Fire is used extensively in the tropics for the destruction of forests. The objectives of burning are site clearing for shifting cultivation, bush and weed control, hunting, grazing, and often simply for fun. Burning is a simple and effective method of land clearing but it has serious consequences for the ecosystem which is being burnt and for its surroundings.
Burning emits large amounts of matter into the atmosphere. Large aerosol particles from burning vegetation spend a short time in the atmosphere but may effectively increase the infra-red re-radiation from the lower atmosphere. Small particles remain for periods which may be as long as several weeks or even years in the upper troposphere. Extensive savanna fires during the dry season lead to heavy dust concentrations in the atmosphere which may even spread into the region of the perhumid equatorial forest. The net effect of smoke pollution at ground surface may be cooling or warming depending on the direction of changes of the surface albedo and on the absorption coefficient of the particles in the atmosphere. Burning also releases nutrients, especially nitrogen, into the atmosphere and into the soil water. Eventually part of the latter will enter the drainage system and be lost. The fate of the former is more complicated and little understood.
The traditional form of shifting cultivation that has been practised for aeons is in equilibrium with the rainforest environment. It is based upon nutrient cycling and the management of weeds and pests through fallows. The influx of large numbers of migrants to the rainforest results in a form of slash-and-burn that is out of balance with this environment and which ultimately turns into a variety of forms of unsustainable agriculture.
In 1970 it was estimated that the number of small-scale farmers living within or on the fringes of tropical forests numbered at least 200 million. In 1993, the best estimates are that this could have increased to 500 million. It is believed that their numbers continue to rise rapidly with the possibility that their number will reach one billion within two decades. It is estimated that these slash-and-burners account for at least 60% of all tropical deforestation. This forest burning contributes about one third of all carbon dioxide emissions causing global warming processes. These people are also the principal cause of the mass extinction of species.
Environmentalists have reported that the majority of some 1000 brush fires pinpointed using satellite images on the island of Borneo in 1998 were on land leased by powerful timber companies that still use slash-and-burn techniques as the fastest and cheapest way to convert rain forests into timber estates and palm-oil plantations. Also small farmers resumed the illegal slashburning practice in order to survive as food shortages and bankruptcies spread in Indonesia.