Although Carbon Dioxide, Methane and Nitrous Oxide occur naturally in the atmosphere, their recent significant atmospheric build-up is largely the result of anthropogenic activities (i.e. human activities). This increase has altered the composition of the Earth’s atmosphere and will have an impact on future global climate.
Carbon dioxide (CO2) is responsible for more than half the human contribution to the greenhouse effect, and concentrations have climbed steadily from around 280 parts per million (0.028 % of the air’s volume) at the beginning of the industrial revolution to around 355 ppm today. About half of the CO2 released by human activities is quickly reabsorbed by the oceans and by growing vegetation. But human-caused CO2 emissions exceed what the oceans and lands are able assimilate. Half the global emission total remains in the atmosphere indefinitely, contributing to the greenhouse effect. The 11 billion tons of carbon dioxide we add each year produces a continuous growth in the gas’s atmospheric concentration of about 0.5 % annually.
The principal anthropogenic source of CO2 is the combustion of fossil fuels, which accounts for about three-quarters of total anthropogenic emissions of carbon world-wide. Biomass burning is also an important contributor. Industrial animal production, through its reliance on mechanisation – both at the site of animal production and at the site of animal feed cultivation – contributes to production of CO2 largely through the use of fossil fuels.
Methane (CH4) is released by livestock as a by-product of digestion. The breakdown of carbohydrates in the digestive tract of herbivores (including insects and humans) results in the production of methane. The volume of methane produced by this process, known as "enteric fermentation", is greatest in ruminant animals, such as cattle, buffalo, sheep, goats, and camels. Ruminant animals possess a rumen or fore-stomach that allows them to digest plant material that monogastric species are unable to digest. The rumen contains as many as 200 species of micro-organisms. A small fraction (5 to 10 percent) of these are methanogenic bacteria. These are responsible for the removal of hydrogen from the rumen. Although non-ruminant animals do produce methane from enteric fermentation in their large intestines, the production is small compared with that which occurs in the rumen of ruminant animals. The majority (some 90 percent) of the methane produced by ruminant animals is exhaled during respiration. The remainder is released during belching or as part of the flatus.
Methane is also released from the solid waste produced by livestock. Methane is produced when methanogenic bacteria decompose the organic material in the solid waste of domesticated animals in an anaerobic environment. The amount of organic material that is susceptible to decomposition is described as the "volatile solids content." The volume of methane produced from a given amount of volatile solids under optimal anaerobic conditions is characterised as the maximum methane- producing capacity of the animal waste. Because conditions are rarely optimal, actual methane production usually is lower than the maximum possible. The share of the maximum methane-producing potential of the waste that is obtained in practice is largely a function of the manner in which the waste is managed. Liquid-based waste management systems provide an anaerobic environment as well as the moisture required for methanogenic bacterial cell production and acidity stabilisation. In contrast, animal waste left to dry in the fields will decompose in the presence of oxygen, minimising methane production.
As a rough guide to the relative importance of these two sources of methane, 1992 figures from the US indicated a production of 2.8 million metric tons of methane from solid wastes and 5.49 million metric tons from enteric fermentation.
The global warming potential of methane is about 24 times higher than that of CO2. Since 1800, methane concentrations in the atmosphere have more than doubled. Livestock contribute to the rise in atmospheric methane through increasing animal populations and accelerating growth in average animal size. Methane emissions from livestock now constitute a significant source of atmospheric methane. It is estimated that domestic animals currently account for about 15% of the annual anthropogenic methane emissions.
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. The global warming potential of Nitrous Oxide (N2O) is considered to be between 170 to 190 times that of CO2. It is a stable compound that does not decay readily in the atmosphere, with a long atmospheric lifetime of over 121 years. The primary sources of N2O from agriculture are mineral fertilisers, legume cropping, and animal waste. As a result, industrial livestock production has an impact on N2O both through the cultivation of crops for feed concentrates and through releases from animal waste (e.g. manure). These losses often are accelerated by poor soil physical conditions. Some N2O also is emitted from biomass burning.
Livestock such as sheep, goats, camels, cattle, and buffalo provide food as well as a supply of manure and power for agriculture. The global livestock population has increased considerably since the 1960s. In addition, there is an increased reliance on industrial livestock production techniques and on improved (often larger) varieties with greater unit productivity.
In summary, therefore, the main pressures on CO2, CH4 and N2O production from livestock result from:
The main determinant of Carbon Dioxide emission from livestock production activities lies in the levels of utilisation of fossil fuels. This topic is dealt with under a separate section.
Ruminants grazing on poor quality rangelands produce more methane per unit of feed consumed compared with high quality feed. However, this is to some extent balanced by the greater size (average live-weight) and higher productivity of "improved" breeds. It is also considered that livestock in developing countries produce less CH4 because of lower feed intakes despite poorer feed quality.
The decomposition of organic material in animal manure in an anaerobic environment produces methane. The most important factor affecting the amount of methane produced is how the manure is managed – since certain types of storage and treatment systems promote an oxygen-free environment. Poor management of solid wastes from livestock contributes to an increased contribution from livestock to levels of atmospheric methane. In general, waste deposited on the ground and left to dry in the fields contributes the lowest levels. In contrast industrial livestock production systems tend to produce the highest levels.
Since nitrogen is the major component of mineral fertiliser, there is mounting concern over the extent to which high-input agriculture (including industrial animal production) 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 fertiliser; in other cases, it can be achieved by increasing yields at the same nitrogen levels.
State indicators are therefore characterised by:
The agriculture sector is characterised 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 countries where rapid increases in fertiliser use and crop production are occurring, substantial increases in emissions of N2O and CH4 are projected. It is considered that even full implementation of potential 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.
Options to mitigate Carbon Dioxide 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 pre-cultivation condition. However, a large-scale reversion or reforestation of agricultural land is only possible if adequate supplies of food, fibre, and energy can be obtained from the remaining area.
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 natural ecosystems. These practices could help reduce agricultural expansion (hence deforestation) in humid zones, especially in Latin America and Africa.
Emissions of methane from domestic ruminant animals can to some extent 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 utilising 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 utilised in this manner is not emitted to the atmosphere. Overall, potential global reduction of CH4 emissions are considered to amount to about 35% of emissions from agriculture.
Improvements in farm technology, such as use of controlled-release fertilisers, nitrification inhibitors, the timing of nitrogen application, and better 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 fertiliser nitrogen (including manure nitrogen) is better used by the crop, less N2O will be produced and less nitrogen will leak from the system. Better management, by improvements in matching nitrogen supply to crop demand and more closely integrating animal waste and crop residue management with crop production, should lead to a decrease in N2O emissions.
Response is therefore characterised by:
OECD (1998) Greenhouse Gas Emission Projections and Estimates of the
Effects of Measures: Moving towards Good Practice. OECD Information Paper.
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CEISIN. Methane Emissions from Livestock. CIESIN Thematic Guides. http://www.ciesin.org/TG/AG/liverear.html
Leng, R. A. 1993. Quantitative ruminant nutrition - A green science. Australian Journal of Agricultural Research 44: 363-80. http://www.ciesin.org/docs/004-180/004-180.html
NASA, 1999. Biomass burning and global change. http://asd-www.larc.nasa.gov/biomass_burn/biomass_burn.html
Levine, J.S. 1994. Biomass burning and the production of greenhosue gases. In: Zepp, R.G. (ed) 1994. Climate Biosphere Interaction: Biogenic Emissions and Environmental Effects of Climate Change. John Wiley and Sons. ISBN 0-471-58943-3. http://asd-www.larc.nasa.gov/biomass_burn/biomass.html
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