In this chapter a brief overview and evaluation of the emissions scenarios is presented. Much of the quantitative analysis of the emissions scenarios is based on the unique scenario database developed for SRES. This database, its structure, and the process of assembling the data that it now contains are described. The database provides a valuable overview of the various emissions modeling approaches and scenarios in the literature. It is the most comprehensive ESD available, and it can be accessed through a website (www-cger.nies.go.jp/cger-e/db/ipcc.html).

The SRES database represents the basis for evaluations of emissions scenarios, their main driving forces, and their uncertainty ranges. The CO₂ emissions trajectories of the scenarios in the SRES database are presented and their distribution and the associated sample statistics assessed. In the same way, the main driving forces of future emissions - population, economic development and energy consumption - are analyzed. Finally, the possible relationships among these driving forces for the collection of emissions scenarios in the database are considered.

Future levels of CO₂ emissions from the energy sector are a function of population, gross world product, the structure and efficiency of the economy, and the costs and availability of different sources of energy.

The factor range for population increases by 2100 across the scenarios is between 1.2 and 3.3 times 1990 levels (see Figure 2-4). This is the smallest factor increase of all the emissions driving forces. This probably reflects a relatively high consensus among demographers as to future population growth. However, this observation is based on a relatively small number of reported population projections in the database (46). The range in projected gross world product values is between 3.2 and 35 times the 1990 levels (see Figure 2-5). The range of the factor increase of primary energy consumption is from 0.9 to 10 times 1990 levels (see Figure 2-8). For energy intensity the range in 2100 is from 9.3 MJ/US$ down to 2.3 MJ/US$.

The range in carbon intensity of primary energy is the widest range of all the driving forces considered here. It varies from 0.025 to 1.1 times 1990 levels (about a factor of 45) in the year 2100 (see Figure 2-11). Emissions trajectories are extremely sensitive to a number of driving forces, which include including population growth, economic growth, and energy intensity improvement. Variation in carbon intensity is the main indicator of the wide variation in energy-related CO₂ . However, it is important to recognize that this is a result of the inputs, assumptions, methods, and types of models used to calculate the scenarios.

The findings of the analysis of the scenarios in the literature suggest the following general conclusions:

• the ranges over which the driving forces vary are large, which contributes to the wide range of future emissions;
• of these driving forces, gross world product and population are often exogenous, while carbon and energy intensity are not;
• the frequency distributions of driving forces and emissions are asymmetric, with long tails and often more than one peak;
• for many driving forces the median and mean are closer to the minimum than to the maximum;
• lack of information as to whether climate intervention policies and measures were included in the scenarios, for about 40 scenarios, means that it is not possible to distinguish between intervention and non-intervention scenarios;
• it is hoped that IPCC or another international institution will, in the future, maintain this or a similar database so as to assure a continuity of knowledge and scientific progress in GHG emissions scenarios (an equivalent database to document narrative and other qualitative scenarios would also be very useful for future climate-change assessments); and
• it would be useful in the future to evaluate the consistency of the driving forces and results of all scenarios included in the scenario database. In addition, it would be helpful to extend the database to include land-use change data where currently only land-use emissions are presented. As noted above, only a small percentage of scenarios contain information on land use.

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