CH4 emissions from rice paddies are an important emission category, and are reviewed in more detail in Wassmann et al. (1993, 1997, 1998), Houghton et al. (1995), and Olivier et al. (1996). CH4 emissions are primarily a function of emission factors and assumed rice cropland area. In turn, emission factors depend on cultivation method (wet versus dry cultivation), water management practices, type of rice variety planted, and cropping patterns. Most long-term scenarios assume that the emission factor of CH4 per unit area of rice cropland remains constant with time, although estimates vary greatly from one scenario reference to another.
The many approaches used to estimate the future extent of rice fields result in increases in the global area of rice fields from a factor of 0.8 to one of 1.8 by 2100 (see Alcamo and Swart, 1998, for a brief review). One of the main factors to affect the future area of rice cropland is the assumed long-term improvement in rice productivity. The typical range of estimates for this variable is between 1.0 and 1.6% per year, depending on the region, time horizon, and reference.
Estimated emissions of CH4 from enteric fermentation depend on assumptions about emission factors per animal and the number of livestock. As summarized by IPCC (1995), emission factors vary greatly depending on the type of cow, their feed regime, and their productivity. Assumptions for the change in meat production from 1990 to 2100 in existing scenarios vary greatly, by a factor of 1.2 to 4.2. Despite the wide range of assumptions about meat production, emissions of the various scenarios do not vary by more than a factor of two, which indicates that other assumptions (e.g., animal productivity) must compensate for the differences in assumed meat production.
As noted above, global estimates can mask significant differences in assumptions about industrial and developing regions. For industrial regions, nearly all scenarios assume a decline in beef production per capita, which is consistent with the current shift away from the consumption of beef to poultry and other protein sources. Meanwhile, the scenarios for developing countries assume a continuing increase in beef consumption, which grew by 3.1% per year between 1982 and 1994, leading to an overall growth of 1.1% per year globally (Rosegrant et al., 1997).
Another factor that influences the future number of livestock is the change in animal productivity, that is, the weight of meat or dairy product per animal. The rate of increase in beef productivity dropped in industrial countries from 1.25% per year in 1967-1982 to 0.69% in 1982-1994, but increased from 0.11% per year to 0.61% per year in the developing countries. Similar to emissions from rice fields, emissions from livestock are influenced not only by number of livestock (equivalent to the extent of rice area), but also by changes in the productivity of animals as these alter the CH4 emission factor.
Some authors doubt that assumed increases in meat production and animal productivity can be sustained indefinitely. For example, Brown and Kane (1995) argue that livestock production cannot be increased greatly because nearly all of the world's suitable rangelands are intensively exploited already. They claim that the rapidly growing demand for meat and dairy products can only be met by livestock production in feedlots, which would result in a rising demand for feed that requires further development of agricultural land and further GHG emissions.
N2O budgets are associated with considerable uncertainties. Agricultural activities and animal production systems are the largest anthropogenic sources of these emissions. Recent calculations using IPCC 1996 revised guidelines indicate that N2O emission from agriculture is 6.2 MtN as N2O per year (IPCC, 1996; Mosier et al., 1998). About one-third is related to direct emissions from the soil, another third is related to N2O emission from animal waste management, and the final third originates from indirect N2O emissions through ammonia (NH3), nitrogen oxides (NOx), and nitrate losses. This compares to earlier estimates of total anthropogenic emissions that range between 3.7 and 7.7 MtN (Houghton et al., 1995). Industrial sources contribute between 0.7 to 1.8 MtN (Houghton et al., 1995; see also Chapter 5, Table 5-3 and Section 3.6.2).
Total natural emissions amount to 9.0 � 3.0 MtN as N2O, so oceans, tropical, and temperate soils are together the most important source of N2O today. Atmospheric concentrations of N2O in 1992 were 311 parts per billion (10 9 ) by volume (ppbv) (Houghton et al., 1995); the 1993 rate of increase was 0.5 ppbv, somewhat lower than that in the previous decade of approximately 0.8 ppbv per year (Houghton et al., 1996).
Among the anthropogenic sources, cultivated soils are the most important, contributing 50 to 70% of the anthropogenic total (see Chapter 5, Table 5-3). This source of N2O is particularly uncertain as the emission level is a complex function of soil type, soil humidity, species grown, amount and type of fertilizer applied, etc. The second largest anthropogenic source of N2O is industry; two processes account for the bulk of industrial emissions - nitric acid (HNO3) and adipic acid production. In both cases N2O is released with the off-gases from the production facilities. Recently, N2O release from animal manure was identified as another significant source of N2O emissions.
N2O emissions from agricultural soils occur through the nitrification and denitrification of nitrogen in soils, particularly that from mineral or organic fertilizers. Emissions are very dependent on local management practices, fertilizer types, and climatic and soil conditions, and are calculated by multiplying an emission factor by the sum of mineral and organic nitrogen applied as fertilizer. The emission factor depends on the fertilizer type and local environmental circumstances, and those used in IPCC (1996) result in an assumed loss of 1.25% (range 0.25 to 2.25%) of nitrogen as N2O per year.
To estimate the trend in fertilizer use, different references employ different approaches. For example, Leggett et al. (1992) directly estimate the amount of fertilizer used, whereas Alcamo et al. (1996) back-calculate fertilizer use from the future amount of agricultural land. Despite these different approaches, estimates of future fertilizer use are quite consistently given as an increase by about a factor 1.4 to 2.8 between 1990 and 2100.
Although the different references are consistent in their findings about future global fertilizer use, the question arises whether these are at all reasonable guesses. Some researchers assume that fertilizer use will increase even more. For example, Kendall and Pimentel (1994) in their "business-as-usual" scenario assume a 300% increase in the use of nitrogen and other fertilizers by 2050. Moreover, most studies of future world food production assume improvements in crop yield. These yield improvements may imply higher overall rates of fertilizer use because many high-yielding crop varieties depend on large amounts of fertilizer.
However, some authors question whether global average fertilizer use will grow. For example, Brown and Kane (1995) note that world fertilizer use has actually fallen in recent years and Kroeze (1993) assumes that per capita N2O emissions from fertilizer consumption decrease by 50% in 2100 relative to 1990 through policies that promote the more efficient use of synthetic fertilizers. Future fertilizer use may also be lower than in the "business-as-usual" scenarios because farmers have other incentives to reduce nitrogen fertilizer use, such as to reduce farming costs and avoid nitrate contamination of groundwater.
This brief review of the literature on prognoses of fertilizer use indicates that the N2O emission scenarios depicted in Figure 3-16 do not take into account the full range of views about future trends in fertilizer use. Additional uncertainty in future emissions occurs because changes in the number of livestock, as discussed above for CH4 emissions, and animal husbandry practices will also affect N2O emissions.
Figure 3-16: Normalized N2O emissions from fertilized soils for (a) global, (b) OECD, (c) former Soviet Union, (d) Asia, (e) Africa, (f) Latin America, based on scenarios in the literature. Scenarios numbers are given in Table 3-7.
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