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Wildfires, also known as wildland fires, are blazes that burn in natural settings, such as grasslands, shrub lands, and forests. Wildfires can incinerate buildings and crops, kill people in adjacent communities, and incur billions of dollars in firefighting costs. But wildfires also play a critical role in many ecosystems, often leading to a greater diversity of plants and animals in the patchy, recovering landscapes they leave behind. Although most wildfires are triggered by lightning strikes, many also are caused by humans, especially in tropical and subtropical areas where humans use fire to clear land for agriculture.

Although wildfire frequency and intensity are partly driven by natural climate cycles, human-caused climate change is playing an increasingly important role. Rising temperatures and changing moisture regimes associated with higher atmospheric greenhouse-gas levels are causing larger, longer-lasting fires in many places and extending fire seasons—those times of year, generally summer through fall, when fires typically occur. This has implications for human safety, the cost of fighting fires, and ecosystem health. More and larger wildfires may also promote more warming through increased release of the greenhouse gases carbon dioxide (CO2) and methane.

Historical Background and Scientific Foundations

Analysis of ancient charcoal deposits from around the world has shown that widespread, severe wildfires first occurred following the establishment of vascular plants and forests, which provided abundant fuel about 350 million years ago. Since then, fire has become an integral part of many ecosystems on varying scales. The chaparral scrub and closed-cone conifer communities common in California, for example, require frequent, regular fires for seed germination and maintaining plant species diversity. Meanwhile, the mixed-conifer forests that cloak mountain slopes at high altitudes and ring Earth at high latitudes are adapted to infrequent, stand-replacing fires.

Human land-use has had a direct impact on fire regimes in some places. In the United States, for example, fire-suppression policies adopted by the federal government since the early 1900s have led to a buildup of woody fuels, such as understory shrubs and small trees, in arid and semi-arid forests, where plants are adapted to frequent, low-intensity fires. These buildups eventually result in catastrophic fires that kill fire-resistant older trees and damage soils.

In the Amazon of South America, drought conditions and the clearing of trees for logging and agriculture have left the rainforest, which is not well-adapted to fire, fragmented and much more vulnerable to blazes, and tropical forest fires have become more common. Similar trends are evident in Southeast Asia.

Analyses of tree rings, which hold climate and fire records dating back centuries and in some cases millennia, have tied wildfires to naturally occurring climatic cycles and shifts, such as droughts. In the forests of the southwestern United States, for example, wet El Niño years, which are caused by the cyclic warming of the tropical Pacific Ocean, can encourage the buildup of vegetation, which may then burn explosively during ensuing dry La Niña years, which occur with cooling of the tropical Pacific.

Rising temperatures and other factors, such as decreasing mountain snowpack and earlier onset of the drier months of spring and summer, which have been linked to rising greenhouse gas levels, are fueling longer, more intense wildfire seasons. These changes are apparent, for example, in the forests of North America.

In the western United States, longer, warmer summers fueled a four-fold increase in the frequency of large wildfires and a six-fold increase in the area of forests burned from 1986 to 2003, as compared to the period from 1970 to 1986. The average duration of large fires also went up, increasing from 7.5 days in 1970–1986 to 37.1 days in 1987–2003. In addition, the average length of the wildfire season increased by 78 days.

These increases in wildfire activity are attributable to a mere 1.6°F (0.9°C) increase in average spring and summer temperatures and to melting of mountain snow-pack one to four weeks earlier. Warmer temperatures during fire seasons have increased the amount of forest burned over the past three decades in Canada, although some scientists predict that climate change may reduce the frequency of fires in some northern forests. Currently, about 12–37 million acres (5–15 million hectares) of boreal forests burn annually, mostly in Siberia, Canada, and Alaska.

As human-caused climate change progresses, computer models show that many countries will experience even greater changes in wildfire regimes. Canada may see a 74–118% increase in forest area burned over the next century. Russia may see similar trends. At temperature increases at or greater than 5.4°F(3°C), more frequent wildfires are 60% more likely in much of South America. Wildfires and hot, dry weather conducive to wildfires will also likely increase across central Australia, the Sahel, southern Africa, central Asia, and at high elevations around the globe.

Increasing atmospheric concentrations of CO2—which plants convert into sugars and biomass during photosynthesis—may also boost wildfires by promoting the growth of more vegetation that can serve as fuel. The potential for igniting fires will also rise as more people move into increasingly dry wild areas and as warming increases the frequency of severe thunderstorms with more cloud-to-ground lightning.

