A wildfire, forest fire, brushfire or a bushfire is an unplanned and uncontrolled fire in an area of combustible vegetation. Some natural forest ecosystems depend on wildfire. Modern forest management often engages in prescribed burns to mitigate fire risk and promote natural forest cycles. Controlled burns can turn into wildfires if they escape containment lines.
Wildfires can be classified by cause of ignition, physical properties, combustible material present, and the effect of weather on the fire. Wildfire severity results from a combination of factors such as available fuels, physical setting, and weather.Graham, et al., 12, 36National Wildfire Coordinating Group Communicator's Guide For Wildland Fire Management, 4–6. Climatic cycles with wet periods that create substantial fuels, followed by drought and heat, often precede severe wildfires. These cycles have been intensified by climate change,Parmesan, C., M.D. Morecroft, Y. Trisurat, R. Adrian, G.Z. Anshari, A. Arneth, Q. Gao, P. Gonzalez, R. Harris, J. Price, N. Stevens, and G.H. Talukdarr, 2022: Chapter 2: Terrestrial and Freshwater Ecosystems and Their Services . In: Climate Change 2022: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change and can be exacerbated by curtailment of mitigation measures (such as budget or equipment funding), or sheer enormity of the event.
Wildfires are a common disaster in many parts of the world, for example in Russia (Siberia), the United States (for example in California, Washington, Oregon, Texas, Florida), Canada and Australia. Wildfires can severely impact humans and their settlements. Effects include for example the direct health impacts of smoke and fire, as well as destruction of property (especially in wildland–urban interfaces), and economic losses. There is also the potential for contamination of water and soil.
At a global level, human practices have made the impacts of wildfire worse, with a doubling in land area burned by wildfires compared to natural levels. Humans have impacted wildfire through climate change (e.g. more intense heat waves and droughts), land-use change, and wildfire suppression. The carbon released from wildfires can add to carbon dioxide concentrations in the atmosphere and thus contribute to the greenhouse effect. This creates a climate change feedback.
Naturally occurring wildfires can have beneficial effects on those ecosystems that have evolved with fire. In fact, many plant species depend on the effects of fire for growth and reproduction.
Ignition The ignition of a fire takes place through either natural causes or human activity (deliberate or not). Over 85% of wildfires in the United States are started by human activity.
Natural causes Natural occurrences that can ignite wildfires without the involvement of humans include lightning, volcanic eruptions, sparks from rock falls, and spontaneous combustions.
Human activity Sources of human-caused fire may include arson, accidental ignition, or the uncontrolled use of fire in land-clearing and agriculture such as slash-and-burn farming.Karki, 7, 11–19. In the tropics, farmers often practice the slash-and-burn method of clearing fields during the dry season.
In middle latitudes, the most common human causes of wildfires are equipment generating sparks (chainsaws, grinders, mowers, etc.), overhead power lines, and arson.
Arson may account for over 20% of human caused fires, although human activities, including campfires, power line failures, and equipment use, are responsible for approximately 85% of wildfires. The combination of these ignition sources with dry conditions leads to more frequent and severe fires. However, in the 2019–20 Australian bushfire season "an independent study found online bots and trolls exaggerating the role of arson in the fires." In the 2023 Canadian wildfires false claims of arson gained traction on social media; however, arson is generally not the main cause of wildfires in Canada. In California, generally 6–10% of wildfires annually are arson.
Coal seam fires burn in the thousands around the world, such as those in Burning Mountain, New South Wales; Centralia, Pennsylvania; and several coal-sustained fires in China. They can also flare up unexpectedly and ignite nearby flammable material.
Spread
The spread of wildfires varies based on the flammable material present, its vertical arrangement and moisture content, and weather conditions.Graham, et al., iv. Fuel arrangement and density is governed in part by topography, as land shape determines factors such as available sunlight and water for plant growth. Overall, fire types can be generally characterized by their fuels as follows: Ground fires are fed by subterranean roots, duff on the forest floor, and other buried organic matter. Ground fires typically burn by smoldering, and can burn slowly for days to months, such as peat fires in Kalimantan and Eastern Sumatra, Indonesia, which resulted from a riceland creation project that unintentionally drained and dried the peat.Graham, et al., 9, 13 Crawling or surface fires are fueled by low-lying vegetative matter on the forest floor such as leaf and timber litter, debris, grass, and low-lying shrubbery.Graham, et al ., iv, 10, 14 This kind of fire often burns at a relatively lower temperature than crown fires (less than ) and may spread at slow rate, though steep slopes and wind can accelerate the rate of spread. This fuel type is especially susceptible to ignition due to spotting . Ladder fires consume material between low-level vegetation and tree canopies, such as small trees, downed logs, and vines. Kudzu, Old World climbing fern, and other invasive plants that scale trees may also encourage ladder fires. Crown, canopy, or aerial fires burn suspended material at the canopy level, such as tall trees, vines, and mosses. The ignition of a crown fire, termed crowning, is dependent on the density of the suspended material, canopy height, canopy continuity, sufficient surface and ladder fires, vegetation moisture content, and weather conditions during the blaze.Graham, et al., iv, 8, 11, 15. Stand-replacing fires lit by humans can spread into the Amazon rain forest, damaging ecosystems not particularly suited for heat or arid conditions.
