Agricultural lands occupy 37% of the Earth’s land surface. Agriculture accounts for 52% and 84% of global anthropogenic methane and nitrous oxide emissions. Agricultural soils may also act as a sink or source for carbon dioxide (CO2), but the net flux is small. Many agricultural practices can potentially mitigate greenhouse gas (GHG) emissions, the most prominent of which are improved cropland and grazing land management and restoration of degraded lands and cultivated organic soils. Lower, but still significant mitigation potential is provided by water and rice management, set-aside, land use change and agroforestry, livestock management and manure management. The global technical mitigation potential from agriculture (excluding fossil fuel offsets from biomass) by 2030, considering all gases, is estimated to be ~5500-6000 megatonnes (Mt) CO2-eq. yr-1, with economic potentials of ~1500-1600, 2500-2700, and 4000-4300 Mt CO2-eq. yr-1 at carbon prices of up to 20, up to 50 and up to 100 US$ t CO2-eq.-1, respectively. In addition, GHG emissions could be reduced by substitution of fossil fuels for energy production by agricultural feed stocks (e.g. crop residues, dung, dedicated energy crops). The economic mitigation potential of biomass energy from agriculture is estimated to be 70-1260 Mt CO2-eq. yr-1 at up to 20 USD t CO2-eq.-1, and 560-2320 Mt CO2-eq. yr-1 at up to 50 USD t CO2-eq. There are no estimates for the additional potential from top down models at carbon prices up to 100 USD t CO2-eq.-1, but the estimate for prices above 100 USD t CO2-eq.-1 is 2720 Mt CO2-eq. yr-1. These potentials represent mitigation of 5-80%, and 20-90% of all other agricultural mitigation measures combined, at carbon prices of up to 20, and up to50 USD t CO2-eq.-1, respectively.
Status of agriculture
Technological developments have allowed remarkable progress in agricultural output per unit of land, increasing per-capita food availability despite a consistent decline in per-capita agricultural land area. However, progress has been uneven across the world with rural poverty and malnutrition remaining in some countries. The share of animal products in the diet has progressively increased in developing countries, whilst remaining constant in the developed world.
Production of food and fiber has more than kept pace with the sharp increase in demand in a more populated world, so that the global average daily availability of calories per-capita has increased, though with regional exceptions. However, this growth has been at the expense of increasing pressure on the environment and dwindling natural resources, and has not solved problems of food security and widespread child malnutrition in poor countries.
The absolute area of global arable land has grown to about 1400 million hectares (Mha), an overall increase of 8% since the 1960s (5% decrease in developed countries, and 22% increase in developing countries). This trend is expected to continue into the future, with a projected additional 500 Mha converted to agriculture from 1997-2020, mostly in Latin America and Sub-Saharan Africa.
Economic growth and changing lifestyles in some developing countries are causing a growing demand for meat and dairy products. From 1967-1997, meat demand in developing countries rose from 11 to 24 kg per capita per year, achieving an annual growth rate of more than 5% by the end of that period. Further increases in global meat demand (about 60% by 2020) are projected, mostly in developing regions such as South and Southeast Asia, and Sub-Saharan Africa.
Agriculture accounts for an estimated emission of 5.1 to 6.1 gigatonnes (Gt) carbon dioxide (CO2)eq/yr in 2005 (10-12 % of total global anthropogenic emissions of GHGs). Methane (CH4) contributes 3.3 Gt CO2 eq/yr and nitrous oxide (N2O) 2.8 Gt CO2eq /yr. Of global anthropogenic emissions in 2005, agriculture accounts for about 60% of N2O and about 50% of CH4. Despite large annual exchanges of CO2 between the atmosphere and agricultural lands, the net flux is estimated to be approximately balanced, with CO2 emissions around 0.04 Gt CO2/yr only (emissions from electricity and fuel use are covered in the buildings and transport sector respectively).
Trends in GHG emissions in agriculture are responsive to global changes: increases are expected as diets change and population growth increases food demand. Future climate change may eventually release more soil carbon (though the effect is uncertain as climate change may also increase soil carbon inputs through high production). Emerging technologies may permit reductions of emissions per unit of food produced, but absolute emissions are likely to grow.
Without additional policies, agricultural N2O and CH4 emissions are projected to increase by 35-60% and ~60%, respectively, up to 2030, thus increasing more rapidly than the 14% increase on non-CO2 GHG observed from 1990 to 2005.
Both the magnitude of the emissions and the relative importance of the different sources vary widely among world regions (Figure 1). In 2005, the group of five regions mostly consisting of non-Annex I countries were responsible for 74% of total agricultural emissions.
