Agriculture accounts for about one-fifth of the projected anthropogenic greenhouse effect, producing about 50 and 70%, respectively, of overall anthropogenic CH4 and N2O emissions; agricultural activities (not including forest conversion) account for approximately 5% of anthropogenic emissions of CO2 (SAR II, Figure 23.1). Total global land under cultivation is estimated to be approximately 1,700 Mha (SAR II, 23.2.2, Table 23-3).
The agriculture sector is characterized by large regional differences in both management practices and the rate at which it would be possible to implement mitigation measures. The effectiveness of various mitigation measures needs to be gauged against the base emission levels and changes in different regions. In non-Annex I countries where rapid increases in fertilizer use and crop production are occurring, substantial increases in emissions of N2O and CH4 are projected. Even full implementation of mitigation measures will not balance these increases. Comprehensive analyses of land use, cropping systems, and management practices are needed at regional and global levels to evaluate changes in emissions and mitigation requirements.
6.2 Technologies for Reducing GHG Emissions in the Agriculture Sector
Technologies for mitigation of GHGs in agriculture and the potential decreases in emissions of CO2, CH4, and N2O are shown in Table 12. Also shown in Table 12 are the equivalent carbon emission reductions for CH4 and N2O based on their respective ratios of global warming potential (SAR I, Table 2.9). Of the total possible reduction in radiation forcing (shown as C-equivalents), approximately 32% could result from reduction in CO2 emissions, 42% from carbon offsets by biofuel production on existing croplands, 16% from reduced CH4 emissions, and 10% from reduced emissions of N2O.
Emissions reductions by the Annex I countries could make a significant contribution to the global total. Of the total potential CO2 mitigation, Annex I countries could contribute 40% of the reduction in CO2 emissions, and 32% of the carbon offset from biofuel production on croplands. Of the global total reduction in CH4 emissions, Annex I countries could contribute 5% of the reduction attributed to improved technologies for rice production, and 21% of reductions attributed to improved management of ruminant animals. These countries also could contribute about 30% of the reductions in N2O emissions attributed to reduced and more efficient use of nitrogen fertilizer, and 21% of the reductions stemming from improved utilization of animal manures. 18
Estimates of potential reductions range widely, reflecting uncertainty in the effectiveness of recommended technologies and the degree of future implementation globally. To satisfy global food requirements and acceptability by farmers, technologies and practices should meet the following general guidelines: (i) Sustainable agricultural production will be achieved or enhanced; (ii) additional benefits will accrue to the farmer; and (iii) agricultural products will be accepted by consumers. Farmers have no incentive to adopt GHG mitigation techniques unless they improve profitability. Some technologies, such as no-till agriculture or strategic fertilizer placement and timing, already are being adopted for reasons other than concern for climate change. Options for reducing emissions, such as improved farm management and increased efficiency of nitrogen fertilizer use, will maintain or increase agricultural production with positive environmental effects.
These multiple benefits will result in high cost-effectiveness of available technologies. Practices that recover investment cost and generate a profit in the short term are preferred over practices that require a long term to recover investment costs; practices that have a high probability associated with expected profits are desired over practices that have less certainty about their returns. When human resource constraints or knowledge of the practice prevent adoption, public education programs can improve the knowledge and skills of the work force and managers to help advance adoption. Comprehensive national and international programs of research, education, and technology transfer will be required to develop and diffuse knowledge of improved technologies. Crop insurance or other programs to share the risk of failure due to natural disaster are needed to aid the adoption of improved practices.
