14.2.2.1. Arable Farming and Tree Crops

Agricultural lands (excluding pastures) represent approximately 19% of the land area of Latin America. Over the past 40 years, the contribution of agriculture to the GDP of Latin American countries has been on the order of 10%. Agriculture remains a key sector in the regional economy because it employs an important segment (30-40%) of the economically active population. It also is very important for the food security of the poorest sectors of the population.

Arable farming is based on annual crops of cereals (wheat, maize, barley, rice, oats), oil seeds (soybean, peanuts, sunflower), vegetables/tubercles (potatoes, cassava), and a variety of perennial grasses, including specialty crops such as cotton, tobacco, tea, coffee, cacao, sugarcane, and sugar beet. Major tree/shrub crops include a large variety of fruits, oil palm, and others. This farm production has given rise to associated activities—such as beekeeping and bee products—as well as important agro-industries that produce valuable incomes in countries that already have developed their own markets and exporting lines.

Although the more important commercial agriculture and agro-industry businesses are well developed in a few countries, many Latin American economies rely on small farming system production. In smaller and poorer countries, such as rural communities in Central America and the Andean valleys and plateaus, agriculture is the basis of subsistence lifestyles and the largest user of human capital. For these countries, agriculture is the main producing sector; it undoubtedly is severely affected by climate variations and would be seriously influenced by climate change (Rosenzweig and Hillel, 1998).

Extremes in climate variability (e.g., the Southern Oscillation) already severely affects agriculture in Latin America. In southeastern South America, maize and soybean yields tend to be higher than normal during the warm Southern Oscillation and lower during the cold phase (Berlato and Fontana, 1997; Grondona et al., 1997; Magrin et al., 1998; Baethgen and Romero, 2000). Contributions to variability as a result of global warming and/or reduction in evapotranspiration from forest loss would be added to this background variability, thereby aggravating losses caused by extreme events.

Land-use choices will be affected by climate change. For example, increasing precipitation in marginal areas could contribute to an increase in cropped lands (Viglizzo et al., 1995). On the other hand, more favorable prices for grain crops relative to those for cattle are causing an increase in cultivated lands (Basualdo, 1995). The continued global trend to replace subsistence with market crops also creates an increasing threat to soil sustainability and enhances vulnerability to climate change.

Global warming and CO₂ fertilization effects on agricultural yields vary by region and by crop. Under certain conditions, the positive physiological effects of CO₂ enrichment could be countered by temperature increases—leading to shortening of the growth season and changes in precipitation, with consequent reductions in crop yields. Reduced availability of water is expected to have negative effects on agriculture in Mexico (Mundo and Martínez-Austria, 1993; Conde et al., 1997b). However, increases in temperature would benefit maize yields at high altitudes and lower the risk of frost damage (Morales and Magaña, 1999). Several studies were carried out in the region to assess the impact of climate change on annual crop yields. Most of these studies use crop simulation models with GCMs and incremental (temperature and precipitation) scenarios as climatic inputs. Baethgen and Magrin (1995) have shown that winter crop yields in Uruguay and Argentina are more sensitive to expected variations in temperature than precipitation. Under nonlimiting water and nutrient conditions and doubled-CO₂, the results for Argentina have shown that maize, wheat, and sunflower yield variations are inversely related to temperature increments, whereas soybean would not be affected for temperature increments up to 3°C (Magrin et al., 1997b, 1999a,b,c). Results obtained under rainfed conditions for different crops and management approaches in the region are summarized in Table 14-5; most of these results predict negative impacts, particularly for maize.

Adaptive measures to alleviate negative impacts have been assessed in the region. In Mexico, Conde et al. (1997a) found that increasing nitrogen fertilization would be the best option to increase maize yields, although it would not be economically feasible at all levels. In Argentina, the best option to improve wheat, maize, and sunflower yields would be to adjust planting dates to take advantage of the more favorable thermal conditions resulting from fewer late frosts (Travasso et al., 1999). However, this adaptive measure would be insufficient for maintaining actual wheat and maize yield levels. Genetic improvement will be necessary to obtain cultivars that are better adapted to the new growing conditions. For wheat and barley crops in Uruguay and Argentina, a longer growth season could be achieved by increasing photoperiodical sensitivity (Hofstadter et al., 1997; Travasso et al., 1999).

Subsistence farming could be severely threatened in some parts of Latin America. The global agricultural model of Rosenzweig et al. (1993) identifies northeastern Brazil as suffering yield impacts that are among the most severe in the world (see Reilly et al., 1996; Canziani et al., 1998; Rosenzweig and Hillel, 1998). Because northeastern Brazil is home to more than 45 million people and is prone to periodic droughts and famines even in the absence of expected climate changes, any changes in this region would have major human consequences.

Climate changes can be expected to lead to changes in soil stocks of carbon and nitrogen. In the Argentinean pampas, chemical degradation of soils, based on climate changes predicted by the GISS GCM (Hansen et al., 1988) at an atmospheric CO₂ concentration of 550 ppm, would reduce organic nitrogen by 6-10% and organic carbon by 7-20% in the topsoil as a result of lower dry-matter production and an increased mineralization rate (Díaz et al., 1997).

Tree crops in locations where frost risk presents a limitation—such as coffee in Paraná, Brazil—benefit from higher minimum temperatures resulting from global warming (Marengo and Rogers, 2000).

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