Friday, December 11, 2009

History

The fossil record of fire first appears with the establishment of a land-based flora in the Middle Ordovician period, 470 million years ago,[99] permitting the accumulation of oxygen in the atmosphere as never before, as the new hordes of land plants pumped it out as a waste product. When this concentration rose above 13%, it permitted the possibility of wildfire. Wildfire is first recorded in the Late Silurian fossil record, 420 million years ago, by fossils of charcoalified plants.[100] Apart from a controversial gap in the Late Devonian, charcoal is present ever since.[100] The level of atmospheric oxygen is closely related to the prevalence of charcoal: clearly oxygen is the key factor in the abundance of wildfire.[101] Fire also became more abundant when grasses radiated and became the dominant component of many ecosystems, around 6 to 7 million years ago;[102] this kindling provided tinder which allowed for the more rapid spread of fire.[101] These widespread fires may have initiated a positive feedback process, whereby they produced a warmer, drier climate more conducive to fire.[101]

Human involvement

Wildfires have been mentioned in human history, from minor allusions in the Bible to classical writers such as Homer, although less focus was placed on uncultivated lands where wildfires occurred.[103][104] Wildfires were also used in battles throughout human history as early thermal weapons. From the Middle ages, accounts were written of occupational burning as well as customs and laws that governed the use of fire. In 14th century Sardinia, firebreaks were used for wildfire protection. In the Atlantic Ocean on the island of Madeira, fire was used to clear the land of Laurisilva (laurel forest) in 1419.[105] In Spain during the 1550s, sheep husbandry was discouraged in certain provinces by Phillip II due to the harmful effects of fires used in transhumance.[103][104] In the countries bordering the Baltic Sea, fire in land use systems was typical during the Neolithic period until World War II.[106] As early as the 1600s, Native Americans were observed using fire for many purposes including cultivation, signaling, and warfare. Scottish botanist David Douglas noted the native use of fire for tobacco cultivation, to encourage deer into smaller areas for hunting purposes, and to improve foraging for honey and grasshoppers. Charcoal sedimentary data off the Pacific coast of Central America also suggests that more burning occurred in the 50 years before the Spanish colonization of the Americas.[107]

Wildfires typically occurred during periods of increased temperature and drought. An increase in fire-related debris flow in alluvial fans of northeastern Yellowstone National Park was linked to the period between AD 1050 and 1200, coinciding with the Medieval Warm Period.[108] However, human influence caused an increase in fire frequency. Dendrochronological fire scar data and charcoal layer data in Finland suggests that, while many fires occurred during severe drought conditions, an increase in the number of fires during 850 BC and 1660 AD can be attributed to human influence.[109] Charcoal evidence from the Americas suggested a general decrease in wildfires between 1 AD and 1750 compared to previous years. However, a period of increased fire frequency between 1750 and 1870 was suggested by charcoal data from North America and Asia, attributed to human population growth and influences such as land clearing practices. This period was followed by an overall decrease in burning in the 20th century, linked to the expansion of agriculture, increased livestock grazing, and fire prevention efforts.[110]

Prevention

Drawing of a grizzly bear with human features. He is wearing blue jeans with a belt and a brimmed hat with the name "Smokey" on the cap, and has a shovel in his left hand. He is pointing to the viewer while the text "Only You" is seen below him.

1985 Smokey Bear poster with part of his admonition, "Only you can prevent forest fires".

Wildfire prevention refers to the preemptive methods of reducing the risk of fires as well as lessening its severity and spread.[111] Effective prevention techniques allow supervising agencies to manage air quality, maintain ecological balances, protect resources,[72] and to limit the effects of future uncontrolled fires.[112] North American firefighting policies may permit naturally-caused fires to burn to maintain their ecological role, so long as the risks of escape onto high-value areas are mitigated.[113] However, prevention policies must consider the role that humans play in wildfires, since, for example, only 5% of forest fires in Europe are not related to human involvement.[114] 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.[115] Landholders with flammable investments such as orchards and tree crops may encourage neighboring landowners to reduce fire risks.[116]

In the mid-1800s, explorers from the HMS Beagle observed Australian Aborigines using fire for ground clearing, hunting, and regeneration of plant food in a method called fire-stick farming.[117] Such careful use of fire has been employed for centuries in the lands protected by Kakadu National Park to encourage biodiversity.[118] In 1937, U.S. President Franklin D. Roosevelt initiated a nationwide fire prevention campaign, highlighting the role of human carelessness in forest fires. Later posters of the program featured Uncle Sam, leaders of the Axis powers of World War II, characters from the Disney movie Bambi, and the official mascot of the U.S. Forest Service, Smokey Bear.[119]