6.2.1 Mitigation of Carbon Dioxide Emissions (SAR II, 23.2)
Options to mitigate CO2 emissions from agriculture include reducing emissions from present sources, and creating and strengthening carbon sinks. Options for increasing the role of agricultural land as a sink for CO2 include carbon storage in managed soils and carbon sequestration after reversion of surplus farm lands to natural ecosystems. However, soil carbon sequestration has a finite capacity over a period of 50-100 years, as new equilibrium levels of soil organic matter are established. Efforts to increase soil carbon levels have additional benefits in terms of improving the productivity and sustainability of agricultural production systems. Soils of croplands taken out of production in permanent set-asides and allowed to revert to native vegetation eventually could reach carbon levels comparable to their precultivation condition. Considering the 640 Mha of land currently under cultivation in the United States, Canada, the former Soviet Union, Europe, Australia, and Argentina, and assuming recovery of the soil carbon originally lost to cultivation, a permanent set-aside of 15% of the land area could sequester 1.5-3 Gt C (over 50-100 years).
A large-scale reversion or afforestation of agricultural land is only possible if adequate supplies of food, fiber, and energy can be obtained from the remaining area. This is currently possible in the European Union and United States through intensive farming systems. However, if farming intensity changes because of environmental concerns or changes in policy, this mitigation option may no longer be available.
Currently, only half of the conversion of tropical forests to agriculture contributes to an increase in productive cropland. The only way to break out of this cycle is through more sustainable use, improved productivity of existing farmland, and better protection of native ecosystems. These practices could help reduce agricultural expansion (hence deforestation) in humid zones, especially in Latin America and Africa.
Management practices to increase soil carbon stocks include reduced tillage, crop residue return, perennial crops (including agroforestry), and reduced bare fallow frequency. However, there are economic, educational, and sociological constraints to improved soil management in much of the tropics. Many tropical farmers cannot afford or have limited access to purchased inputs such as fertilizer and herbicides. Crop residues are often needed for livestock feed, fuel, or other household uses, which reduces carbon inputs to soil. To the extent that improved management is based on significantly increased fossil fuel consumption, benefits for CO2 mitigation will be decreased.
Energy use by agriculture, per unit of farm production, has decreased since the 1970s. Fossil fuel use by agriculture in industrialized Annex I countries, constituting 3-4% of overall consumption, can be reduced through the use of minimum tillage, irrigation scheduling, solar drying of crops, and improved fertilizer management.
Both conventional food and fiber crops and dedicated biofuel crops, such as short-rotation woody crops and perennial herbaceous energy crops, produce biomass that is valuable as a feedstock for energy supply. Dedicated biofuel crops require similar soils and management practices as conventional agricultural crops, and would compete with food production for limited resources (SAR II, 23.2.4). The extent to which their production will be expanded depends on the development of new technologies, their economic competitiveness with traditional food and fiber crops, and social and political pressures. Dedicated energy plants, including short-rotation woody crops, perennial herbaceous energy crops, and annuals such as whole-plant cereal crops or kenaf, could be sustainably grown on 8-11% of the marginal to good cropland in the temperate zone. For example, in the European Union it has been estimated that 15-20 Mha of good agricultural land will be surplus to food production needs by the year 2010. This would be equivalent to 20-30% of the current cropland area.
Due to increasing agricultural demand in the tropics, a lower percentage of land is likely to be dedicated to energy crops, so a reasonable estimate may be 5-7%. In total, however, there could be a significant amount of land available for biofuel production, especially from marginal land and land in need of rehabilitation. The CO2 mitigation potential of a large-scale global agricultural biofuel program could be significant. Assuming that 10-15% of the world's cropland area could be made available, fossil fuel substitutions in the range of 300-1300 Mt C have been estimated. This does not include the indirect effects of biofuel production through increasing carbon storage in standing woody biomass or through increasing soil carbon sequestration. Recovery and conversion of 25% of total crop residues (leaving 75% for return to the soil) could substitute for an additional 100-200 Mt fossil fuel C/yr. However, the possible offsets by increased N2O emissions need to be considered. Generally, crops from which only the oil, starch, or sugar are used are of limited value in reducing CO2 emissions, due to the low net energy produced and the relatively high fossil fuel inputs required. The burning of whole plant biomass as an alternative to fossil fuel results in the most significant CO2 mitigation.
