Download Wildfire Book [PORTABLE]
Julie Ahlstedt
A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation.[1][2] Depending on the type of vegetation present, a wildfire may be more specifically identified as a bushfire (in Australia), desert fire, grass fire, hill fire, peat fire, prairie fire, vegetation fire, or veld fire.[3] Some natural forest ecosystems depend on wildfire.[4] Wildfires are distinct from beneficial human usage of wildland fire, called controlled or prescribed burning, although controlled burns can turn into wildfires. Modern forest management often engages in prescribed burns to mitigate risk and promote natural forest cycles.
Wildfires are often classified by characteristics like cause of ignition, physical properties, combustible material present, and the effect of weather on the fire.[5] Wildfire behavior and severity result from a combination of factors such as available fuels, physical setting, and weather.[6][7][8][9] Climatic cycles with wet periods that create substantial fuels, followed by drought and heat, often proceed severe wildfires.[10] These cycles have been intensified by climate change.[11]
Naturally occurring wildfires have beneficial effects on native vegetation, animals, and ecosystems that have evolved with fire.[12][13][14] Many plant species depend on the effects of fire for growth and reproduction.[15] Some natural forests are dependent on wildfire.[16] High-severity wildfires may create complex early seral forest habitat (also called "snag forest habitat"), which may have higher species richness and diversity than an unburned old forest.
Wildfires are among the most common forms of natural disaster in some regions, including Siberia, California, British Columbia, and Australia.[17][18][19][20] Areas with Mediterranean climates or in the taiga biome are particularly susceptible. 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.[11] Humans have impacted wildfire through climate change, land-use change, and wildfire suppression.[11] The increase in severity of fires in the US creates a positive feedback loop by releasing naturally sequestered carbon back into the atmosphere, increasing the atmosphere's greenhouse effect thereby contributing to climate change.[11]
The spread of wildfires varies based on the flammable material present, its vertical arrangement and moisture content, and weather conditions.[35] 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:
Dense forests usually provide more shade, resulting in lower ambient temperatures and greater humidity, and are therefore less susceptible to wildfires.[46] 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.[47] Plants continuously lose water by evapotranspiration, but water loss is usually balanced by water absorbed from the soil, humidity, or rain.[48] When this balance is not maintained, often as a consequence of droughts, plants dry out and are therefore more flammable.[49][50]
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.[51] 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 100 C (212 F). Next, the pyrolysis of wood at 230 C (450 F) releases flammable gases. Finally, wood can smolder at 380 C (720 F) or, when heated sufficiently, ignite at 590 C (1,000 F).[52][53] Even before the flames of a wildfire arrive at a particular location, heat transfer from the wildfire front warms the air to 800 C (1,470 F), which pre-heats and dries flammable materials, causing materials to ignite faster and allowing the fire to spread faster.[47][54] High-temperature and long-duration surface wildfires may encourage flashover or torching: the drying of tree canopies and their subsequent ignition from below.[55]
Wildfires have a rapid forward rate of spread (FROS) when burning through dense uninterrupted fuels.[56] They can move as fast as 10.8 kilometres per hour (6.7 mph) in forests and 22 kilometres per hour (14 mph) in grasslands.[57] Wildfires can advance tangential to the main front to form a flanking front, or burn in the opposite direction of the main front by backing.[58] 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.[59][60] Torching and fires in tree canopies encourage spotting, and dry ground fuels around a wildfire are especially vulnerable to ignition from firebrands.[61] 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 20 kilometres (12 mi) from the fire front.[62]
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.[63] 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 80 kilometres per hour (50 mph).[64][65][66] Rapid rates of spread, prolific crowning or spotting, the presence of fire whirls, and strong convection columns signify extreme conditions.[67]
Climate variability including heat waves, droughts, and El Niño, and regional weather patterns, such as high-pressure ridges, can increase the risk and alter the behavior of wildfires dramatically.[74][75][76] Years of high precipitation can produce rapid vegetation growth, which when followed by warmer periods can encourage more widespread fires and longer fire seasons.[77] High temperatures dry out the fuel loads and make them more flammable, increasing tree mortality and posing significant risks to global forest health.[78][79][80] Since the mid-1980s, in the Western US, earlier snowmelt and associated warming has also been associated with an increase in length and severity of the wildfire season, or the most fire-prone time of the year.[81] A 2019 study indicates that the increase in fire risk in California may be partially attributable to human-induced climate change.[82]
In 2019, extreme heat and dryness caused massive wildfires in Siberia, Alaska, Canary Islands, Australia, and in the Amazon rainforest. The fires in the latter were caused mainly by illegal logging. The smoke from the fires expanded on huge territory including major cities, dramatically reducing air quality.[88]
As of August 2020, the wildfires in that year were 13% worse than in 2019 due primarily to climate change, deforestation and agricultural burning. The Amazon rainforest's existence is threatened by fires.[89][90][91][92] Record-breaking wildfires in 2021 occurred in Turkey, Greece and Russia, thought to be linked to climate change.[93]
Wildfires release large amounts of carbon dioxide, black and brown carbon particles, and ozone precursors such as volatile organic compounds and nitrogen oxides (NOx) into the atmosphere.[94][95] These emissions affect radiation, clouds, and climate on regional and even global scales. Wildfires also emit substantial amounts of semi-volatile organic species that can partition from the gas phase to form secondary organic aerosol (SOA) over hours to days after emission. In addition, the formation of the other pollutants as the air is transported can lead to harmful exposures for populations in regions far away from the wildfires.[96] While direct emissions of harmful pollutants can affect first responders and residents, wildfire smoke can also be transported over long distances and impact air quality across local, regional, and global scales.[97]
In June and July 2019, fires in the Arctic emitted more than 140 megatons of carbon dioxide, according to an analysis by CAMS. To put that into perspective this amounts to the same amount of carbon emitted by 36 million cars in a year. The recent wildfires and their massive CO2 emissions mean that it will be important to take them into consideration when implementing measures for reaching greenhouse gas reduction targets accorded with the Paris climate agreement.[103] Due to the complex oxidative chemistry occurring during the transport of wildfire smoke in the atmosphere,[104] the toxicity of emissions was indicated to increase over time.[105][106]
Wildfire prevention refers to the preemptive methods aimed at reducing the risk of fires as well as lessening its severity and spread.[113] Prevention techniques aim to manage air quality, maintain ecological balances, protect resources,[114] and to affect future fires.[115] Prevention policies must consider the role that humans play in wildfires, since, for example, 95% of forest fires in Europe are related to human involvement.[116]
Strategies for wildfire prevention, detection, control and suppression have varied over the years.[121] One common and inexpensive technique to reduce the risk of uncontrolled wildfires is controlled burning: intentionally igniting smaller less-intense fires to minimize the amount of flammable material available for a potential wildfire.[122][123] Vegetation may be burned periodically to limit the accumulation of plants and other debris that may serve as fuel, while also maintaining high species diversity.[124][125] While other people claim that controlled burns and a policy of allowing some wildfires to burn is the cheapest method and an ecologically appropriate policy for many forests, they tend not to take into account the economic value of resources that are consumed by the fire, especially merchantable timber.[126] Some studies conclude that while fuels may also be removed by logging, such thinning treatments may not be effective at reducing fire severity under extreme weather conditions.[127]
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