of more than 80 kilometres per hour (50 mph). Wildfires are also often stronger during the day. Burning logs that smolder can burn up to five times faster because of lower humidity, higher temperatures, and stronger winds. A study published in Science Advances estimated that from 1975 to 2024, the number of hours each year when wildfires could burn in North America rose by 36%. This change was linked to climate changes.

Climate change effects

Wildfire disasters have become much more common in recent decades. Climate change makes heat waves and droughts stronger. These dry out plants, which helps wildfires grow. The area burned by the 2023 Canadian wildfires was much larger than in any year since 1983. A study published in June 2024 found that extreme fire events became much more common between 2003 and 2023.

Warmer weather and drier land make wildfires more likely. Fires can spread quickly through very dry plants. In Australia, the fires in 2019 became more extreme than before. That same year, huge fires also burned in Siberia, Alaska, the Canary Islands, and the Amazon rainforest. Wildfires release carbon dioxide and smoke into the air. Over the past century, wildfires have added a large share of the carbon that warms our planet. Wildfire smoke can also harm people's health and is expected to cause more harm in the future.

Prevention and mitigation

Suppression

How people stop wildfires depends on the tools they have. In places with fewer resources, people may use simple methods, such as throwing sand or beating the fire with sticks or palm fronds. In wealthier countries, silver iodide can be used to encourage snow to fall, while fire retardants and water can be dropped from unmanned aerial vehicles, planes, and helicopters. Putting out every fire is no longer expected, but most wildfires are stopped before they grow out of control. Each year, more than 99% of the roughly 10,000 new wildfires are contained. Fires that escape during extreme weather are hard to stop unless the weather changes. Fighting wildfires takes up a large part of a country's gross domestic product. In the United States, U.S. Forest Service costs in California alone are high, as firefighters work to put out most wildfires each year.

Fire retardant

Fire retardants slow wildfires by making it harder for them to burn. Scientists are still studying their effects on water quality, including through eutrophication. Fighting wildfires can also be dangerous. Fires can suddenly change direction and jump across fire breaks. Heat and smoke can make people lose track of where the fire is. During the Australian February 2009 Victorian bushfires, some people died, and many homes and buildings were destroyed. Wildland firefighters face hazards such as heat stress, fatigue, smoke and dust, and injuries such as burns, cuts, scrapes, animal bites, and rhabdomyolysis. Working in heat can lead to illnesses such as heat stroke, so rest, cool water, and breaks are important. Smoke can contain gases such as carbon monoxide, sulfur dioxide, and formaldehyde, along with tiny particles such as ash and silica. Some of these particles are particulates.

Categorization

In the US, the National Wildfire Coordinating Group uses a system to describe how serious a wildfire is. The fire is checked in several ways. These include the threat to buildings and things people care about, and the social and economic concerns. Other checks look at how likely the fire is to spread, how fast it grows, and how much fuel it has. People also look at how hard the fire is to manage because of weather, access, or land shape, and how long it may last.

Based on these checks, each fire gets a type number. Type 5 wildfires have low risk and are simple to handle. Type 1 wildfires have high or overwhelming risk and are very complex.

Impacts on the natural environment

Fire is a big force that can damage or destroy forests. Large wildfires can send smoke, soot, and tiny particles high into the air, sometimes as high as the stratosphere. Wildfires also give off carbon dioxide and can make it harder to breathe and cause heart problems.

Some living things depend on fire. Many plants and animals in North America grew up alongside fire, and some need it to make new plants. But some places, such as the chaparral in southern California, have too much fire. This upsets natural cycles and helps unwanted weeds spread. In the Amazon rainforest, fires put many kinds of living things at risk.

Forest fires are a growing problem for farms. In 2024 and 2025, fire seasons on several continents set new records. Fires can also make drinking water unsafe. Ash and chemicals can wash into rivers and streams. After wildfires, some chemicals, such as benzene, have been found in drinking water systems.

