The SRES emissions scenarios also have different emissions for other GHGs and chemically active species such as CO, NO_x , and volatile organic compounds. The uncertainties that surround the emissions sources of these gases, and the more complex set of driving forces behind them are considerable and unresolved. Hence, model projections of these gases are particularly uncertain and the scenarios presented here are no exception. Improved inventories and studies linking driving forces to changing emissions in order to improve the representation of these gases in global and regional emission models remain an important future research task.

The emissions of other gases follow dynamic patterns much like those shown in Figure TS-7 for carbon dioxide emissions. Further details about GHG emissions are given in Chapter 5.

9.4.1. Nitrogen Oxides Emissions

Some models of the six SRES models do not provide a comprehensive description of NO_x emissions or include only specific sectors (e.g., energy-related sources) and have adopted other source categories from corresponding model runs derived from other models. Even with a simplified model representation, future NO_x emission levels are mainly determined by two set of variables: levels of fossil energy use (see Chapter 4), and level and timing of emission controls, inspired by local air quality concerns.

As a result the spread of NO_x emissions is largest within the A1 scenario family (28 to 151 MtN/yr by 2100), almost as large as the range across all 40 SRES scenarios (see Table TS-4). Only in the highest emission scenarios (the fossil fuel intensive scenarios within the A1 scenario family and the high population, coal intensive A2 scenario family) do emissions rise continuously throughout the 21^st century. In the A1B ("balanced") scenario group and in the B2 scenario family, NO_x emission levels rise less. NO_x emissions tend to increase up to 2050 and stabilize thereafter, the result of a gradual substitution of fossil fuels by alternatives as well as of the increasing diffusion of NO_x control technologies. Low emission futures are described by various B1 family scenarios, as well as in the A1T scenario group, that describe futures in which NO_x emissions are controlled because of either local air quality concerns or rapid technological change away from conventional fossil technologies. Overall, the SRES scenarios describe a similar upper range of NO_x emissions as the previous IS92 scenarios (151 MtN versus 134 MtN, respectively, by 2100), but extend the IS92 uncertainty range toward lower emission levels (16 versus 54 MtN by 2100 in the SRES and IS92 scenarios, respectively).

9.4.2. Volatile Organic Compounds, Excluding Methane

NMVOCs arise from fossil fuel combustion (as with NO_x , wide ranges of emission factors are typical for internal combustion engines), and also from industrial processes, fuel storage (fugitive emissions), use of solvents (e.g., in paint and cleaners), and a variety of other activities. In this report NMVOCs are discussed as one group. As for NO_x emissions, not all models include the NMVOCs emissions category or all of its sources.

A relatively robust trend across all 40 scenarios (see Chapter 5) is a gradual increase in NMVOC emissions up to about 2050, with a range between 190 and 260 Mt. Beyond 2050, uncertainties increase with respect to both emission levels and trends. By 2100, the range is between 58 and 552 Mt, which extends the IS92 scenario range of 136 to 403 Mt by 2100 toward both higher and lower emissions (see Table TS-4). As for NO_x emissions, the upper bounds of NMVOC emissions are formed by the fossil fuel intensive scenarios within the A1 scenario family, and the lower bounds by the scenarios within the B1 scenario family. Characteristic ranges are between 60 and 90 Mt NMVOC by 2100 in the low emissions cluster and between 370 and 550 Mt NMVOC in the high emissions cluster. All other scenario families and individual scenarios fall between these two emissions clusters; the B2 marker scenario (B2-MESSAGE) closely tracks the median of global NMVOC emissions from all the SRES scenarios (see Chapter 5).

9.4.3. Carbon Monoxide Emissions

The same caveats as stated above for NO_x and NMVOC emissions also apply to CO emissions - the number of models that represent all the emission source categories is limited and modeling and data uncertainties, such as emission factors, are considerable. As a result, CO emission estimates across scenarios are highly model specific and future emission levels overlap considerably between the four SRES scenario families (see Table TS-4). Generally, emissions are highest in the high growth fossil fuel intensive scenarios within the A1 scenario family. Lowest emission levels are generally associated with the B1 and B2 scenario families. By 2100, emissions range between 363 and 3766 Mt CO, a considerably larger uncertainty range, particularly toward higher emissions, than in IS92, for which the 2100 emission range was between 450 and 929 Mt CO (see Table TS-4).

Table TS-4 (see later) summarizes the emissions of GHGs, sulfur dioxide and other radiatively active species by 2100 for the four markers and the ranges for other 36 scenarios. Combined with Tables TS-2 and TS-3, the tables provide a concise summary of the new SRES scenarios. Data are given for both the harmonized and all scenarios.

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