Forest fires have raged periodically throughout history. However, in recent decades, due to a variety of factors -- human activities, the prevailing weather and degradation of natural resources -- their frequency and intensity has increased. Fire has special uses under expert control as a management tool (clearing fuel by control burning, removing debris from clearing operations, back-burning for fire suppression), but uncontrolled wildfire destroys wildlife, damages timber resources, weakens trees, paves the way for attack by insects and diseases, and increases soil deterioration and surface run-off of water. Forest fires also threaten local communities, destroying lives and property. More indirectly, they damage watersheds and recreational resources. Depleted groundwater can dry ecosystems, leading to conditions that encourage fires to spread.
A major consequence of forest fires is their potential impact on global atmospheric problems, including climate change. The burning of standing forest can release 10 to 80% of forest biomass to the atmosphere as heat—trapping carbon dioxide and particulate air emissions. Only in the past decade have researchers realized the important contributions of biomass burning to the global budgets of carbon dioxide, methane, nitric oxide, tropospheric ozone, methyl chloride and elemental carbon particulates. This is another destructive positive feedback cycle where climate change accelerates drought, hence bushfires.
Climate and the nature of the forest are important determinants of the incidence of forest fires. Fires are common wherever there is a long dry season (such as in parts of Latin America, Africa and Australia) and especially where thunderstorms are accompanied by little rain (such as in parts of Asia and North America). Wildfire is most common in drier tropical and temperate forests, and uncommon where forests are divided into small, isolated units as in central Europe.
Coniferous forests burn much more readily than broad-leaved forests. The boreal forest region dominated by evergreen conifers regularly experiences some of the most extensive natural and human-caused forest fires on earth. For the period 1981-1989 an estimated 3.0 million ha burned annually in the former Soviet Union, nearly all in the taiga region of Russia. Satellite observations suggest this is greatly underestimated (since fires are monitored only on protected forest and pasture lands) and is more like 14.5 million ha for the 1987 season alone. In the same fire season about 1.3 million ha of forests were affected by fire in the montane-boreal forests of Northeast China, south of the Amur (Heilongjiang) River.
In Canada, an average of 9000 fires have occurred yearly, burning an average of 2.8 million hectares annually, the great majority in the less accessible boreal forest in the north and west of the country. The annual area burned is highly episodic and has varied by an order of magnitude (e.g. 0.67 million hectares in 1997, 7.28 million hectares in 1995). Likewise, in years with prolonged hot and dry periods of summer weather, Alaska experiences millions of hectares burned, mostly in a few very large fires.
Natural wildfires were virtually unknown in the humid tropical evergreen forests and may, under exceptional circumstances, destroy tropical evergreen forests (as happened in Ivory Coast and Samoa). In 1991-92, however, and again in 1995-97, hundreds of fires were alight for several months in Indonesia's tropical rainforests on Kalimantan (Borneo) and Sumatra, fuelled by burning underground peats and coal seams and highly inflammable debris from questionably managed logging operations (which may explain the Indonesian government's long silence before seeking help). The fires spread over 10 million hectares, causing losses of $9 billion. The fires recurred on a smaller scale in 1998 and cost Indonesia's neighbours an estimated $4 billion through disruption of tourism and transport as well as damage to public health. In 1999 the haze was still recorded at "extremely dangerous levels" in Sumatra.
In addition to the Indonesian fires, for six months in 1997 the fires blanketed an area of about 3 million square kilometres, which included Singapore, Brunei much of Malaysia, parts of Thailand, the Philippines and Australia. During 1997-98 the worst fires on record also raged in the Amazon rainforests of Brazil. They threatened the lives and crops of the Yanomami, who are also suffering from a lack of water. The damage to the environment was incalculable; it is estimated that the forest will take at least 100 years to recuperate.
The forests of Southeast Asia and of the Brazilian Amazon were especially vulnerable to fire in 1997 and 1998 because of a severe drought probably related to the strong El NiÃ±o of the same period and/or changing global weather patterns. After the severe El NiÃ±o of 1982, the largest fires then on record raged across Kalimantan. The 1997 and 1998 fires were far more extensive and coincided with an even more severe El NiÃ±o.
Despite the fact that hundreds of millions of dollars were spent to save the rain forest, burnings in Amazon region were up 28 percent in 1996, according to satellite data, and deforestation increased by 34 percent between 1991 and 1994. Roughly a fifth of the fires that rage annually between June and October cause new deforestation, and another tenth is burning off ground cover in virgin forests.
The recent prevalence of massive forest fires in certain countries is attributed to commercial logging, the rapid expansion of plantations and government sponsored transmigration programmes. Taking advantage of the dry conditions fires are started to clear land for agricultural use. There is a scramble for land at the forest frontier. Extinguishing the fires is costly and inefficient; the best solution is prevention. Effective reform is difficult in countries like Indonesia, where government rules are laxly applied or not enforced at all, especially in remote areas.
According to a 2000 report, fires in 1997 and 1998 burned nearly 10 million hectares (24.7 million acres) of Indonesian forest, an area similar in size to Hungary or South Korea. Studies using satellite photographs showed that on Kalimantan, Sulawesi and Sumatra - three of the largest forested islands of Indonesia—more than 17 million hectares of forests disappeared in 12 years, from 1986 to 1997. The forestry ministry estimated that the nationwide annual deforestation rate was at least 1.5 million hectares, nearly twice the estimate published by the World Bank in 1994.
The connection between groundwater depletion and increasingly destructive and frequent forest fires has been observed across the globe in recent years. Not only do groundwater dynamics affect fires, making the ecosystem more or less resilient to fire, but fires affect groundwater too. Groundwater dependent ecosystems (where tree roots access the groundwater table) are resilient against water stress, but they begin to decline, sometimes irreversibly, if there is little groundwater available and when there is also a shortage of surface water. For example, losing forest canopy to fire eliminates shade, leading to hotter ground, which could affect evaporation and aquifer recharge; sediment from fires may block routes that stormwater takes on its return to groundwater. This loss of habitat cascades into trouble for the structure and function of the ecosystem by interfering with reproduction, growth, recruitment and mortality.
During the decade 2010 to 2020, about 200 million trees have died in California forests due to wildfires exacerbated by climate change, drought and bark beetle infestation. Tree loss on this scale has never before been recorded, and each dead tree provides more fuel for wildfires.
South-East Asian rainforests have been drying up for the past 20 years, creating conditions conducive to fires that have led to many deaths related to smoke pollution. Some estimates place the number of casualties as high as 400,000. During lengthy dry periods, researchers in Borneo have found that groundwater levels become so depleted that underground capillary action is effectively halted, creating a “hydrological drought.” When fires happen during hydrological drought, there’s a tenfold increase in the area of forest loss.