Changing pressures on the environment

 Changing pressures on the environment Agriculture’s contribution to air pollution and climate   Public attention tends to focus on the more visible signs of agriculture’s impact on the environment,

 whereas it seems likely that the non-visible or less obvious impacts of air pollution cause the greatest economic costs.

 Changing pressures on the environment

 Agriculture affects air quality and the atmosphere in four main ways:

 particulate matter and GHGs from land clearance by fire (mainly rangeland and forest) and the burning of rice residues; methane from rice and livestock production; nitrous oxide from fertilizers and manure; and ammonia from manure and urine.

Pollution from biomass burning

Soot, dust, and trace gases are released by biomass burning during the forest, bush, or rangeland clearance for agriculture.

Burning is traditionally practiced in “slash and burn” tropical farming, in the firing of savannah regions by pastoralists to stimulate forage growth and in the clearing of fallow land and disposing of crop residues, particularly rice.

 This burning has had major global impacts and has caused air pollution in tropical regions far away from the source of the fires.

Two developments should result in an appreciable fall in air pollution from biomass burning. Deforestation is often achieved by burning, or fire is used after timber extraction to remove the remaining vegetation.

 The projected reduction in the rate of deforestation will slow down the growth in air pollution. The shift from extensive to intensive livestock production systems will reduce the practice of rangeland burning under extensive grazing systems, although the latter systems seem likely to remain dominant
in parts of sub-Saharan Africa.

 The growth in the contribution of crop residues may also slow down because of the projected very slow growth in rice production.

 Climate change itself, however, may cause temperatures to rise in the dry season, increasing fire risks, and thus increasing pollution from biomass burning in some areas (Lavorel et al., 2001).

Greenhouse gas emissions.

 For some countries, the contribution from agriculture to GHG emissions is a substantial share of the national total emissions, although it is seldom the dominant source.

 Its share may increase in importance as energy and industrial emissions grow less rapidly than in the past while some agricultural emissions continue to grow.

 There is an increasing concern not just with carbon dioxide but also with the growth of agricultural emissions of other gases such as methane, nitrous oxide, and ammonia arising from crop and livestock production.

 In some countries, these can account for more than 80 percent of GHG emissions from agriculture.

The conversion of tropical forests to agricultural land, the expansion of rice and livestock production, and the increased use of nitrogen fertilizers have all been significant contributors to GHG emissions.

Agriculture now contributes about 30 percent of total global anthropogenic emissions of GHGs, although large seasonal and annual variations make a precise assessment difficult (Bouwman, 2001).

 Tropical forest clearance and land-use change were major factors in the past for carbon dioxide emissions, but are likely to play a smaller role in the future.

 More attention is now being given to methane (CH4) and nitrous oxide (N2O), since agriculture is responsible for half or more of total global anthropogenic emissions of these GHGs.


change. Its warming potential is about 20 times more powerful than carbon dioxide.1 Global methane emissions amount at present to about 540 million tonnes p.a., increasing at an annual rate of 20-30 million tonnes.

 Rice production currently contributes about 11 percent of global methane emissions. Around 15 percent comes from livestock (from enteric fermentation by cattle, sheep, and goats and from animal excreta).

 The livestock contribution can be higher or lower at the national level depending on the extent and level of intensification.

Changing pressures on the environment

 In the United Kingdom and Canada, the share is over 35 percent. The production structure for ruminants in developing countries is expected to increasingly shift towards that prevalent in the industrial countries.

 The major share of cattle and dairy production will come from the feedlot, stall-fed, or other restricted grazing systems, and by 2030 nearly all pig and poultry production will also be concentrated in appropriate housings.

 Much of it will be on an industrial scale with potentially severe local impacts on air and water pollution. The livestock projections in this report entail both positive and negative implications for methane emissions.

 The projected increase in livestock productivity, in part related to improvements in feed intake and feed digestibility, should reduce emissions per animal.

 Factors tending to increase emissions are the projected increase in cattle, sheep and goat numbers, and the projected shift in production systems from grazing to stall-feeding.

 The latter is important because storage of manure in a liquid or waterlogged state is the principal source of methane emissions from manure, and these conditions are typical of the lagoons, pits and storage tanks used by intensive stall-feeding systems.