A prescribed burn in a Pinus nigra stand in Portugal

Wildfires are caused by a combination of factors such as topography, fuels, and weather. Other than reducing human infractions, only fuels may be altered to affect future fire risk and behavior.[43] Wildfire prevention programs around the world may employ techniques such as wildland fire use and prescribed or controlled burns.[1][120][121] Wildland fire use refers to any fire of natural causes that is monitored but allowed to burn. Controlled burns are fires ignited by government agencies under less dangerous weather conditions.[122] Vegetation may be burned periodically to maintain high species diversity, and frequent burning of surface fuels limits fuel accumulation, thereby reducing the risk of crown fires.[123][124] Using strategic cuts of trees, fuels may also be removed by handcrews in order to clean and clear the forest, prevent fuel build-up, and create access into forested areas.[125] Chain saws and large equipment can be used to thin out ladder fuels and shred trees and vegetation to a mulch.[126] Multiple fuel treatments are often needed to influence future fire risks, and wildfire models may be used to predict and compare the benefits of different fuel treatments on future wildfire spread.[43] However, controlled burns are reportedly "the most effective treatment for reducing a fire’s rate of spread, fireline intensity, flame length, and heat per unit of area" according to Jan Van Wagtendonk, a biologist at the Yellowstone Field Station.[127] Additionally, while fuel treatments are typically limited to smaller areas, effective fire management requires the administration of fuels across large landscapes in order to reduce future fire size and severity.[128]

Building codes in fire-prone areas typically require that structures be built of flame-resistant materials and a defensible space be maintained by clearing flammable materials within a prescribed distance from the edifice.[129][130] Communities in the Philippines also maintain fire lines 5 to 10 meters (16 to 33 ft) wide between the forest and their village, and patrol these lines during summer months or seasons of dry weather.[131]

Three photos of the same forest region. The first features a central tree with other trees in the distance. A man and two mounted horses are seen at varying distances behind the central tree. The forest floor features low-lying vegetation such as grasses. The second and third photos feature the same central tree but with increasing amounts of trees in the mid- and foregrounds. The central tree is almost completely blocked from view in the third picture.

A ponderosa pine stand in the Bitterroot National Forest in Montana in 1909, 1948, and 1989. The increase in vegetation density was attributed to fire prevention efforts since 1895.[132]

Detection

A four-legged tower with a small enclosure at the top, next to two one-story buildings. The tower is four stories tall. Trees are at either side, and in the foreground there are rocks, some vegetation, and a rough trail.

Dry Mountain Fire Lookout in the Ochoco National Forest, Oregon, circa 1930

Fast and effective detection is a key factor in wildfire fighting.[133] Early detection efforts were focused on early response, accurate day and nighttime use, the ability to prioritize fire danger, and fire size and location in relation to topography.[134] Fire lookout towers were used in the United States in the early 1900s and fires were reported using telephones, carrier pigeons, and heliographs.[135] Aerial and land photography using instant cameras were used in the 1950s until infrared scanning was developed for fire detection in the 1960s. However, information analysis and delivery was often delayed by limitations in communication technology. Early satellite-derived fire analyses were hand-drawn on maps at a remote site and sent via overnight mail to the fire manager. During the Yellowstone fires of 1988, a data station was established in West Yellowstone, permitting fire information delivery in approximately four hours.[134]

Currently, public hotlines, fire lookouts in towers, and ground and aerial patrols can be used as a means of early detection of forest fires. However, accurate human observation may be limited by operator fatigue, time of day, time of year, and geographic location. Electronic systems have gained popularity in recent years as a possible resolution to human operator error. These systems may be semi- or fully-automated and employ systems based on the risk area and degree of human presence, as suggested by GIS data analyses. An integrated approach of multiple systems can be used to merge satellite data, aerial imagery, and personnel position via GPS into a collective whole for near-realtime use by wireless Incident Command Centers.[136][137]

A small, high risk area that features thick vegetation, a strong human presence, or is close to a critical urban area can be monitored using a local sensor network. Detection systems may include wireless sensor networks that act as automated weather systems: detecting temperature, humidity, and smoke.[138][139][140] These may be battery-powered, solar-powered, or tree-rechargeable: able to recharge their battery systems using the small electrical currents in plant material.[141] Larger, medium-risk areas can be monitored by scanning towers that incorporate fixed cameras and sensors to detect smoke or additional factors such as the infrared signature of carbon dioxide produced by fires. Brightness and color change detection and night vision capabilities may be incorporated also into sensor arrays.[142][143][144]

A satellite view of the Balkans and Greece. Clouds and smoke trails are seen above the Balkans and trailing south into the Ionian Sea.

