Global climate changes caused by increased greenhouse gas emissions to the atmosphere from anthropogenic activities have direct influence on natural and agrosystem functioning (Lal, 2002). Modifications in hydrologic regimes and atmosphere temperature due to anthropogenic greenhouse effects provoke variations in plant productivity, and therefore affect food production (Intergovernmental Panel on Climate Change [IPCC], 1998).

Crop simulation models driven by future climate scenarios from global circulation models suggest that reduction in agricultural production would be more severe in tropical regions (IPCC, 2001), where there is still a shortage of food production. Brazil, located almost entirely in the tropical zone, is not an exception to this rule, and therefore is susceptible to reductions in agricultural and livestock production. Moreover, agriculture comprises the largest single sector of the Brazilian economy, representing 29% of gross domestic product in 2002, and about 47.5% of export revenue in 2003. Consequently, understanding the possible impacts of climate change on Brazilian agriculture is crucial to government decision makers.

Research about the impact of climate change on Brazilian agriculture is scanty, and has focused on grain production (Siqueira et al., 1994, 2001). Simulations of grain production are usually done by coupling a crop growth model with a climate change scenario and projected increases in CO2 from a future emission scenario, using historical climate data and current CO2 levels as a base scenario. Siqueira et al. (1994, 2001) presented results of wheat (Triticum vulgare Vill), maize (Zea mays L. ), and soybean (Glycine max L. Merr) production simulations with the crop growth model CERES and SOYGRO for 13 situations under climate change scenarios generated by the Goddard Institute for Space Studies, Geophysical Fluid Dynamic Laboratory, and United Kingdom Meteorological Office general circulation models (GCMs) run with 330 and 555 ppm CO2.

Siqueira et al. (2001) reported that simulations show an increase in the mean air temperature between 3°C to 5°C and an increase of about 11% in the mean precipitation for the Center-South region throughout the year 2050. This scenario would cause a reduction of 30% and 16% of wheat and corn production, respectively; and an increase of about 21% in soybean production. These figures correspond to a reduction of 1 million metric tons of wheat and 2.8 million metric tons of corn and an increase of 3.5 metric tons of soybeans. The major problems resultant from additional rainfall are related to higher probability of disease incidence, greater difficulty in cultivation management, and higher risks of soil water erosion (Siqueira et al., 1994).

Most studies concerning the impact of climate change on food security deal with grain production only. But beef production, with a herd of 175 million cattle in 2001, represents a large component of Brazilian agriculture. About 30% of the total is in the Amazon region where pasturage is typically extensively managed on low-fertility soils. The sustainability of these fragile ranching systems can be evaluated through soil organic matter (SOM) status. A changing climate can induce losses of SOM that upset the input-output nutrient balance and provoke losses in plant grass productivity, and subsequently sustainability of the overall system.

The main objective of this chapter is to simulate changes induced by potential climate change on SOM stocks in extensive pasturage of the Brazilian Amazon region.

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