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, http://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; Rosegrant et al., 1995; Antle et al., 1999; Johnson, 1999). Furthermore, the severest challenge to the ability of global agricultural capacity to expand apace with demand, with or without climate change, will come in the next 25-40 yr, 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 25-40 yr, with the rest of the century being anti-climactic. The situation for forest ecosystems and the services they provide may be somewhat different due to the longer lifetime of the species involved. The response of forest ecosystems will play out over longer time frames of a century or more, with reliability of the forecasted change critically tied to the accuracy of the future climate predictions. Here, however, it is expected that economic systems will change more rapidly in response to ecosystem changes in resource supply than those ecosystem responses themselves, and arguably with greater uncertainty attached to them.
Agricultural production in the latter half of the 20th century increased the global food supply outstripping the increase in global demand for food - this was accomplished in spite of increases in global population and incomes. As a result, prices for most major crops declined when adjusted for inflation. Wheat and feed corn declined at an annual average rate of 1-3% over the period (Johnson, 1999; Antle et al., 1999). In the absence of climate change, several analysts (e.g., World Bank, 1993; Rosegrant et al., 1995; Johnson, 1999) expect inflation-adjusted food prices to remain stable or slowly to decline over the next two decades. Confidence in this outcome is high over the next two decades.
Declining food prices will likely ease but not fully erase problems of food security, particularly in low-income countries where lack of access to food, political instability, and inadequate physical and financial resources will remain major challenges. In some instances, especially in the small number of nations with little immediate prospect for a successful transition from agricultural economies to manufacturing or service-based economies, lower global food prices could be stressful. However, the anticipated spread of technology and science-based production practices even to the poorest agricultural economies will likely reduce costs of production to help farmers cope with lower prices. Agricultural trade policies tend to decrease the efficiency of production both in high and low-income countries. In high-income countries, policies tend to subsidize production in order to protect the agriculture sector while in low-income countries policies tend to tax and discourage production (Schiff and Valdez, 1996).
Much of the optimism for future growth in agricultural production hinges on anticipated technological progress that increases crop yields. Rosegrant and Ringler (1997) argue that considerable unexploited capacity to raise crop yields exists in current crop varieties. Other analysts (e.g., Pingali, 1994; Tweeten, 1998) argue that the declining supply of new agricultural land combined with large-scale degradation of soil and water resources will slow the increase in global agricultural output, which may slow or negate the expected decline in real food prices. Approximately 50% of cereal production in developing countries is irrigated and, although it accounts only for 16% of the world's crop land, irrigated land produces 40% of the world's food. It appears that the rate of expansion of irrigation is slowing and 10 to 15% of irrigated land is degraded to some extent by waterlogging and salinization (Alexandratos, 1995). It is questionable whether or not the irrigation water supplies necessary to meet future irrigation demands will be available. The two conflicting views represented in this paragraph make the future trend in prices beyond the first third of the century highly uncertain.
Forests cover nearly 30% (3,500 Mha) of the world's land area (excluding Greenland and Antarctica), of which 60% are located in seven countries (in order): Russia, Brazil, Canada, United States, China, Indonesia and the Democratic Republic of the Congo. Although 57% of the global forest is in developing countries, only 81 Mha of these were classified as plantations in 1995; developed countries account for a further 80-100 Mha (FAO, 1997). Between 50 and 95% of the industrial roundwood production for many countries is met from plantations that cover as little as 1-17% of their total forest area (Sedjo, 1999), and plantations in developing countries typically occur in community woodlots or agro-forestry operations.
FAO (1997) estimated that there was a human-induced net loss of about 5% (180 Mha) in forest area between 1980 and 1995. The estimated 200 Mha conversion of forests to subsistence agriculture, cash crops and ranching was in part offset by an estimated 40 Mha of new plantations and 20 Mha of afforestation and old-farm abandonment in developed nations. Deforestation appears to have accounted for an overall loss of native forest estimated at 65 Mha between 1990 and 1995 (FAO, 1997), while degradation, in the form of fragmentation, non-sustainable logging of mature forests, and infrastructure development, has occurred on large scales, leading to a net loss of forest biomass in existing forest. For example, 1 Mha yr-1 of degraded forests result from damaging logging practices and surface fires in Brazil and effectively account for as much loss of forest areas as the annual deforestation in these same regions (Cochrane et al., 1999, Nepstad et al., 1999). These human encroachments are not limited to the tropical forests: Robinson et al. (1999) have shown that in boreal Canada, road development, survey lines and wellheads have led to a loss of 54,000 to 81,000 ha yr-1 forest cover.
Forests and woodlands provide many goods and services that society depends upon, including marketable timber and non-timber products (food, fuel, and fiber) as well as a host of environmental, recreational and spiritual values. Changes in the global climate, as well as the direct pressures by human activities, are likely to have an impact on most of these goods and services, with significant socioeconomic impacts (Winnett, 1998). Separation of the influence of climate change on forests and their associated goods and services from other global change factors such as land-use change, land-use practices and changes in atmospheric chemistry (CO2 and pollution) is difficult. There are, however, strong signals emerging both from carefully controlled experiments (e.g., Luo et al., 2002, Shaver et al., 2000) and from transect studies across strong climatic gradients (e.g., McGuire et al., 2001, Yu et al., 2002, Canadell et al., 2002). These signals appear across a range of scales from responses of individual organisms and species, to changes in landscape patterns associated with a combination of competitive advantage and altered disturbance regimes. Associated changes in carbon stocks (i.e., source-sink relationships), a focus for many of the studies (e.g., Apps et al., 2002), are strongly linked with the global change induced alteration of forest dynamics. Alteration of natural disturbance patterns (such as fire, insect disease and storm damage) and human disturbances (land-use change) appear to be especially important in northern and some temperate forest ecosystems. Not only are there direct effects from the disturbances themselves, but they may also facilitate other ecosystem responses to climate change through altered successional pathways (Overpeck et al., 1990). Because of the importance of this matter to the land use, land-use change and forestry components of the Kyoto protocol (Watson et al., 2000), for example, the IPCC is presently convening an expert task group to factor out direct human-induced changes in carbon stocks from those due to indirect human induced and natural effects.
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