5.4.2. Nitrous Oxide

Uncertainties in the estimates of current N₂O emissions (Table 5-3) are also reflected in base-year 1990 differences, between 4.8 and 6.9 MtN across the six models used to develop the SRES scenarios. The range across the four markers (Table 5-3) is not as wide, between 6.0 and 6.9 MtN, and leans more toward the higher end of the uncertainty range reported in Houghton et al. (1995). After standardization, 1990 N₂O emissions in the SRES scenarios were set to 6.7 MtN, well within the IPCC WGI SAR range (Table 5-3). Even more so than for CH₄ , food supply is assumed to be a key determinant of future N₂O emissions. Size, age structure, and regional spread of the global population are likely to affect future emission trajectories, together with diets and rates of improvement in agricultural productivity. The representation of how these driving forces translate into N₂O emissions varies across the models because of differences in both sectoral coverage and emission factors. As a result, differences in emission trajectories are not only scenario dependent, but also model dependent, which illustrates additional uncertainties in our understanding and representation of driving forces and their influence on N₂O emissions.

The range of emissions across all 40 SRES scenarios increases continuously over the 21^st century (Figure 5-7). By 2100, N₂O emissions range between 5 and 20 MtN. Emission ranges also tend to be comparable across the four scenario storylines, an indication of the decisive impact of uncertainty in the modeling (mentioned above). Future N₂O emissions in the SRES scenarios tend to cluster into two broad groups - those that project relatively flat, even slightly declining, emissions and those that indicate continuously rising trends toward high emission levels. Some other scenarios (e.g. A1B-ASF, B1-ASF, and B1-IMAGE) indicate the possibility of transitional emission patterns in which N₂O emissions peak around 2050 and decline thereafter, more or less in step with the population size. As a result of differences in modeling approaches, no individual scenario tracks the mean or median across all 40 scenarios of the SRES set.

5.4.2.1. A1 Scenario Family

The A1 family of scenarios covers close to the full range of N₂O emissions from the SRES scenarios (Figure 5-7). The A1G-MiniCAM scenario is at the high end of the A1 range (Figure 5-8a). Most of the emissions increase in this scenario is associated with the agriculture sector, primarily with animal manure management. Emissions are driven by a rapid increase in income that induces steep increases in meat and dairy consumption. The A1B-AIM marker scenario has emissions near the low end of the range. This scenario assumes that fertilizer use in developing countries is nearly saturated and that increased productivity comes from better management. A portion of nitrogen from fertilizers is also assumed to be stored indefinitely in underground water. However, the emissions from energy use in A1B-AIM increase until the third quarter of the 21^st century, and reach a level about three times the 1990 volume. In the middle of the A1 family range lie emissions from the A1B-ASF and A1B-IMAGE scenarios (Figure 5-8a). In these scenarios, emissions growth to the 2050s is driven by growth in the agricultural and transportation sectors. Thereafter, emissions decline, primarily because of declining population levels and increases in agricultural productivity (lower production factor input per unit of output).

As suggested by Figure 5-8a, differences in the energy supply system reflected by the four A1 scenario groups (A1G, A1C, A1T, and A1 or "balance") are only partially translated into differences in N₂O emissions. For example, scenarios that rely on coal as the primary energy source span the range from 6 MtN (A1C-MESSAGE) to 16 MtN (A1C-MiniCAM). At the same time, trajectories of A1T "technology" scenarios are located in the lower end of the emissions range.

5.4.2.2. A2 Scenario Family

N₂O emission trajectories of the A2 scenario family fall into two major groups. The first group includes the A2-AIM and A2-MESSAGE scenarios, and the rest of scenarios form the second group. The A2-AIM N₂O emissions increase at a relatively low rate and do not exceed 10 MtN in 2100 (Figure 5-8b), because of a relatively slow increase in nitrogen fertilizer input and omission of N₂O sources associated with animal wastes. N₂O emissions in the second group of scenarios (which includes the family marker A2-ASF) grow steadily throughout the 21^st century, driven by an increased demand for food and associated increases in the animal waste and nitrogen fertilizer use (Table 5-6). In 2100 the emissions from this group of scenarios range from 15 to 20 MtN.