Mitigation technologies, practices, options, potentials and costs
|Table 1. Estimates of the global agricultural economic GHG mitigation potential (Mt CO2-eq yr-1) by 2030 under different assumed prices of CO2-equivalents for a SRES B2 baseline.|
|Price of CO2-eq. (USD t CO2-eq.-1)|
|Up to 20||Up to 50||Up to 100|
|OECD||330 (60-470)||540 (300-780)||870 (460-1280)|
|EIT||160 (30-240)||270 (150-390)||440 (230-640)|
|Non-OECD / EIT||1140 (210-1660)||1880 (1040-2740)||3050 (1610-4480)|
|Note: figures in brackets show 1 standard deviation around the mean estimate, potential excluding energy efficiency measures and fossil fuel offsets from bio-energy.|
Considering all gases, economic potentials for agricultural mitigation by 2030 are estimated to be about 1600, 2700 and 4300 Mt CO2-eq. yr-1 at carbon prices of up to 20, 50 and 100 US$ t CO2-eq.-1, respectively for a Intergovernmental Panel on Climate Change (IPCC) Special Report on Emissions Scenarios (SRES) B2 baseline (see Table 1).
Improved agricultural management can reduce net GHG emissions, often affecting more than one GHG. The effectiveness of these practices depends on factors such as climate, soil type, and farming system.
About 90% of the total mitigation arises from sink enhancement (soil carbon sequestration) and about 10% arises from emission reduction (medium agreement/medium evidence. The most prominent mitigation options in agriculture (with potentials shown in Mt CO2-eq. yr-1 for carbon prices up to 100 USD t CO2-eq.-1 by 2030) are (see also Figure 2):
- restoration of cultivated organic soils (1260),
- improved cropland management (including agronomy, nutrient management, tillage/residue management and water management (including irrigation and drainage) and set-aside/agroforestry: (1110),
- improved grazing land management (including grazing intensity, increased productivity, nutrient management, fire management and species introduction: (810), and
- restoration of degraded lands (using erosion control, organic amendments and nutrient amendments: (690).
Lower, but still substantial mitigation potential is provided by:
- rice management: (210), and
- livestock management (including improved feeding practices, dietary additives, breeding and other structural changes, and improved manure management [improved storage and handling and anaerobic digestion]: (260).
In addition, 770 Mt CO2-eq. yr-1 could be provided by 2030 by improved energy efficiency in agriculture. This amount is, however, for a large part included in the mitigation potential of buildings and transport.
At lower carbon prices, low-cost measures most similar to current practice are favored (e.g., cropland management options), but at higher carbon prices, more expensive measured with higher per-area mitigation potentials are favored (e.g., restoration of cultivated organic / peaty soils.
GHG emissions could also be reduced by substitution of fossil fuels with energy production from agricultural feed stocks (e.g. crop residues, dung, energy crops), which are counted in energy end-use sectors (particularly energy supply and transport). There are no accurate estimates of future agricultural biomass supply, with figures ranging from 22 exajoules (EJ)/yr in 2025 to above 400 EJ/yr in 2050. The actual contribution of agriculture to the mitigation potential by using bioenergy depends, however, on relative prices of fuels and the balance of demand and supply. Top-down assessments, that include assumptions on such a balance, estimate the economic mitigation potential of biomass energy supplied from agriculture to be 70-1260 Mt CO2-eq. yr-1 at up to 20 USD t CO2-eq.-1, and 560-2320 Mt CO2-eq. yr-1 at up to 50 USD t CO2-eq. There are no estimates for the additional potential from top-down models at carbon prices up to 100 USD t CO2-eq.-1, but the estimate for prices above 100 USD t CO2-eq.-1 is 2720 Mt CO2-eq. yr-1. These potentials represent mitigation of 5-80%, and 20-90% of all other agricultural mitigation measures combined, at carbon prices of up to 20, and up to 50 USD t CO2-eq.-1, respectively. Above the level where agricultural products and residues form the sole feedstock, bioenergy competes with other land-uses for available land, water and other resources. The mitigation potentials of bio-energy and improved energy efficiency are counted in the user sectors, predominantly transport and buildings, respectively.
Interactions of mitigation options with vulnerability and adaptation
Agricultural actions to mitigate GHGs could (a) reduce vulnerability (e.g., if soil carbon sequestration reduces the impact of drought) or (b) increase vulnerability (e.g., if heavy dependence on biomass energy makes the energy supply more sensitive to climatic extremes). Policies to encourage mitigation and/or adaptation in agriculture may need to consider these interactions. Similarly, adaptation-driven actions may either (a) favor mitigation (e.g., residue return to fields to improve water holding capacity will also sequester carbon) or (b) hamper mitigation (e.g., use of more nitrogen fertilizer to overcome falling yields, leading to increased nitrous oxide (N2O) emissions). Strategies that simultaneously increase adaptive capacity, reduce vulnerability and mitigate climate change are likely to present fewer adoption barriers than those with conflicting impacts. For example increasing soil organic matter content can both improve fertility and reduce the impact of drought, improving adaptive capacity, making agriculture less vulnerable to climate change whilst also sequestering carbon.
Effectiveness of climate policies: opportunities, barriers and implementation issues
Actual levels of GHG mitigation practices in the agricultural sector are below the economic potential for the measures as reported above. Little progress in implementation has been made because of costs of implementation and other barriers, including: pressure for agricultural land, demand for agricultural products, competing demands for water as well as various social, institutional and educational barriers. Soil carbon sequestration in European croplands, for instance, is likely to be negligible by 2010, despite significant economic potential. Many of these barriers will not be overcome without policy/economic incentives.
