In many countries, and especially in the developing countries of the southern hemisphere, systematic burning, grazing and cutting of forest-land is carried out in order to provide new land for agricultural or livestock purposes. It is often carried out without factors such as climate and topography having been sufficiently studied and on lands where slope, nature of the soil or other physiographic characteristics clearly indicate that the land involved is suitable only for forest. 

Although this practice may lead to a temporary increase in productivity, there are also many indications that in the long run there is usually a decrease in productivity per unit of surface and that erosion and irreversible soil deterioration often accompany this process.

Forests on which humans and other animals depend heavily take in carbon dioxide gas, provide oxygen and clean the air. Their water holding capacity maintains soil and water levels, preventing disasters such as landslides, floods and droughts. Tropical forests, the most important surviving woodlands, contain about two thirds of all plant and animal species. Tropical plants are the basis for several useful drugs, but vast numbers have not been tested for their medical properties. Tropical forests are also stores of genetic material to feed back into cultivated plants susceptible to disease and pests. At the present rate of deforestation, an estimated 15% of all species could disappear within the next two decades.

Many factors  contribute to deforestation: timber production, clearance for agriculture (including livestock production), cutting for firewood and charcoal, fires, droughts, strip mining, pollution, urban development, population pressures and warfare.

It is estimated that the annual rate of deforestation is of the order of 17 million hectares.  Other estimates include:

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