Impacts and Issues

The increased incidence of longer, larger wildfires and longer wildfire seasons may raise firefighting costs substantially and put more homes and communities at risk for damage or destruction, with a greater potential for human casualties where people live in rural or fire-prone areas. Fire-management agencies in Canada already spend $500 million a year on firefighting. Governmental appropriations for firefighting in the United States, meanwhile, have increased from an average of $1.1 billion dollars between 1996 and 2001 to $2.9 billion between 2001 and 2005. About 50–95% of that increased cost came from protecting the increasing number of homes built close to wild areas. Increasingly severe fire seasons, stemming from heightened drought and severe weather conditions and a buildup in fuels due to past fire suppression, are another major factor.

Increasingly severe wildfire seasons will alter ecosystems, potentially causing some to collapse. In eastern Canada, intensified wildfire regimes driven by human-caused global warming already appear to be changing some forests from spruce-dominated to pine-dominated ecosystems, and have led to 75–95% reductions in tree density. More-extensive fires may also reduce the extent and connectivity of old-growth forests, further threatening already endangered species such as the northern spotted owl and favoring species that thrive in disturbed areas (e.g., pocket gophers). Altered fire regimes will likely favor greater incidence of tree pests, such as bark beetles, that reproduce in dead or stressed trees and spread to healthier stands, killing more trees and leading to still greater potential for stand-replacing fires.


BIOMASS: The sum total of living and once-living matter contained within a given geographic area. Plant and animal materials that are used as fuel sources.

CARBON SEQUESTRATION: The uptake and storage of carbon. Trees and plants, for example, absorb carbon dioxide, release the oxygen, and store the carbon. Fossil fuels were at one time biomass and continue to store the carbon until burned.

DROUGHT: A prolonged and abnormal shortage of rain.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

PHOTOSYNTHESIS: The process by which green plants use light to synthesize organic compounds from carbon dioxide and water. In the process, oxygen and water are released. Increased levels of carbon dioxide can increase net photosynthesis in some plants. Plants create a very important reservoir for carbon dioxide.

SAHEL: The transition zone in Africa between the Sahara Desert to the north and tropical forests to the south. This dry land belt stretches across Africa and is under stress from land use and climate variability.

TREE RINGS: Marks left in the trunks of woody plants by the annual growth of a new coat or sheath of material. Tree rings provide a straightforward way of dating organic material stored in a tree trunk. Tree-ring thickness provides proxy data about climate conditions: most trees put on thicker rings in warm, wet conditions than in cool, dry conditions.

In chaparral, grasslands, and shrub lands, increasingly frequent fires may favor dense colonization by invasive grasses and weeds. In some cases, these invaders may, in turn, promote even more frequent fires because they are more flammable and grow more densely than native cover. Increased loss of sagebrush and scrub habitats would be damaging for many songbird species as well as for sage grouse and other animals.

Forests provide a number of ecosystem services— species habitat, timber, climate regulation, cultural and spiritual benefits, and soil and water protection and purification. (Globally, more than 75% of usable freshwater supplies come from forested catchments.) All this may be altered or hindered by intensifying fire regimes. Not the least is carbon sequestration. Forests around the world hold 1.81 trillion tons (1.64 trillion metric tons) of carbon, equivalent to about 220% of atmospheric carbon. Currently, wildfires release about 3.86 billion tons (3.5 billion metric tons) of carbon annually, roughly 40% of current fossil-fuel emissions.

Thus, more and larger fires mean more carbon emissions, which could further accelerate global warming, and lead to still more fires. However, potential changes in vegetation structure around the globe brought on by climate change make it difficult for scientists to predict how this feedback mechanism will play out in detail.

See Also Forests and Deforestation.



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Web Sites

“Charcoal Reveals Wildfire History.” BBC News,July 14, 2006. <http://news.bbc.co.uk/2/hi/science/nature/5180924.stm> (accessed October 28, 2007).

“Cost of Fighting Fires on Public Lands.” Headwaters Economics, September 18, 2007. <http://www.headwaterseconomics.org/wildfire/cost_of_fire.pdf> (accessed November 5, 2007).

“Learning to Live with Fire,” State of California, August 1999. <http://www.fire.ca.gov/education_content/downloads/live_w_fire.pdf> (accessed October 28, 2007).

“The World on Fire.” NOVA Online, June 2002.<http://www.pbs.org/wgbh/nova/fire/world.html> (accessed October 28, 2007).

Sarah Gilman