Physical properties
Wildfires occur when all the necessary elements of a fire triangle come together in a susceptible area: an ignition source is brought into contact with a combustible material such as vegetation that is subjected to enough heat and has an adequate supply of oxygen from the ambient air. A high moisture content usually prevents ignition and slows propagation, because higher temperatures are needed to evaporate any water in the material and heat the material to its fire point.
Dense forests usually provide more shade, resulting in lower ambient temperatures and greater humidity, and are therefore less susceptible to wildfires.Graham, et al., 12. Less dense material such as grasses and leaves are easier to ignite because they contain less water than denser material such as branches and trunks.National Wildfire Coordinating Group Communicator's Guide For Wildland Fire Management, 3. Plants continuously lose water by evapotranspiration, but water loss is usually balanced by water absorbed from the soil, humidity, or rain. When this balance is not maintained, often as a consequence of droughts, plants dry out and are therefore more flammable.
A wildfire front is the portion sustaining continuous flaming combustion, where unburned material meets active flames, or the smoldering transition between unburned and burned material.Glossary of Wildland Fire Terminology, 74. As the front approaches, the fire heats both the surrounding air and woody material through convection and thermal radiation. First, wood is dried as water is vaporized at a temperature of . Next, the pyrolysis of wood at releases flammable gases. Finally, wood can smolder at or, when heated sufficiently, ignite at .de Sousa Costa and Sandberg, 229–230. Even before the flames of a wildfire arrive at a particular location, heat transfer from the wildfire front warms the air to , which pre-heats and dries flammable materials, causing materials to ignite faster and allowing the fire to spread faster. High-temperature and long-duration surface wildfires may encourage flashover or torching: the drying of tree canopies and their subsequent ignition from below.Graham, et al., 10–11.
Wildfires have a rapid forward rate of spread (FROS) when burning through dense uninterrupted fuels. They can move as fast as in forests and in grasslands.Billing, 5–6 Wildfires can advance tangentially to the main front to form a flanking front, or burn in the opposite direction of the main front by backing.Graham, et al., 12 They may also spread by jumping or spotting as winds and vertical convection columns carry firebrands (hot wood embers) and other burning materials through the air over roads, rivers, and other barriers that may otherwise act as firebreaks.Graham, et al., 16. Torching and fires in tree canopies encourage spotting, and dry ground fuels around a wildfire are especially vulnerable to ignition from firebrands.Graham, et al., 9, 16. Spotting can create spot fires as hot embers and firebrands ignite fuels downwind from the fire. In Australian bushfires, spot fires are known to occur as far as from the fire front.
Especially large wildfires may affect air currents in their immediate vicinities by the stack effect: air rises as it is heated, and large wildfires create powerful updrafts that will draw in new, cooler air from surrounding areas in thermal columns.National Wildfire Coordinating Group Communicator's Guide For Wildland Fire Management, 4. Great vertical differences in temperature and humidity encourage pyrocumulus clouds, strong winds, and fire whirls with the force of tornadoes at speeds of more than .Graham, et al., 16–17.Olson, et al., 2 Rapid rates of spread, prolific crowning or spotting, the presence of fire whirls, and strong convection columns signify extreme conditions.Glossary of Wildland Fire Terminology, 69.
Intensity variations during day and night Intensity also increases during daytime hours. Burn rates of smoldering logs are up to five times greater during the day due to lower humidity, increased temperatures, and increased wind speeds.de Souza Costa and Sandberg, 228 Sunlight warms the ground during the day which creates air currents that travel uphill. At night the land cools, creating air currents that travel downhill. Wildfires are fanned by these winds and often follow the air currents over hills and through valleys.National Wildfire Coordinating Group Communicator's Guide For Wildland Fire Management, 5. Fires in Europe occur frequently during the hours of 12:00 p.m. and 2:00 p.m.San-Miguel-Ayanz, et al., 364. Wildfire suppression operations in the United States revolve around a 24-hour fire day that begins at 10:00 a.m. due to the predictable increase in intensity resulting from the daytime warmth.Glossary of Wildland Fire Terminology, 73. Based on climate-driven weakening of day-night weather constraints, a study published in Science Advances estimated that from 1975 to 2024, annual potential burning hours for wildfires in North America rose 36%.
Climate change effects
Increasing risks due to climate change