After nearly two decades of research on the potential impacts of climate change on agriculture, attention is now turning to the assessment of appropriate responses. A distinction can be made between two types of responses: mitigation and adaptation. Mitigation is action to check the rising atmospheric concentration of greenhouse gases, thereby moderating their effects. Adaptation refers to actions that reduce the negative effects or enhance the beneficial effects of climate changes that are already occurring or are projected to occur in the future. Adaptations may be either autonomous or planned (IPCC, 2001). Research on these two types of responses have proceeded on parallel tracks. Here we suggest that they be considered conjunctively.
The practice of agriculture plays a major role in the global carbon cycle (Figure 10.9) (Rosenzweig and Tubiello, 2004; Rosenzweig and Hillel, 2000). On a global scale, the process of photosynthesis by agricultural crops fixes about 2 Gt C year-1, with about 1 Gt C year1 providing sustenance for the world's population that is respired back to the atmosphere as it is consumed; about 1 Gt C is returned to the soil annually as plant residues. Some of the latter carbon, however, subsequently is returned to the atmosphere by soil microbial activity, and some is stored in the soil matrix. Furthermore, the fossil fuel that powers the machinery to sow, irrigate, harvest, and dry crops worldwide is responsible for atmospheric emissions of about 150 MT (million metric tons) C year-1. Large amounts of fossil fuel energy are used to produce fertilizers, especially nitrogenous compounds. Rice cultivation, livestock production, and soil processes are also responsible for considerable methane and nitrous oxide emissions (Rosenzweig and Hillel, 1998).
The agricultural carbon cycle offers several entry points for mitigation of greenhouse gas accumulation in the atmosphere. An important one is the potential for agricultural soils to store carbon, particularly to the extent that its "active" carbon stores had been depleted by past soil management practices (Rosenzweig and Hillel, 2000). Other ways that agriculture may help to mitigate the enhanced greenhouse effect
are through the production of biofuels, the development of more efficient rice and livestock production systems, and the reduction of fossil fuel use by farm machinery.
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