The problem gases for animal farming are methane and nitrous oxide. Biological processes produce both naturally, but modern agricultural methods greatly increase the emissions. Both are more potent greenhouse gases than carbon dioxide: methane has about 21 times and nitrous oxide about 310 times the potency (radiative forcing power) of an equivalent mass of carbon dioxide. Both undergo slow reactions in the atmosphere, methane being converted into carbon dioxide with a 12±3-year half-life and nitrous oxide into the non-greenhouse gas nitrogen with half life of about a120-years. These figures combine to result in a methane emission producing 23 times and a nitrous oxide emission 296 times the effect of the emission of the same mass of carbon dioxide, over a 100-year period. The concentrations of both gases in the atmosphere are steadily increasing, methane having increased by 150% and nitrous oxide by 16% since industrialization began. The current effective radiative forcing by the major greenhouse gases is: carbon dioxide 1.46, methane 0.48 and nitrous oxide 0.14 watts per square metre.
Methane
Atmospheric methane is mainly formed by anaerobic microbiological activity, so is released wherever microbes grow in the absence of air. Wetlands and swamps are primary natural sources, with wetland agricultural practices such as rice paddies increasing the amount produced. Enteric digestion in the gut of most animals (including humans) involves anaerobic microbial fermentation as part of the normal digestive process, with resulting formation of methane. Ruminant animals (cattle, sheep, deer and goats in New Zealand) have developed digestive systems that enhance this process with increased emissions of methane, as eructations (burping) rather than flatulance (the politically unacceptable farting). In particular they have a stomach which functions as a very effective anaerobic digester, with a microbial population that has the ability to digest hemicellulose and some cellulose in grasses. Ruminants such as cows, sheep and buffaloes can thereby extract more nutrients from vegetation than non-ruminants, such as horses, but as a consequence produce more methane.
The increasing scale of dairying in New Zealand has led to increased numbers of dairy cattle over recent years, though the figures 1994 to 2002 show that a 26% increase in dairy cattle numbers (5.1 million in 2002,) and a 25% increase in deer (1.6 million), was balanced by a 12% decrease in beef cattle (4.5 million) and a 25% decrease in sheep numbers (40 million). More recently, conversion of previously non-animal farming land to dairying, and the intensification of dairying with more irrigation and additional fertilizer applications, has led to increasing cow numbers and methane emissions, which now contribute about 17% of New Zealand green-house gas emissions.
There is considerable interest in research into methods by which methane emissions from ruminants might be reduced. Cattle might be bred, or genetically modified, to somehow reduce emissions. Strains of ruminant microbes might be developed which produce less methane. Possibly something could be fed to the animals to modify the rumination process to produce less methane. Grasses and/or other feed could be developed which digest with less methane production
Methane production is an economic cost, representing wasted nutritional value in the feed, which provides additional incentive to find an answer. However, the current outlook doesn’t look too promising and as long as ruminant animals contribute significantly to the New Zealand economy their methane emissions look set to be a problem area for reduction of greenhouse gas emissions for the foreseeable future.
Methane largely arises by biological activity from organic material formed by photosynthesis from atmospheric carbon dioxide. In the atmosphere it is slowly converted back to carbon dioxide, so methane emissions do not lead to a long-term increase in atmospheric carbon dioxide and thus produce only a transient increase in greenhouse activity.
Nitrous Oxide
Nitrous oxide is a potent greenhouse gas that contributes about 9% to total New Zealand greenhouse emissions (as carbon dioxide equivalents). Nitrous oxide is formed by denitrifying bacteria as part of the natural nitrogen cycle. Prior to current intensive agriculture, nitrogen in the atmosphere was “fixed” (that is converted into ammonia or nitrates and so made available to plants) by lightning and by nitrogen-fixing bacteria such as present in the root nodules of legumes. These processes provided all the nitrogen for the biological nitrogen cycle for all plants and animals. The nitrogen fixation was balanced by bacterial denitrification that converted nitrates in soil or water to nitrous oxide, so that the nitrogen status was stable, and little nitrogen found its way from soil into waterways. Until recently in New Zealand the bulk of grassland agricultural nitrogen was provided by clover, but synthetic nitrogen fertilizers, such as urea and ammonium salts, are readily available and now relatively cheap, and their use in New Zealand has increased four-fold since 1990. Nitrogenous fertilizers are manufactured by using natural gas, with a consequent release of carbon dioxide. Their use thus leads not only to the serious but transient greenhouse burden of increased nitrous oxide, but also the continuously increasing atmospheric burden of carbon dioxide derived from fossil carbon as natural gas. About 0.3% of New Zealand carbon dioxide emissions arise from ammonia/urea manufacture from natural gas at Kapuni.
The use of synthetic nitrogen fertilizers reduces New Zealand farming dependence on clover, which has been seriously impacted by the clover root-weevil. Added nitrogen fertilizer greatly increases grass growth, and hence farming productivity. However increased nitrogen levels in the soil lead to increased denitrifying bacterial activity and hence greater nitrous oxide emissions. Saturation of the soil nitrogen system also leads to leaching of excess nitrate into waterways, with environmentally disastrous consequences. Animal excreta give rise to about 80% of New Zealand nitrous oxide emissions. Nitrous oxide emission and nitrate leaching are both wasteful of nitrogen fertilizer, and the solution is for farming methods to be developed which better control nitrogen applications in order to maximize production while minimizing emissions and leaching into waterways. In particular, animal excreta need to be much more efficiently recycled. Reversion to a less intensive and more clover-based grassland farming would significantly reduce New Zealand greenhouse gas emissions, at a cost of lower productivity. Soil treatments that reduce the activity of denitrifying bacteria have been developed and are increasingly being used.
Conclusions
Natural gas usage in urea production would be simple to include in the carbon-trading scheme, but usage would be reduced only if less urea was manufactured. Fossil fuel used in farming, and farming related transport will be captured early by the carbon-trading scheme. Emissions of methane and nitrous oxide will be much more difficult to quantify and attribute to individual “producers” and it is difficult to see how such a scheme would be developed and administered, except at an industry level. Until such details are determined and carbon costs allocated, the tax-payer will have to meet Kyoto Protocol charges arising from farming emissions.
The bad news is that unless radical changes in farming methods are developed, it seems probable that green-house gas emissions resulting from animal farming could be reduced only by reducing animal-based production, particularly of dairy products. The good news is that emissions from farming mainly produce transient green-house effects and add little fossil carbon to the biosphere.
For more details see
Intergovernmental Panel on Climate Change, interim report 4 (2007), particularly Chapter 2.3; Chemically and Radiatively Important Gases.
http://ipcc-wg1.ucar.edu/wg1/wg1-report.html
New Zealand’s Greenhouse Gas Inventory 1950-2005, particularly Annex 1; Key Categories. http://www.mfe.govt.nz/publications/climate/nir-jul07/nir-jul07.pdf
Green-house Gas Emissions from Grassland Animal Farming in New Zealand
http://www.mfe.govt.nz/issues/climate/about/greenhouse-gas-inventory.html
Climate Change 
