S. K. Guru
1.2 Agriculture as a contributor
At the same time, agriculture has not only been shown to pro- duce significant effects on climate change primarily through the production and release of GHGs, such as carbon dioxide, methane and nitrous oxide, but also by altering the Earth’s land cover that can change its ability to absorb or reflect heat and light, thus contributing to radiative forcing. Land use change such as deforestation and desertification, together with the use of fossil fuels are the major anthropogenic sources of carbon dioxide. Besides the problems associated with land use through deforestation for example, can translate into increased ero- sion. Agriculture itself is the major contributor to increasing methane and nitrous oxide concentrations in the Earth’s atmo- sphere. Agricultural practices themselves have often added to the water shortage problem as in Africa or other arid/semi- arid areas more than anywhere else due to the differences in property rights. More precisely, because farmers are often not the owners of the land they work on, the preservation of natu- ral resources is generally viewed as a secondary objective. In addition, pressures represented by increasing populations and changing technology add to the problem of land deterioration related to agricultural practices (Drechsel et al., 2001). Another illustration of environment-damaging agricultural practices is the intense use of fertilisers in low-quality lands. As yields increase, so will water consumption, thus creating a vicious circle (Gommes and Petrassi, 1996).
Agriculture releases into the atmosphere significant amounts of GHGs, that is, CO2, CH4 and N2O (Cole et al., 1997;
Anonymous, 2001a,b; Paustian et al., 2004). CO2 is released from microbial decay or burning plant litter and soil organic matter (Janzen, 2004; Smith, 2004a–c); CH4 from fermenta- tive digestion by ruminants, stored manures, paddy cultivation or decomposition of organic materials in anaerobic conditions (Mosier et al., 1998) and N2O from microbial transformation of nitrogen in soils and manures especially under wet condi- tions where available nitrogen exceeds plant requirements (Smith and Conen, 2004; Oenema et al., 2005). Direct agri- cultural emissions were 10–12% of the total anthropogenic GHG emissions in 2005, that is, 5.1–6.2 Pg CO2-eq. (Smith et al., 2007a). CH4 contributes 3.3 GtCO2-eq. year−1 and N2O 2.8 GtCO2-eq. year−1. Of the global anthropogenic emissions in 2005, agriculture accounted for about 60% of N2O and about 50% of CH4 (Denman et al., 2007). Globally, agricultural Global trends
CH4 and N2O emissions have increased by nearly 17% from 1990 to 2005, an average annual emission increase of about 60 MtCO2-eq. year−1 (Anonymous, 2006a,b). Three sources—bio- mass burning (N2O and CH4), enteric fermentation (CH4) and soil N2O emissions—together explained 88% of the increase.
Livestock (cattle and sheep) account for about one-third of global anthropogenic emission of CH4 (Murray et al., 1976;
Kennedy and Milligan, 1978; Crutzen, 1995; Anonymous, 2006a). Agricultural lands generate very large CO2 fluxes both to and from the atmosphere (Anonymous, 2001a) but the net flux is small (Smith et al., 2007a), which amounts to 40 MtCO2-eq. in 2000, less than 1% of global anthropogenic CO2
emissions (Anonymous, 2006b). GHG emissions from defor- estation mainly in tropical countries contributed an additional 5.9 Pg CO2-eq. per year (with an uncertainty range of ±2.9 Pg CO2-eq.), thus equalling or exceeding emissions from all other agricultural sources combined.
Agricultural N2O emissions will increase 35–60% till 2030 due to increasing use of nitrogenous fertiliser and animal manure production (Mosier and Kroeze, 2000; Anonymous, 2003, 2006a). If the demands for food increase and the diet shifts as projected, then annual emissions of GHGs from agriculture may escalate further (Smith et al., 2007a). If CH4
emissions increase proportionately with increasing livestock, then it is projected that CH4 emission will increase by 60% till 2030 (Anonymous, 2003) while both enteric fermentation and manure management will increase CH4 emission by 21% from 2005 to 2020 (Anonymous, 2006a). Further, although global rice production areas will increase to 4.5% by 2030, substantial CH4 emission is not expected, which may be due to less rice grown in continuous flooding under future water-scarce condi- tions or due to rice cultivars emitting less CH4 (Wang et al., 1997). But a sustained increase in the area of irrigated rice between 2005 and 2020, a 16% increase in CH4 emission is pro- jected (Anonymous, 2006a). The baseline 2020 emissions for non-CO2 GHGs is 7250 MtCO2-eq. Non-CO2 GHG emissions in agriculture are projected to increase by about 13% from 2000 to 2010 and by 13% from 2010 to 2020 (Anonymous, 2006b).