Integrated and non-climate policies affecting emissions of greenhouse gases
The adoption of mitigation practices will often be driven largely by goals not directly related to climate change. This leads to varying mitigation responses among regions, and contributes to uncertainty in estimates of future global mitigation potential. Policies most effective at reducing emissions may be those that also achieve other societal goals. Some rural development policies undertaken to fight poverty, such as water management and agroforestry, are synergistic with mitigation. For example, agroforestry undertaken to produce fuel wood or to buffer farm income against climate variation, may also increase carbon sequestration. In many regions, agricultural mitigation options are influenced most by non-climate policies, including macro-economic, agricultural, and environmental policies. Such policies may be based on United Nations (UN) conventions (e.g., Biodiversity and Desertification) but are often driven by national or regional issues. Among the most beneficial non-climate policies are those that promote sustainable use of soils, water and other resources in agriculture since these help to increase soil carbon stocks and minimize resource (energy, fertilizer) waste.
Co-benefits of greenhouse gas mitigation policies
Some agricultural practices yield purely ‘win-win’ outcomes but most involve trade-offs. Agro-ecosystems are inherently complex. The co-benefits and trade-offs of an agricultural practice may vary from place to place because of differences in climate, soil, or the way the practice is adopted.
In producing bioenergy, for example, if the feedstock is crop residue, soil organic matter may be depleted as less carbon is returned, thus reducing soil quality; conversely, if the feedstock is a densely-rooted perennial crop, soil organic matter may be replenished, thereby improving soil quality.
Many agricultural mitigation activities show synergy with the goals of sustainability. Mitigation policies that encourage efficient use of fertilizers, maintain soil carbon, and sustain agricultural production are likely to have the greatest synergy with sustainable development.
For example, increasing soil carbon can also improve food security and economic returns. Other mitigation options have more uncertain impact on sustainable development, e.g., the use of some organic amendments may improve carbon sequestration, but impacts on water quality may vary depending on the amendment. Co-benefits often arise from improved efficiency, reduced cost and environmental co-benefits. Trade-offs relate to competition for land, reduced agricultural productivity, and environmental stresses.
Technology research, development, deployment diffusion and transfer
Many of the mitigation strategies outlined for the agriculture sector employ existing technology. For example, reduction of emissions per unit of production will be achieved by increases in crop yields and animal productivity. Such increases in productivity can occur through a wide range of practices—better management, genetically modified crops, improved cultivars, fertilizer recommendation systems, precision agriculture, improved animal breeds, improved animal nutrition, dietary additives and growth promoters, improved animal fertility, bioenergy feed stocks, anaerobic slurry digestion and methane capture systems—all of which reflect existing technology. Some strategies involve new uses of existing technologies. For example, oils have been used in animal diets for many years to increase dietary energy content, but their role and feasibility as a methane suppressant is still new and not fully defined. For some technologies, more research and development will be needed
Long term outlook
Global food demands may double by 2050, leading to intensified production practices (e.g., increasing use of nitrogen fertilizer). In addition, forecast increases in consumption of livestock products will increase methane (CH4) and nitrous oxide (N2O) emissions if livestock numbers increase, leading to growing emissions in the baseline after 2030. Agricultural mitigation measures will help to reduce GHG emissions per unit of product, relative to the baseline. However, until 2030 only about 10% of the mitigation potential is related to CH4 and N2O. Deployment of new mitigation practices for livestock systems and fertilizer applications will be essential to prevent an increase in emissions from agriculture after 2030.
Projecting long-term mitigation potentials is also hampered by other uncertainties. For example, the effects of climate change are unclear; yields and soil carbon sequestration could increase with elevated carbon dioxide (CO2) and warmer temperatures (at least in some regions), but the effects may be offset by accelerated soil carbon decomposition, changes in precipitation, and unpredictable adaptive responses. Some studies have suggested that technological improvements could potentially counteract the negative impacts of climate change on cropland and grassland soil carbon stocks, implicating technological improvement as a key factor in future GHG mitigation. Such technologies could, for example, act through increasing production, thereby increasing carbon returns to the soil and reducing the demand for fresh cropland.
- Smith, P., Martino, D., Cai, Z., Gwary, D., Janzen, H.H., Kumar, P., McCarl, B., Ogle, S., O’Mara, F., Rice, C., Scholes, R.J., Sirotenko, O., Howden, M., McAllister, T., Pan, G., Romanenkov, V., Schneider, U., Towprayoon, S., Wattenbach, M. & Smith, J.U. 2007. Greenhouse gas mitigation in agriculture. Philosophical Transactions of the Royal Society, B. (in press).
- Smith, P., Martino, D., Cai, Z., Gwary, D., Janzen, H.H., Kumar, P., McCarl, B., Ogle, S., O’Mara, F., Rice, C., Scholes, R.J., Sirotenko, O., Howden, M., McAllister, T., Pan, G., Romanenkov, V., Schneider, U. & Towprayoon, S. 2007. Policy and technological constraints to implementation of greenhouse gas mitigation options in agriculture. Agriculture, Ecosystems & Environment 118:6-28.