 When appropriate technologies are introduced to use methane in local power production, as has been done in some South and East Asian countries, the changes can be beneficial.

 If emissions grow in direct proportion to the projected increase in livestock numbers and in carcass weight or milk output, global methane emissions could be 60 percent higher by 2030.

Growth in the developed regions will be slow but in East and South Asia emissions
could more than double, largely because of the rapid growth of pig and poultry production in these regions.

Rice cultivation is the other major agricultural source of methane.

 The harvested area of rice is projected to expand by only about 4.5 percent by 2030  depending on yield growth rates, and possibly on the ability of technological improvements to compensate for climate-change-induced productivity loss if this becomes serious (Wassmann, Moya, and Lantin, 1998).

Total methane emissions from rice production will probably not increase much in the longer term and could even decrease, for two reasons.

 First, about half of rice is grown using almost continuous flooding, which maintains anaerobic conditions in the soil and hence results in high methane emissions.

 However, because of water scarcity, labor shortages, and better water pricing, an increasing proportion of rice is expected to be grown under controlled irrigation and better nutrient management, causing methane emissions to fall.

 Second, up to 90 percent of the methane from rice fields is emitted through the rice plant. New high-yielding varieties exist that emit considerably less methane than some of the widely used traditional and modern cultivars, and this property could be widely exploited over the next ten to 20 years (Wang, Neue, and Samonte, 1997).

Nitrous oxide

 Nitrous oxide (N2O) is the other powerful GHG for which agriculture is the dominant anthropogenic source.

 Mineral fertilizer use and cattle production are the main culprits. N2O is generated by natural biogenic processes, but the output is enhanced by agriculture through nitrogen fertilizers, the creation of crop residues, animal urine and feces, and nitrogen leaching and runoff.

 N2O formation is sensitive to climate, soil type, tillage practices, and type and placement of fertilizer. It is also linked to the release of nitric oxide and ammonia, which contribute to acid rain and the acidification of soils and drainage systems (Mosier and Kroeze, 1998).

The current agricultural contribution to total global nitrogen emissions is estimated at 4.7 million tonnes p.a., but it is great under 1 Power is measured in terms of the global warming potential (GWP) of gas, taking account of the ability of a gas to absorb infrared radiation and its lifetime in the atmosphere.

336 dainty about the magnitude because of the wide range in estimates of different agricultural sources.

Changing pressures on the environment

Nitrogen fertilizer is one major source of nitrous oxide emissions. The crop projections to 2030 imply slower growth of nitrogen fertilizer use compared with the past.

Depending on progress in raising fertilizer-use efficiency, the increase between 1997/99 and 2030 in total fertilizer use could be as low as 37 percent.

 This would entail similar or even smaller increases in the direct and indirect N2O emissions from fertilizer and from nitrogen leaching and runoff.

 Current nitrogen fertilizer use in many developing countries is very inefficient. In China, for example, which is the world’s largest consumer of nitrogen fertilizer, it is not uncommon for half to be lost by volatilization and 5 to 10 percent by leaching.

 Better on-farm fertilizer management, wider regulatory measures, and economic incentives for balanced fertilizer use and reduced GHG emissions, together with technological improvements such as more cost-effective slow-release formulations should reduce these losses in the future.

Livestock is the other major source of anthropogenic nitrous oxide emissions (Mosier et al., 1996; Bouwman, 2001). These emissions arise in three ways. First, from the breakdown of manure applied as fertilizer, primarily to crops but also to pastures.

 The proportion of manure thus used is difficult to estimate, but it is probably less than 50 percent. Moreover, there are opposite trends in its use. In the developed countries, growing demand for organic foods, better soil nutrition management, and greater recycling is favoring the increased use of manure.

 In the developing countries, with strong growth in industrial-scale livestock production separate from crop production, and with decreasing labor availability, there is a trend to rely more on mineral fertilizers to maintain or raise crop yields.

The second source is dung and urine deposited by grazing animals. The emissions from this source are higher for intensively managed grasslands than for extensive systems (Mosier et al., 1996). Similarly, emissions from animals receiving low-quality feeds are likely to be less than with higher-quality feeds.

