IPCC Fourth Assessment Report: Climate Change 2007
Climate Change 2007: Working Group III: Mitigation of Climate Change


Agricultural lands (lands used for agricultural production, consisting of cropland, managed grassland and permanent crops including agro-forestry and bio-energy crops) occupy about 40-50% of the Earth’s land surface.

Agriculture accounted for an estimated emission of 5.1 to 6.1 GtCO2-eq/yr in 2005 (10-12% of total global anthropogenic emissions of greenhouse gases (GHGs)). CH4 contributes 3.3 GtCO2-eq/yr and N2O 2.8 GtCO2-eq/yr. Of global anthropogenic emissions Errata in 2005, agriculture accounts for about 60% of N2O and about 50% of CH4 (medium agreement, medium evidence). 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 GtCO2/yr only (emissions from electricity and fuel use are covered in the buildings and transport sector, respectively) (low agreement, limited evidence).

Globally, agricultural CH4 and N2O emissions have increased by nearly 17% from 1990 to 2005, an average annual emission increase of about 60 MtCO2-eq/yr. During that period, the five regions composed of Non-Annex I countries showed a 32% increase, and were, by 2005, responsible for about three-quarters of total agricultural emissions. The other five regions, mostly Annex I countries, collectively showed a decrease of 12% in the emissions of these gases (high agreement, much evidence).

A variety of options exists for mitigation of GHG emissions in agriculture. The most prominent options are improved crop and grazing land management (e.g., improved agronomic practices, nutrient use, tillage, and residue management), restoration of organic soils that are drained for crop production and restoration of degraded lands. Lower but still significant mitigation is possible with improved water and rice management; set-asides, land use change (e.g., conversion of cropland to grassland) and agro-forestry; as well as improved livestock and manure management. Many mitigation opportunities use current technologies and can be implemented immediately, but technological development will be a key driver ensuring the efficacy of additional mitigation measures in the future (high agreement, much evidence).

Agricultural GHG mitigation options are found to be cost competitive with non-agricultural options (e.g., energy, transportation, forestry) in achieving long-term (i.e., 2100) climate objectives. Global long-term modelling suggests that non-CO2 crop and livestock abatement options could cost-effectively contribute 270–1520 MtCO2-eq/yr globally in 2030 with carbon prices up to 20 US$/tCO2-eq and 640–1870 MtCO2-eq/yr with C prices up to 50 US$/tCO2-eq Soil carbon management options are not currently considered in long-term modelling (medium agreement, limited evidence).

Considering all gases, the global technical mitigation potential from agriculture (excluding fossil fuel offsets from biomass) by 2030 is estimated to be ~5500-6,000 MtCO2-eq/yr (medium agreement, medium evidence). Economic potentials are estimated to be 1500-1600, 2500-2700, and 4000-4300 MtCO2-eq/yr at carbon prices of up to 20, 50 and 100 US$/tCO2-eq, respectively About 70% of the potential lies in non-OECD/EIT countries, 20% in OECD countries and 10% for EIT countries (medium agreement, limited evidence).

Soil carbon sequestration (enhanced sinks) is the mechanism responsible for most of the mitigation potential (high agreement, much evidence), with an estimated 89% contribution to the technical potantial. Mitigation of CH4 emissions and N2O emissions from soils account for 9% and 2%, respectively, of the total mitigation potential (medium agreement, medium evidence). The upper and lower limits about the estimates are largely determined by uncertainty in the per-area estimate for each mitigation measure. Overall, principal sources of uncertainties inherent in these mitigation potentials include: a) future level of adoption of mitigation measures (as influenced by barriers to adoption); b) effectiveness of adopted measures in enhancing carbon sinks or reducing N2O and CH4 emissions (particularly in tropical areas; reflected in the upper and lower bounds given above); and c) persistence of mitigation, as influenced by future climatic trends, economic conditions, and social behaviour (medium agreement, limited evidence).

The role of alternative strategies changes across the range of prices for carbon. At low prices, dominant strategies are those consistent with existing production such as changes in tillage, fertilizer application, livestock diet formulation, and manure management. Higher prices elicit land-use changes that displace existing production, such as biofuels, and allow for use of costly animal feed-based mitigation options. A practice effective in reducing emissions at one site may be less effective or even counterproductive elsewhere. Consequently, there is no universally applicable list of mitigation practices; practices need to be evaluated for individual agricultural systems based on climate, edaphic, social setting, and historical patterns of land use and management (high agreement, much evidence).

GHG emissions could also be reduced by substituting fossil fuels with energy produced from agricultural feed stocks (e.g., crop residues, dung, energy crops), which would be counted in sectors using the energy. The contribution of agriculture to the mitigation potential by using bioenergy depends on relative prices of the fuels and the balance of supply and demand. Using top-down models that include assumptions on such a balance the economic mitigation potential for agriculture in 2030 is estimated to be 70-1260 MtCO2-eq/yr at up to 20 US$/tCO2-eq, and 560-2320 MtCO2-eq/yr at up to 50 US$/tCO2-eq There are no estimates for the additional potential from top down models at carbon prices up to 100 US$/tCO2-eq, but the estimate for prices above 100 US$/tCO2-eq is 2720 MtCO2-eq/yr. 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 US$/tCO2-eq, respectively. An additional mitigation of 770 MtCO2-eq/yr could be achieved by 2030 by improved energy efficiency in agriculture, though the mitigation potential is counted mainly in the buildings and transport sectors (medium agreement, medium evidence).

Agricultural mitigation measures often have synergy with sustainable development policies, and many explicitly influence social, economic, and environmental aspects of sustainability. Many options also have co-benefits (improved efficiency, reduced cost, environmental co-benefits) as well as trade-offs (e.g., increasing other forms of pollution), and balancing these effects will be necessary for successful implementation (high agreement, much evidence).

There are interactions between mitigation and adaptation in the agricultural sector, which may occur simultaneously, but differ in their spatial and geographic characteristics. The main climate change benefits of mitigation actions will emerge over decades, but there may also be short-term benefits if the drivers achieve other policy objectives. Conversely, actions to enhance adaptation to climate change impacts will have consequences in the short and long term. Most mitigation measures are likely robust to future climate change (e.g., nutrient management), but a subset will likely be vulnerable (e.g., irrigation in regions becoming more arid). It may be possible for a vulnerable practice to be modified as the climate changes and to maintain the efficacy of a mitigation measure (low agreement, limited evidence).

In many regions, non-climate policies related to macro-economics, agriculture and the environment, have a larger impact on agricultural mitigation than climate policies (high agreement, much evidence). Despite significant technical potential for mitigation in agriculture, there is evidence that little progress has been made in the implementation of mitigation measures at the global scale. Barriers to implementation are not likely to be overcome without policy/economic incentives and other programmes, such as those promoting global sharing of innovative technologies.

Current GHG emission rates may escalate in the future due to population growth and changing diets (high agreement, medium evidence). Greater demand for food could result in higher emissions of CH4 and N2O if there are more livestock and greater use of nitrogen fertilizers (high agreement, much evidence). Deployment of new mitigation practices for livestock systems and fertilizer applications will be essential to prevent an increase in emissions from agriculture after 2030. In addition, soil carbon may be more vulnerable to loss with climate change and other pressures, though increases in production will offset some or all of this carbon loss (low agreement, limited evidence).

Overall, the outlook for GHG mitigation in agriculture suggests that there is significant potential (high agreement, medium evidence). Current initiatives suggest that synergy between climate change policies, sustainable development and improvement of environmental quality will likely lead the way forward to realize the mitigation potential in this sector.