Ranges in estimates of potential mitigation reflect uncertainty about the effectiveness of management options and about the degree of future implementation globally. A primary issue in evaluating these options is whether the world can continue to support an increasing population with its growing needs for food and fiber and, at the same time, expand the amount of land used for production of biomass for energy (SAR II, 23.2.5, 25.3.3).
6.2.2 Mitigation of Methane Emissions (SAR II, 188.8.131.52)
The largest agricultural sources of CH4 are managed ruminant animals and rice production. Rice cultivation will continue to increase at its current rate to meet food requirements. Flooded rice fields produce CH4 emissions, which can be reduced by improved management measures. The ranges of potential reductions shown indicate uncertainty about the effectiveness of mitigation measures and the degree of additivity of effects as, for example, in rice production. Successful implementation of available mitigation technologies will depend on demonstration that (i) grain yield will not decrease or may increase; (ii) there will be savings in labor, water, and other production costs; and (iii) rice cultivars that produce lower CH4 emissions are acceptable to local consumers.
Emissions of CH4 from domestic ruminant animals can be reduced as producers use improved grazing systems with higher quality forage, since animals grazing on poor quality rangelands produce more CH4 per unit of feed consumed. Confined feeding operations utilizing balanced rations that properly manage digestion of high energy feeds also can reduce direct emissions, but can increase indirect emissions from feed production and transportation. CH4 produced in animal waste disposal systems can provide an on-farm energy supply, and the CH4 utilized in this manner is not emitted to the atmosphere. Overall, potential global reduction of CH4 emissions amounts to about 35% (15-56%) of emissions from agriculture.
6.2.3 Mitigation of Nitrous Oxide Emissions (SAR II, 184.108.40.206)
Nitrogen is an essential plant nutrient; however, it is also a component of some of the most mobile compounds in the soil-plant-atmosphere system. Since nitrogen is the major component of mineral fertilizer, there is mounting concern over the extent to which high-input agriculture loads nitrogen compounds into the environment. Nitrogen budgeting, or an input/output balance approach, provides a basis for policies to improve nitrogen management in farming and livestock systems, and for mitigating its environmental impact. Management systems can decrease the amount of nitrogen lost to the environment through gaseous losses of ammonia or N2O, or through leaching of nitrate into the subsoil. In some cases, improved efficiency is achieved by using less fertilizer; in other cases, it can be achieved by increasing yields at the same nitrogen levels.
The primary sources of N2O from agriculture are mineral fertilizers, legume cropping, and animal waste. These losses often are accelerated by poor soil physical conditions. Some N2O also is emitted from biomass burning. Improvements in farm technology, such as use of controlled-release fertilizers, nitrification inhibitors, the timing of nitrogen application, and water management should lead to improvements in nitrogen use efficiency and further limit N2O formation. The underlying concept in reducing N2O emissions is that if fertilizer nitrogen (including manure nitrogen) is better used by the crop, less N2O will be produced and less nitrogen will leak from the system. By better matching nitrogen supply to crop demand and more closely integrating animal waste and crop residue management with crop production, N2O emissions could be decreased by about 0.36 Mt N2O-N or about 17% (9-26%) of the current emission rate in agriculture.
6.3 Measures for Reducing GHG Emissions in the Agriculture Sector
Measures that can have significant effects on the mitigation of GHGs in the agriculture sector include the following (see Table 13 for sample technical options):
The primary objectives of many of these measures are usually not related solely to climate change issues, but rather to such aims as reducing environmental pollution and natural resource degradation. Governments could promote more efficient fertilizer use by changing commodity programs to allow more flexibility and to encourage farmers to grow crops and adopt practices that rely less on commercial fertilizers. Support and encouragement of the best management practices to reduce soil degradation and environmental pollution would be consistent with mitigation measures for reduction of GHGs.
Measures to encourage improved land-use practices can increase carbon
storage. These could include permanent set-aside provisions for marginal and
degraded lands. Incentives could be provided for managing existing croplands
in a sustainable and environmentally sound manner. Government programs can
support the development of practices that maintain or increase crop yields and
reduce emissions per unit of crop yield.
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