The extent to which global climate change is occurring is not clear, but it is well documented that carbon dioxide (CO2) levels in the atmosphere are steadily increasing. The increased levels of CO2 will likely have a positive effect on food and fiber production in areas where water does not become more limited. However, in areas that become hotter and drier as a result of global climate change, yields will likely decline even though CO2 levels for photosynthesis are more favorable. Rosenzweig (2000) surmises that if atmospheric buildup of greenhouse gases continues without limit, sooner or later it is bound to warm the Earth's surface. Such a warming trend cannot but affect the regional panoply of temperature and precipitation governing natural and agricultural systems. Some areas — possibly the northern United States,

Canada, and Russia — may become more productive, but other areas such as the Middle East and parts of Asia may become less productive. Dryland regions are already severely challenged because of high temperatures, and additional warming will indeed be a challenge to these regions unless average precipitation is significantly increased. There is also the possibility that the already highly variable precipitation amounts in these regions will become even more variable. This would result not only in more severe droughts but also more water erosion and flooding during extreme precipitation events.

Jones and Thornton (2003) stated that climate change could potentially lead to a 10% drop in developing-country maize production over the next 50 years. They projected that the lost production would not be across the board or evenly spread, but that losses would vary widely from one agro-ecosystem to another. For example, they predicted that large maize-producing countries such as Brazil and Mexico will be hit hard, while Chile and Ecuador will likely be relatively unaffected, and other countries may even experience increased yields. Latin America will likely see a reduction approaching 25% due to higher temperatures and decreasing rainfall. These authors forecast that Nigeria, South Africa, and Tanzania will lose upward of 20% of total production. Maize is particularly important in many parts of the world, and less maize means less grain for poor people, less feed for farm animals, and less milk and meat for hungry households. The researchers, however, emphasized that the projected decreases in maize production could be lessened by better crop varieties and land management systems.

Global food and fiber production may not be severely threatened because gradual adjustments can be made to produce more in favored areas. There will, however, be areas where food and fiber production will be limited, as is already the situation today in many dryland regions. Policies should be identified and implemented that can mitigate the threat of global climate change. Conservation agriculture (FAO, 2002), based on the principles of avoiding mechanical soil disturbance, maintaining a permanent soil cover, and crop rotation is a sound strategy that is being promoted and applied in most parts of the world. Conservation agriculture is practiced on 45 million ha, mostly in South and North America (FAO, 2002). This accounts, however, for only about 3% of the 15 billion ha of arable land worldwide. Conservation agriculture is practiced from the humid tropics to almost the Arctic Circle and on all kinds of soils (FAO, 2002). According to the FAO (2002), "So far the only area where the concept has not been successfully adapted is the arid areas with extreme water shortage and low production of organic matter. In these areas both humans and animals compete with the soil for crop residues."

Although applying all conservation agriculture principles to dryland areas is a challenge, substantial benefits can occur by reducing tillage and managing crop residues as already illustrated in Figure 14.3. Some C can be sequestered in these regions to slowly begin restoring some of the lost SOM. Increases will likely be slow and in many cases insignificant because of high temperatures that hasten decomposition and the shortage of N, and perhaps P, that must be available to form SOM. Even small increases in dryland regions must be pursued because continued soil degradation will make these already water-deficient areas even more deficient.

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