5.4.2.3. B1 Scenario Family

The spread of N₂O emissions in the B1 family is quite large. The high-end reflects the agricultural assumptions used in the MiniCAM model, in which continued growth in meat production is driven by increases in per capita consumption that more than offset the reduction in population size (Figure 5-8c). The B1-AIM scenario has emissions near the low end of the range, because fertilizer use in developing countries is assumed to level off and better management increases productivity. Emissions from animal waste, mobile sources, and other sources in B1-AIM are nearly constant. The B1- IMAGE scenario (B1 family marker) has slowly increasing emissions through to 2050, which predominately originate from increases in livestock and the use of synthetic fertilizer (Table 5-6). After 2050, these emissions decline as the population size and associated demand for animal products declines (Figure 5-8c). The B1-ASF scenario has an intermediate emission trajectory with a maximum of 11.5 MtN in 2050 and a subsequent decline to 9.3 MtN by 2100 (Figure 5-8c). The two illustrative scenarios A1G-MiniCAM and A1T-MESSAGE display similar patterns of methane emissions as the A2 and B1 marker scenarios, respectively, and are therefore not discussed separately here.

5.4.2.4. B2 Scenario Family

The B2 family of scenarios covers a large range of N₂O emissions, similar to that found for the A1 family (see Figure 5-7). In the B2-MESSAGE scenario (B2 family marker), which has the lowest emissions in the family, improvements in agricultural productivity exceed increases in the demand for agricultural products and significantly slow emission growth in the sector (Table 5-6). Slow growth in agriculture and energy-related emissions and declines of emissions from other sectors result in a relatively flat emissions profile (Figure 5-8d). Emissions in the B2-ASF scenario are between the middle of the family range and its high end. The continuous increase in emissions in the B2-ASF scenario (which lie between the middle and high end of the family range) occurs through growth in nitrogen fertilizer use, in animal wastes, and in mobile sources. It is driven by increases in population and, to a smaller extent, by per capita increases in consumption of meat and dairy products.

5.4.2.5. Inter-Family Comparison

The allocation of N₂O emissions into source categories for the SRES marker scenarios is shown in Table 5-6. Separate emissions estimates for fertilized soils and manure were not available for all scenarios. As the emissions in Table 5-6 have not been standardized (see Box 5-1), base-year 1990 (and 2000) emissions are not the same for the different scenarios. In general, base-year emissions are likely to have greater differences at finer levels of detail. These differences result from different model calibrations, different model methodologies, and different classification schemes.

Emissions from fertilized soils and manure from agricultural animals dominate N₂O emissions in all the SRES markers. The fraction of total emissions from these two sources increases to about 80% in the A2, B1, and B2 marker scenarios, while the agricultural fraction remains roughly the same over the 21^st century in the A1B marker. As discussed in Section 5.4.2, the range of future emissions is similar across each scenario family when all storyline interpretations are considered. Therefore, while it is useful to discuss emissions from the marker scenarios individually, clearly differences in modeling approaches and model assumptions have a particularly strong effect on future N₂O emissions.

Agricultural emissions in the A1B-AIM marker scenario decrease over the 21^st century, with increased productivity offsetting additional food demand. Agricultural emissions in the A2-ASF marker scenario increase substantially because of the food demands of a large population, coupled with less technological change. Emissions in the B1-IMAGE marker scenario increase and subsequently fall. In the B2-MESSAGE marker, agricultural emissions of N₂O increase steadily throughout most of the 21^st century, with a decrease between 2000 and 2010. These patterns differ because of the different driving forces (population and demand), model assumptions, and modeling approaches.

Emissions from the categories "Industry/Fossil Fuels" and "Other" show a monotonic rise in the A2-ASF marker, while emissions in the B1-IMAGE marker rise and subsequently fall. Industry and fossil fuel emissions fall and then increase slightly in the B2-MESSAGE marker scenario, while "Other" emissions, which largely arise from land-use changes, fall. Industry and fossil fuel emissions in the A1B-AIM marker scenario increase through the 21^st century, but are countered by a decrease in emissions from the "Other" category.

The combined dynamics of N₂O emissions in the A1, B1, and B2 markers leads to nearly stable or declining emissions during most of the 21^st century (Figure 5-7). The B2 marker shows the lowest emission level (from the year 2010 to 2080), despite a larger population than in A1 and B1. One possible reason is inter-model differences in treatment of land-use changes. Unlike the other markers, the A2-ASF scenario yields a continuous growth in emissions, which corresponds to its assumptions of high population and slow technological change.

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