Page images
PDF
EPUB
[blocks in formation]

6. AGRICULTURE SECTOR17

Agriculture accounts for about one-fifth of the projected anthropogenic greenhouse effect, producing about 50 and 70%, respectively, of overall anthropogenic CH, 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 1700 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 NO and CH, 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.

[blocks in formation]

Technologies for mitigation of GHGs in agriculture and the potential decreases in emissions of CO2, CH, and N2O are shown in Table 12. Also shown in Table 12 are the equivalent carbon emission reductions for CH, 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 CH, emissions and 10% from reduced emissions of NO.

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 CH, 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 programmes can improve the knowledge and skills of the work force and managers to help advance adoption. Comprehensive national and international programmes of research, education and technology transfer will be required to develop and diffuse knowledge of improved technologies. Crop insurance or other programmes to share the risk of failure due to natural disaster are needed to aid the adoption of improved practices.

[blocks in formation]

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

17This section is based on SAR II, Chapter 23, Agricultural Options for Mitigation of Greenhouse Gas Emissions (Lead Authors: V. Cole, C. Cerri, K. Minami, A. Mosier, N. Rosenberg, D. Sauerbeck, J. Dumanski, J. Duxbury, J. Freney, R. Gupta, O. Heinemeyer, T. Kolchugina, J. Lee, K. Paustian, D. Powlson, N. Sampson, H. Tiessen, M. van Noordwijk and Q. Zhao).

"Annex I countries' share of emission reductions is based on production data in the Food and Agriculture Organization (FAO) 1994 Production Yearbook, Vol. 48, FAO Statistics Series. Rome, Italy.

50

Technologies, Policies and Measures for Mitigating Climate Change

Table 12: Agricultural technologies for mitigation of GHG emissions and potential reductions of annual emissions of carbon dioxide, methane and nitrous oxide (based on SAR II. Tables 23-4, 23-5, 23-6, 23-10 and 23-11).

[blocks in formation]

-Increasing soil C through better management of existing agricultural soilsb

-Increasing soil C through permanent set-aside of surplus agricultural land in temperate regions
Restoration of soil C on degraded landsd

[blocks in formation]

Mt Clyr

10-50

400-600

21-42 24-240

[ocr errors][merged small][ocr errors][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

⚫ Based on current use of 3-4.5% of the total fossil C emission (28 Gt Clyr, OECD, 1991) by industrialized countries and an arbitrary reduction range of 10-50%.

*Assuming a recovery of one-half to two-thirds of the estimated historic loss (44 G) of C from currently cultivated soils (excluding wetland soils) over a 50-year period.

• Based on an estimated C sequestration of 15-3 Gt over a 100-year period, from a 15% set-aside of cultivated soils (~640 Mha), in industrialized countries with current or potential production surpluses; annual and cumulative rates given as I and 50%, respectively. Based on restoration of 10-20% of former wetland area (8 Mha) now under cultivation in temperate regions.

4 Assuming potential C sequestration of 1-2 kg C/m2 over a 50-year period, on an arbitrary 10-50% of moderately to highly degraded land (1.2x10 ha globally). • Assuming about 10-15% of world cultivated lands to be available for biofuels.

Based on 25% recovery of crop residues and assumptions on energy conversion and substitution.

C-equivalent of CH, emissions based on 100-year GWP (SAR L. Table 2.9).

⚫C-equivalent of N2O emissions based on 100-year GWP (SAR L, Table 2.9).

Technologies, Policies and Measures for Mitigating Climate Change

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, fibre 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 fibre 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 fibre crops, and social and political pressures. Dedicated energy plants, including short-rotation woody crops, perennial herbaceous energy crops, and annuals such as wholeplant cereal crops or kenaf, could be sustainably grown on 8-11% of the marginal to good cropland in the temperate zone.

[ocr errors][subsumed]

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 CO, mitigation potential of a large-scale global agricultural biofuel programme 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.

[blocks in formation]

The largest agricultural sources of CH, 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 CH, 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 labour, water and other production costs; and (ii) rice cultivars that produce lower CH, emissions are acceptable to local consumers.

Emissions of CH, from domestic ruminant animals can be reduced as producers use improved grazing systems with higher quality forage, since animals grazing on poor-quality rangelands

52

Technologies. Policies and Measures for Mitigating Climate Change

Table 13: Selected examples of technical options to mitigate GHG emissions in the agricultural sector.

[blocks in formation]

Increase N Fertilizer
Use Efficiency

- Better application methods
- Match N supply with
crop needs

-Maximize manure use - Optimize tillage, imiga

Programmes

- Agricultural fuel taxes

Voluntary Agreements -Technology transfer

Voluntary Agreements - Change commodity programmes to allow more flexibility and support of best management practices -Technology transfer

Market-based
Programmes

- Energy pricing

- Removal of market barriers

Regulatory Measures Regulation of animal density

Voluntary Agreements -Technology transfer

Voluntary Agreements - Technology transfer

[blocks in formation]
[blocks in formation]

Technologies, Policies and Measures for Mitigating Climate Change

produce more CH, 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. CH, produced in animal waste disposal systems can provide an on-farm energy supply, and the CH, utilized in this manner is not emitted to the atmosphere. Overall, potential global reduction of CH, emissions amounts to about 35% (15–56%) of emissions from agriculture.

[blocks in formation]

Nitrogen is an essential plant nutrient; however, it is also a component of some of the most mobile compounds in the soil-plantatmosphere 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 NO 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 NO 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

53

[blocks in formation]
« PreviousContinue »