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Latin America has about 23% of the world's potential arable land, although-in
contrast with other regions-it maintains a high percentage of natural ecosystems
(UNEP, 1992). The total cropland (about 134 million ha), excluding pastures,
represents less than 10% of the total area of the Latin American region (see
Annex D). About 306 million ha (72.7%) of the agricultural drylands in South
America (i.e., irrigated lands, rain-fed croplands, and rangelands) suffer from
moderate to extreme deterioration (UNEP, 1992), and about 47% of the soils in
grazing lands have lost their fertility (LAC CDE, 1992). These land degradation
figures include erosion and soil degradation on hillsides and in mountain areas,
as well as desertification by overgrazing, salinization, and alkalinization
of irrigated soils in tropical pasture lands. The area of irrigated land has
increased substantially: Data for the periods 1966-68 and 1986-88 show that
irrigated land has increased from 8,674,000 ha to 14,040,000 ha (UNEP, 1992).
A preliminary calculation by the Global Assessment of Soil Degradation (GLASOD,
1990) suggests that approximately 14% of the land area in South America is affected
by some form of human-induced soil degradation. The percentage distribution
of this land area, by type and degree of soil degradation, is summarized in
Table 6-6.
Historical shifts in land-use patterns are important factors to consider in
the design of adaptive strategies aimed at maintaining or increasing production
in the face of climate change while preventing soil erosion and other undesirable
environmental consequences (Viglizzo et al., 1995).
Studies of the dynamics of climate and land use during the past century have been performed by Viglizzo et al. (1997), with the aim of quantifying the impact of climate on land-use change and discussing adaptive land-use strategies for the Argentine Pampas. These studies show a high and positive degree of association between rainfall and crop growth in humid zones. This relationship declines, however, in the transition from humid to semi-arid zones. In these marginal environments, land-use change appears to be highly correlated to the crop yields (which are a combined function of climate and technology) and less correlated to the rainfall pattern (Viglizzo et al., 1995; Viglizzo et al., 1997).
The contribution of agriculture (10-12%) to the GDP of Latin American countries in the past 40 years has been secondary to other human activities (Baethgen, 1994). Agriculture is still a key sector in the region's economy, however, because it occupies an important proportion (30-40%) of the economically active population. Moreover, agriculture has generated the largest export income in countries having neither oil nor mineral production. Finally, in the smaller and poorer countries of the region, agriculture is the basis of subsistence lifestyles, the largest manpower user, and the main producing and exporting sector (Baethgen, 1994; IPCC 1996, WG II, Section 13.7).
Because agricultural production in lower-latitude and lower-income countries is more likely to be negatively affected by climate change (IPCC 1996, WG II , Section 13.8), large Latin American communities would be vulnerable to global warming. Climatic variations that result in shorter rainy seasons and/or increased frequency of rainless years would have extremely negative consequences for the region. However, yield impacts also depend on other factors, such as cultivars and specific environmental conditions.
In Mexico, any shift toward warmer, drier conditions could bring nutritional and economic disaster because agriculture already is stressed by low and variable rainfall. More than one-third of the Mexican population works in agriculture-a sector whose prosperity is critical to the nation's debt-burdened economy. Although only one-fifth of Mexico's cropland is irrigated, this area accounts for half of the value of the country's agricultural production, including many export crops (Liverman and O'Brien, 1991). The principal conclusion of Liverman and O'Brien's work is that Mexico is likely to be warmer and drier. According to each of the models used, potential evaporation is likely to increase; in most cases, moisture availability will decrease, even where the models project an increase in precipitation. Sensitivity analyses of evaporation and moisture-deficit calculations indicate that water availability would be expected to increase only if the model results produced much higher rainfall and relative humidities or significantly less solar radiation and wind. Such changes are possible, of course, as the modeling of clouds and synoptic conditions improves; at present, however, it seems that a moisture decrease is more likely.
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