Wildfire

A wildfire is an uncontrolled fire that burns in vegetation. It may move through forests, grasslands, brushlands or croplands and can spread into places where people live and work. Terms such as forest fire, brush fire and bushfire describe particular settings; usage differs by region. A planned, managed burn is generally called a prescribed fire while it remains under control, although any planned fire that escapes may become a wildfire.

Ignitions can be natural, especially from lightning, or result from human activity. Accidents such as power-line failures or escaped campfires, and deliberate ignition, are among the possible human sources. Which sources dominate varies among places and circumstances, so a single global proportion should not be assumed.

Fire behaviour depends on the available fuel, weather and topography. Dry vegetation, heat and wind can increase the chance of ignition or intensify a burning fire; slopes and terrain also influence its movement. In forest settings, fire may burn organic material below the surface, debris and low vegetation at the surface, or the crowns of trees. These ground, surface and crown forms can interact during one incident.

Ecologists describe a fire regime as a pattern that includes how often fires occur, their intensity and how much fuel they consume. The regime differs among ecosystems because vegetation, moisture, ignition sources, weather and landform differ. A fire's effects therefore cannot be judged by size alone: its frequency, intensity and place in the ecosystem's history matter as well.

Fire is a natural process in many ecosystems, and some plants and habitats are adapted to periodic burning. Depending on the fire regime, fire can clear accumulated vegetation, make nutrients available, reduce competition and help some fire-adapted plants regenerate. It can also create a changing mosaic of habitats.

The effects are not always beneficial. Fires that are unusually severe or frequent for a particular ecosystem can kill organisms, alter habitat and change vegetation structure. In its assessment of North America, the Intergovernmental Panel on Climate Change describes ecosystem impacts from increases in burned area and fire severity, while emphasizing that responses vary with vegetation, moisture, disturbance and human land use. In some dry, non-forest ecosystems of the western United States, the IPCC notes that limited fuel can constrain fire activity; it assesses that warming may further reduce plant productivity there, limiting burned area. This illustrates why climate effects on fire are not uniform.

Wildfires can destroy homes and infrastructure, cause evacuations, disrupt livelihoods and threaten human and animal life. Smoke is an important effect beyond the burn area: it can travel long distances and worsen air quality. The IPCC's North America assessment reports that fire-related poor air quality caused increased respiratory distress and that exposure can extend far from the fire source. Impacts on water resources, cultural sites and local economies can continue after flames are out.

Risk is shaped not only by fire weather but also by where homes and infrastructure are built, preparedness, access to warnings and evacuation routes, and the capacity of local response systems. Those factors differ widely across communities.

Climate change can alter temperature, precipitation and drought conditions that influence fire weather, but its effects interact with vegetation, land use and ignition. For North America, the IPCC concludes with high confidence that human-caused climate change has produced warmer and drier conditions that favour wildland fires; it also attributes part of the increase in burned area in western North America to climate change. This regional finding should not be read as a uniform prediction for every ecosystem: some fuel-limited areas may respond differently.

Fire management includes prevention and risk reduction, readiness, emergency response and recovery. The Food and Agriculture Organization describes an integrated approach through five elements: review and analysis, risk reduction, readiness, response and recovery. Tools can include fire detection, public preparedness, suppression, fuel management and—where appropriate—prescribed burning. In some fire-adapted forests in the western United States, prolonged fire exclusion has allowed fuels to accumulate. Prescribed fire and decisions to suppress or manage a wildfire require local expertise, suitable conditions and careful consideration of risks to people and ecosystems.