Agriculture and forests account for approximately 41% of the Earth's land covers (Houghton, 1990). According to the United Nation's Food and Agriculture Organization (FAO)
(www.fao.org/DOCREP/003/X7470E/X7470E00.HTM), global exports of their commodities and services were valued at $440 billion in 1999. As noted above, unlike less managed ecosystems, the products of agriculture and forests are traded as commodities on world markets. Those products possess critical life-giving properties and are part of the Earth's life support system. There is a consensus that the global food and fiber enterprise will be challenged over the coming decades to expand capacity in step with anticipated expansion in global demand (World Bank, 1993; Alexandratos, 1995; Roseg-rant et al., 1995; Antle et al., 1999; Johnson, 1999). Furthermore, the most severe challenge to the ability of global agricultural capacity to expand space with demand, with or without climate change, will come in the next 25 to 40 years, with the challenge abating after that, as population growth is projected to slow and global income elasticity of food demand is projected to decline. That is, the real story of climate change impacts on global agriculture is likely to be played out over the next 25 to 40 years, with the rest of the century being anticlimactic.
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