[Forest Fires: Their Destructive Work, Causes And Prevention (Classic Reprint) W. W. Ashe
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A wildfire (or forest fire, bushfire) 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 different from controlled or prescribed burning, which are carried out to provide a benefit for people. Modern forest management often engages in prescribed burns to mitigate fire risk and promote natural forest cycles. However, controlled burns can turn into wildfires by mistake.
Forest Fires: Their Destructive Work, Causes And Prevention (Classic Reprint) W. W. Ashe
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Naturally occurring wildfires can have beneficial effects on those ecosystems that have evolved with fire.[12][13][14] In fact, 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). These types of forest may have higher species richness and biodiversity than an unburned old forest.
Sources of human-caused fire may include arson, accidental ignition, or the uncontrolled use of fire in land-clearing and agriculture such as the slash-and-burn farming in Southeast Asia.[24] In the tropics, farmers often practice the slash-and-burn method of clearing fields during the dry season.
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.[35]
The spread of wildfires varies based on the flammable material present, its vertical arrangement and moisture content, and weather conditions.[36] 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:
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.[45][46]
Dense forests usually provide more shade, resulting in lower ambient temperatures and greater humidity, and are therefore less susceptible to wildfires.[47] 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.[48] Plants continuously lose water by evapotranspiration, but water loss is usually balanced by water absorbed from the soil, humidity, or rain.[49] When this balance is not maintained, often as a consequence of droughts, plants dry out and are therefore more flammable.[50][51]
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.[52] 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).[53][54] 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.[48][55] High-temperature and long-duration surface wildfires may encourage flashover or torching: the drying of tree canopies and their subsequent ignition from below.[56]
Wildfires have a rapid forward rate of spread (FROS) when burning through dense uninterrupted fuels.[57] 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.[58] 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.[59] 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.[60][61] Torching and fires in tree canopies encourage spotting, and dry ground fuels around a wildfire are especially vulnerable to ignition from firebrands.[62] 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.[63]
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.[64] 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).[65][66][67] Rapid rates of spread, prolific crowning or spotting, the presence of fire whirls, and strong convection columns signify extreme conditions.[68]
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.[69] 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.[70] Fires in Europe occur frequently during the hours of 12:00 p.m. and 2:00 p.m.[71] 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.[72]
Weather conditions are raising the risks of wildfires. But the total area burnt by wildfires has decreased. This is mostly because savanna has been converted to cropland, so there are fewer trees to burn.[76]
Climate variability including heat waves, droughts, and El Nio, and regional weather patterns, such as high-pressure ridges, can increase the risk and alter the behavior of wildfires dramatically.[77][78][79] Years of high precipitation can produce rapid vegetation growth, which when followed by warmer periods can encourage more widespread fires and longer fire seasons.[80] High temperatures dry out the fuel loads and make them more flammable, increasing tree mortality and posing significant risks to global forest health.[81][82][83] 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.[84] A 2019 study indicates that the increase in fire risk in California may be partially attributable to human-induced climate change.[85]
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.[91]
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.[92][93][94][95] Record-breaking wildfires in 2021 occurred in Turkey, Greece and Russia, thought to be linked to climate change.[96]
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.[97][98] 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.[99] 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.[100]
The health effects of wildfire smoke, such as worsening cardiovascular and respiratory conditions, extend beyond immediate exposure, contributing to nearly 16,000 annual deaths, a number expected to rise to 30,000 by 2050. The economic impact is also significant, with projected costs reaching $240 billion annually by 2050, surpassing other climate-related damages.[101]
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.[107] Due to the complex oxidative chemistry occurring during the transport of wildfire smoke in the atmosphere,[108] the toxicity of emissions was indicated to increase over time.[109][110]
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