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. 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, 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. 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. 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. 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. This kind of fire often burns at a relatively lower temperature than crown fires (less than 400 °C or 750 °F) 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 (see below). 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. 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
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