 Since shifts are expected from Million tonnes N p.a. Mean value Range Natural sources
Oceans 3.0 1-5 Soils total, of which: 6.0 3.3-9.7 Tropical soils Wet forest 3.0 2.2-3.7 Dry savannahs 1.0 0.5-2.0 Temperate soils Forests 1.0 0.1-2.0 Grasslands 1.0 0.5-2.0
Subtotal natural sources 9.0 4.3-14.7

Anthropogenic sources

Agriculture total, of which: 4.7 1.2-7.9 Agricultural soils, manure, fertilizer 2.1 0.4-3.8 Cattle and feedlots 2.1 0.6-3.1 Biomass burning 0.5 0.2-1.0 Industry 1.3 0.7-1.8
Subtotal anthropogenic sources 6.0 1.9-9.6
Total all-sources 15.0 6.2-24.3
Source: Mosier and Kroeze (1998), modified using Mosier et al. (1996).
Table 12.2 Global N2O emissions


extensive to intensive production systems and from low- to higher-quality feeds, it can be assumed that there will be an increase in N2O emissions from deposited dung and urine.

 The third source is from the storage of excreta produced in stallfeeding or in intensive production units. This may produce a reduction in emissions since, on average, stored excreta produce about half as much N2O as excreta deposited on pastures (Mosier et al., 1996).

Changes in manure production over time have been estimated using the projected growth in livestock populations (allowing for differences between cattle, dairy, sheep and goats, pigs, and poultry). The amounts per head have been adjusted on a regional basis to allow for projected changes in carcass weight and milk output.

 Emission rates have been adjusted for the assumed shifts from extensive to stall-fed systems. Based on these assumptions and estimates, the total production of manure is projected to rise by about 60 percent between 1997/99 and 2030.

However, N2O emissions are projected to rise slightly more slowly (i.e. by about 50 percent) because of the switch from extensive to stall-fed systems.

 This relative environmental gain from intensification has to be seen against the rise in ammonia and methane emissions and the probable growth in point-source pollution that the intensive livestock units will generate.

This latter cannot be quantified but is a very serious problem in a number of developed and developing countries (de Haan, Steinfeld, and Blackburn, 1998).

 Agriculture is the dominant source of anthropogenic ammonia emissions, which are around four times greater than natural emissions.

Livestock production, particularly cattle, accounts for about 44 percent, mineral fertilizers for 17 percent and biomass burning, and crop residues for about 11 percent of the global total (Bouwman et al., 1997; Bouwman, 2001).

 Volatilization rates from mineral fertilizers in developing countries are about four times greater than in developed countries because of higher temperatures and lower quality fertilizers.

Losses are even higher from manure (about 22 percent of the nitrogen applied). Ammonia emissions are potentially even more acidifying than emissions of sulfur dioxide and nitrogen oxides (Galloway, 1995).

 Moreover, future emissions of sulfur dioxide are likely to be lower as efforts continue to reduce industrial and domestic emissions and improve energy-use efficiency, whereas there is little action on reducing agriculture-related emissions.

The ammonia released from intensive livestock systems contributes to both local (Pitcairn et al., 1998) and longer-distance deposition of nitrogen (Asman, 1994).

 This causes damage to trees and acidification and eutrophication of terrestrial and aquatic ecosystems, leading to decreased nutrient availability, disruption of nitrogen fixation and other microbiological processes, and declining species richness (UNEP/RIVM, 1999).

The livestock projections of this study are based on changes in both animal numbers and in productivity, as determined by changes in carcass weight or milk output per animal.

 It is assumed that the volume of excreta per animal, which is the main source of the ammonia, increases over time in proportion to carcass weight, which in turn is a reflection of the increase in the use of feed concentrates.

 estimates of ammonia emissions in 1997/99 and 2030 using these assumptions. The projected increase for the developing countries (80 percent) is significantly higher than the increase (50 percent) given in Bouwman et al. (1997).

 These projections have three main environmental implications. First, all the developing regions potentially face ammonia emission levels that have caused serious ecosystem damage in the developed countries.

 Second, emissions may continue to rise in the developed countries, adding to the already serious damage in some areas. And third, in East Asia and Latin America, a high proportion of the emissions will come from intensive pig production systems, in which emission reduction is more difficult.


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