80 percent of these CO2 emissions are estimated to come from conversion of land to agriculture, making this “indirect� source of agricultural greenhouse gases about as important as the direct effects discussed above. South America and Asia are each responsible for about 40 percent of land conversion, with Africa accounting for most of the rest (Smith et al 2007). In addition, agriculture makes several other indirect contributions to greenhouse gas emissions. For example, CO2 is emitted in the manufacture of fertilizer, pesticides, and machinery, while fuel is burned in transport of farm products, farm inputs and in the use of farm machinery, also emitting CO2. Taking into account direct activities of agriculture and the clearing of natural vegetation for agricultural land use, all world agriculture in total accounts for about one-third of world greenhouse gas emissions.
1.3 climate change trends Scientists have developed global climate models to project likely future paths of climate change. These models accurately replicate observed recent climate change, but their projections of future climate lead to a wide range of results due, in large part, to the complexity of the factors affecting climate. Scenarios of future climate are also heavily dependent on greenhouse gas emissions that in turn depend on human decisions, making climate projections uncertain.
Warming drives other climate changes Despite their limitations, global climate models consistently project a future of global warming (IPCC 2007a). The consensus of scientists is that global warming is not reversible in the next decades. Due to the lifespan of CO2 in the atmosphere and heat stored in the oceans, temperatures will only slowly decrease even after CO2 emissions stabilize or begin to fall. Higher temperatures drive other changes such as increasing the rate of evaporation and moisture-holding capacity of a warmer atmosphere, leading to higher total global precipitation. Rainfall variability and intensity is expected to increase but, due to complex changes in atmospheric and oceanic circulation systems, changes in rainfall will not be the same everywhere in all seasons. While some regions will be wetter, others, especially the tropics, will experience more prolonged and frequent droughts that are more severe at higher temperatures.
CLIMATE CHANGE: FROM CONCEPTS TO ACTION
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