A rainforest is a forested biome with high annual rainfall. Some cite a minimum normal annual ranfall of 2500 mm (about 100 inches), with normal rainfall at least 60 mm during each of the twelve months of the year. Others set the minimum annual rainfall barrier as low as 1700 mm (about 67 inches). The soil can be poor because high rainfall tends to leach out soluble nutrients. This type of biome is found in both temperate and tropical climates. As well as prodigious rainfall, many rainforests are characterized by a high number of resident species and tremendous biodiversity.
The undergrowth in a rainforest is restricted in many areas by the lack of sunlight at ground level. This makes it possible for humans and other animals to walk through the forest. If the leaf canopy is destroyed or thinned for any reason, the ground beneath is soon colonised by a dense tangled growth of vines, shrubs and small trees called jungle.
It is estimated that rainforests provide up to 40% of the oxygen currently found in the atmosphere. And yet, tropical and temperate rain forests have been subjected to heavy logging and agricultural clearance throughout the 20th century, so that the area covered by rainforest around the world is rapidly shrinking. It is estimated that the rainforest was reduced by about 58,000 km² annually in the 1990s. Rainforests used to cover 14% of the Earth's surface. This percentage is now down to 6% and it is estimated that the remaining rainforests could disappear within 40 years (mid-21st century) at the present rate of logging. Many scientists seriously dispute these estimates, especially considering the rapid growth of new tropcial rainforests in cleared areas. Biologists have estimated that large numbers of species are being extirpated (driven to extinction)—possibly more than 50,000 a year—due to the removal of habitat with destruction of the rain forests.