Unfortunately, for non-CO2 GHG emission estimates, there is no baseline for 2030. Assuming a similar rate of increase from 2000 to 2020, the 2030 global agricultural non-CO2 GHG emissions were projected to increase 13% during 2000–2010 and 2010–2020, while 10–15% increase were projected for 2020–2030, that is, from 8000 to 8400 with a mean of 8300 MtCO2-eq. by 2030 (Anonymous, 2006a). Moreover, the future
evolution of CO2 emissions from agriculture is uncertain (Smith et al., 2007a). Fortunately, stable/declining deforestation (Anonymous, 2003) and increased adoption of conservation tillage practices (Anonymous, 2001c) will decrease CO2 emis- sion (Smith et al., 2007a).
The magnitude of emissions and relative importance of the dif- ferent sources vary widely among 10 world regions: developing countries of South Asia, developing countries of East Asia, sub- Saharan Africa, Latin America and the Caribbean, Middle East and North Africa, Caucasus and Central Asia, Western Europe (EU 15, Norway and Switzerland), Central and Eastern Europe, OECD Pacific (Australia, New Zealand, Japan and Korea) and OECD North America, that is, Canada, the United States and Mexico (Anonymous, 2006a). Non-Annex I countries compris- ing five regions contributed 74% of total agricultural emissions.
N2O emissions from soils primarily due to N fertilisers and manures were the main GHG source from seven regions, while CH4 from enteric fermentation was the main GHG source in the other three regions (Latin America and Caribbean, the countries of Eastern Europe, the Caucasus and Central Asia and OECD Pacific). This was due to 24% and 36% of global sheep and cattle population in these three regions (Anonymous, 2003).
Rice production emitted 97% and biomass burning emitted 92% of the total world CH4 emissions in developing countries, while South and East Asia dominated the emissions from rice production with 82% and emissions from biomass burning dominated with 74% in sub-Saharan Africa, Latin America and the Caribbean. Developed regions with 52% of total emissions from only manure management were higher than the develop- ing regions with 48% of total emissions (Anonymous, 2006a).
However, CO2 emissions and removal from agricultural lands in these 10 regions are uncertain as some countries reported net emissions while some reported net removals, but countries from Eastern Europe, the Caucasus and Central Asia had an annual emission of 26 MtCO2 year−1 in 2000 (Anonymous, 2006b).
The Middle East, North Africa and sub-Saharan Africa were the highest emitters of GHGs with a combined 95%
increase in the period 1990–2020 (Anonymous, 2006a). The per capita food production is either declining or at levels lesser than adequate in sub-Saharan Africa (Scholes and Biggs, 2004) due to low and declining soil fertility along with inadequate fertiliser inputs (Sanchez, 2002; Smith et al., 2007a). The rising wealth of urban populations in this region (South and Central Africa, including Angola, Zambia, Democratic Republic of Regional trends
Congo, Mozambique and Tanzania) will increase the demand for livestock products, intensifying and expanding agriculture to still largely unexploited areas, thereby resulting in higher GHG emissions (Smith et al., 2007a). In East Asia, with a 4 and 12 times increase of milk and meat production, respectively, from 1961 to 2004 (Anonymous, 2006c) and its projected continued increase in consumptions, the GHG emissions are expected to increase 86% and 153%, respectively, from enteric fermentation and manure management, during 1990 to 2020 (Anonymous, 2006a). In a pursuit to ensure food security for its teeming population, South Asia will be using more and more nitrogenous fertiliser and manure, thereby increasing its GHG emission (Anonymous, 2006a).
Deforestation of cropland and grassland in the Latin America and Caribbean resulted in increased emissions of GHG, mainly CO2 and N2O (Anonymous, 2006c). N2O emissions have sig- nificantly reduced in the countries of Central and Eastern Europe, the Caucasus and Central Asia for their decreased use of nitrogenous fertilisers since 1990. However, driven by favour- able economic conditions in these countries, the use of nitrog- enous fertilisers may shoot up again, which will result in 32%
increase of N2O emissions from soils by 2020 (Anonymous, 2006c). Non-CO2 GHG emissions increased consistently from the agricultural sector between 1990 and 2020 in OECD North America and OECD Pacific with an 18% and 21% increase, respectively. These emissions were from manure manage- ment and N2O from soils. In Oceania, emission increased due to exponential increases of nitrogenous fertiliser use, while in North America it increased due to management of manure from cattle, poultry and swine production along with manure applica- tion to soils. Fortunately, CO2 emission from land conversion has been reduced in both these regions having active vegeta- tion policies restricting further clearing (Anonymous, 2006a).
The only region in the globe with decreased projection of GHG emissions from agriculture till 2020 is Western Europe due to its adoption of climate-specific and environmental policies along with economic constraints on agriculture (Anonymous, 2006a).
1.3 Global agricultural land use change