Agriculture accounts for the major share of human use of land. Pasture and crops alone took up 37 percent of the earth's land area in 1999. Over two-thirds of human water use is for agriculture. In Asia the share is four-fifths. Crop and livestock production have a profound effect on the wider environment. They are the main source of water pollution by nitrates, phosphates and pesticides. They are also the major anthropogenic source of the greenhouse gases methane and nitrous oxide, and contribute on a massive scale to other types of air and water pollution. The extent and methods of agriculture, forestry and fishing are the leading causes of loss of the world's biodiversity. The overall external costs of all three sectors can be considerable.