Conditions and Changing Capacity
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Since 1970, livestock products have tripled and crop outputs have doubled, a sign of rising incomes and living standards. Food production, which was worth US$1.3 trillion in 1997, is likely to continue to increase significantly as demand increases. Nonetheless, soil degradation is widespread and severe enough to reduce productivity on about 16 percent of agricultural land, especially cropland in Africa and Central America and pastures in Africa. Although global inputs and new technologies may offset this decline in the foreseeable future, regional differences are likely to increase.
Production intensification has limited the capacity of agroecosystems to provide clean freshwater, often significantly. Both irrigated and rain-fed agriculture can threaten downstream water quality by leaching fertilizers, pesticides, and manure into groundwater or surface water. Irrigated agriculture also risks both soil and water degradation through waterlogging and salinization, which decreases productivity. Salinization is estimated to reduce farm income worldwide by US$11 billion each year.
Irrigation accounts for fully 70 percent of the water withdrawn from freshwater systems for human use. Only 30-60 percent is returned for downstream use, making irrigation the largest net user of freshwater globally. Although only 17 percent of croplands now depend on irrigation, that share has grown; global irrigated area increased 72 percent from 1966 to 1996. Competition with other kinds of water use, especially for drinking water and industrial use, will be stiffest in developing countries, where populations and industries are growing fastest.
Agricultural land, which supports far less biodiversity than natural forests, has expanded primarily at the expense of forest areas. As much as 30 percent of the potential area of temperate, subtropical, and tropical forests has been lost to agriculture through conversion. Intensification also diminishes biodiversity in agricultural areas by reducing the space allotted to hedgerows, copses, or wildlife corridors and by displacing traditional varieties of seeds with modern high-yield but uniform crops. Nonetheless, certain practices, including fallow periods and intercropping, can encourage diversity as well as productivity.
In agricultural areas the amount of carbon stored in soils is nearly double that stored in the crops and pastures that the soils support. The share of carbon stored in agroecosystems (about 26-28 percent of all carbon stored in terrestrial systems) is about equal to the share of land devoted to agroecosystems (28 percent of all land). Agricultural emissions of both carbon dioxide and methane are increasing because of conversion of forests and woody savannas to agricultural uses, deliberate burning of crop stubble and pastures to control pests or promote fertility, and paddy rice cultivation.
Source note: Data are from the companion book, World Resources 2000-2001: Ecosystems and People: The Fraying Web of Life and from Pilot Analysis of Global Ecosystems: Agroecosystems (World Resources Institute, 2000).
The Value of Ecosystems
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