Wildfires across the Balkans in late July 2007 (MODIS image)

Satellite and aerial monitoring can provide a wider view and may be sufficient to monitor very large, low risk areas. These more sophisticated systems employ GPS and aircraft-mounted infrared or high-resolution visible cameras to identify and target wildfires.[145][146] Satellite-mounted sensors such as Envisat's Advanced Along Track Scanning Radiometer and European Remote-Sensing Satellite's Along-Track Scanning Radiometer can measure infrared radiation emitted by fires, identifying hot spots greater than 39 °C (102 °F).[147][148] The National Oceanic and Atmospheric Administration's Hazard Mapping System combines remote-sensing data from satellite sources such as Geostationary Operational Environmental Satellite (GOES), Moderate-Resolution Imaging Spectroradiometer (MODIS), and Advanced Very High Resolution Radiometer (AVHRS) for detection of fire and smoke plume locations.[149][150] However, satellite detection is prone to offset errors, anywhere from 2 to 3 kilometers (1 to 2 mi) for MODIS and AVHRR data and up to 12 kilometers (7.5 mi) for GOES data.[151] Satellites in geostationary orbits may become disabled, and satellites in polar orbits are often limited by their short window of observation time. Cloud cover and image resolution and may also limit the effectiveness of satellite imagery.[152]

Suppression

A three-engine red-and-white cargo plane in-flight, releasing a large quantity of water from its undercarriage storage tanks. The water trails behind the aircraft in a continuous, fan-shaped drop pattern.

Tanker 910 during a drop demonstration in December, 2006

Wildfire suppression may include a variety of tools and technologies, including throwing sand and beating fires with sticks and palm fronds in rural Thailand, using silver iodide to encourage snow fall in China, and full-scale aerial assaults by ALTUS II unmanned aerial vehicles, planes, and helicopters using drops of water and fire retardants.[153][154][155][156] Complete fire suppression is no longer an expectation, but the majority of wildfires are often extinguished before they grow out of control. While more than 99% of the 10,000 new wildfires each year are contained, escaped wildfires can cause extensive damage. Worldwide damage from wildfires is in the billions of euros annually.[157] Wildfires in Canada and the US consume an average of 54,500 square kilometers (13,000,000 acres) per year.[158][159]

Fuel buildup can result in costly, devastating fires as new homes, ranches, and other development are built adjacent to wilderness areas. Continued growth in fire-prone areas and rebuilding structures destroyed by fires has been met with criticism.[160] However, the population growth along the wildland-urban interface discourages the use of current fuel management techniques. Smoke is an irritant and attempts to thin out the fuel load is met with opposition due to desirability of forested areas, in addition to other wilderness goals such as endangered species protection and habitat preservation.[161] The ecological benefits of fire is often overridden by the economic benefits of protecting structures and lives.[162] Additionally, government policies that cover the wilderness usually differs from local and state policies that govern urban lands.[163][164]

Modeling

A dark region shaped like a shield with a pointed bottom. An arrow and the text "propagation axis (wind)" indicates a bottom-to-top direction up the body of the shield shape. The shape's pointed bottom is labeled "fire start". Around the shield shape's top and thinning towards its sides, a yellow-orange region is labeled "left front", "right front", and (at the top) "head of the fire".

Fire Propagation Model

Wildfire modeling is concerned with numerical simulation of wildfires in order to understand and predict fire behavior.[165][166] Wildfire modeling can ultimately aid wildfire suppression, namely increase the safety of firefighters and the public, reduce risk, and minimize damage. Using computational science, wildfire modeling involves the statistical analysis of past fire events to predict spotting risks and front behavior. Various wildfire propagation models have been proposed in the past, including simple ellipses and egg- and fan-shaped models. Early attempts to determine wildfire behavior assumed terrain and vegetation uniformity. However, the exact behavior of a wildfire's front is dependent on a variety of factors, including windspeed and slope steepness. Modern growth models utilize a combination of past ellipsoidal descriptions and Huygens' Principle to simulate fire growth as a continuously expanding polygon.[167][168] Extreme value theory may also be used to understand large wildfires. However, large fires that exceed suppression capabilities are often regarded as statistical outliers in standard analyses, even though fire policies are influenced by catastrophic wildfires more so than small fires.

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