Impacts on humans

Main article: Carbon monoxide poisoning

Main article: Firefighting

The 2009 Station Fire burns in the foothills of the San Gabriel Mountains above the Jet Propulsion Laboratory, near Pasadena, California

Wildfire risk is the chance that a wildfire will start in an area or spread to it, and the harm it could cause to things people care about. This risk depends on human activities, the weather, the plants and trees that can burn, and whether there are enough people and tools to stop a fire. Wildfires have always been a danger to people. Changes that humans have made to land and climate now put more people in their path. Global warming also makes temperatures higher and droughts more common, which adds to the risk.

Wildfire smoke is mostly carbon dioxide and water vapor. It also has smaller amounts of carbon monoxide, acrolein, and other chemicals. Tiny particles in the smoke, called particulate matter, are a major threat to health. After the 2007 California wildfires, scientists found heavy metals in the ash, including lead, arsenic, cadmium, and copper. After the Camp Fire (2018), lead levels near Chico rose sharply in the hours after the fire. The U.S. Environmental Protection Agency (EPA) created the air quality index to help people understand how clean or dirty the air is.

Smoke can make breathing harder. This is especially true for people with asthma, young children, older adults, and people with long-term illnesses. The smallest particles can travel deep into the lungs and the bloodstream. They can cause coughing, sore throats, wheezing, and shortness of breath. Firefighters face greater risk from smoke, and hypoxic conditions, where the body gets too little oxygen, are one danger they face. Residents can also be affected through soil contamination and polluted water.

Hazards remain after a fire is out. People returning home may be at risk from trees that the fire weakened, and ash pits can cause injuries. Burned land also soaks up rain less easily, so heavy rain can lead to more serious flooding and mud slides. Wildfires can also affect mental health. They can cause conditions such as post-traumatic stress disorder (PTSD), depression, anxiety, and phobias.

History

Further information: Fossil record of fire

Scientists who study ancient plants, called Paleobotany, have found the first evidence of wildfires in fossils of giant fungi called Prototaxites. These fossils were preserved as charcoal in South Wales and Poland. They are about 430 million years old, from the Silurian period. Fires burned on the ground starting about 405 million years ago, before the Early Devonian period. Fires continued through the Carboniferous period. Later, as the air gained more oxygen, wildfires became more common by the Late Permian. Their decrease from the late Permian to the Triassic periods is explained by lower oxygen levels. Wildfires in the ancient Paleozoic and Mesozoic periods were a lot like fires today. Some trees, such as Eucalyptus and Sequoia, have thick bark that helps them survive fires.

Human involvement

People have used fire for farming and hunting since the Paleolithic and Mesolithic ages. This changed the land around them. Wildfires were sometimes used in battles throughout human history as early thermal weapons. As early as the 1600s, Native Americans were seen using fire for growing crops, signaling, and warfare. In the mid-1800s, explorers saw Aboriginal Australians using fire to clear ground, hunt, and grow plant food. This method was later called fire-stick farming. Wildfires usually happen when it is hot and drought is present. According to a paper published in the journal Science, the number of fires, both natural and caused by people, went down between 1998 and 2015. Some invasive species, such as gamba grass in Australia, have in some cases made wildfires more intense.

Society and culture

Wildfires have a place in many cultures. "To spread like wildfire" is a common English saying. It means something that quickly reaches more and more people. Wildfires have been linked to the development of Ancient Greece. In modern Greece, as in many other places, wildfires are the most common disaster caused by a natural hazard. They play a big part in the social and economic lives of the people there.

In 1937, U.S. President Franklin D. Roosevelt started a nationwide campaign to prevent fires. It said that people's carelessness could cause forest fires. Later posters from the program showed Uncle Sam, characters from the Disney movie Bambi, and the official mascot of the U.S. Forest Service, Smokey Bear. The Smokey Bear campaign created one of the most popular characters in the United States. For many years, a real bear was the Smokey Bear mascot, and he has appeared on postage stamps. Wildfires also affect society in other ways. For example, they lead power companies to keep their equipment from starting fires. They also lead some insurance companies to stop offering or renewing home insurance for people who live where wildfires are likely.