Emissions Scenarios

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2.4.11. Carbon Intensity and Decarbonization

Figure 2-11: Global decarbonization of primary energy - historical development and future scenarios, shown as an index (1990 = 1). The median (50th), 5th, 25th, 75th and 95th percentiles of the frequency distribution are shown. Statistics associated with scenarios from the literature do not imply probability of occurrence. Data source: Nakicenovic, 1996; Morita and Lee, 1998.

Decarbonization denotes the declining average carbon intensity of primary energy over time (see Kanoh, 1992). Although the decarbonization of the world's energy system shown in Figure 2-11 is comparatively slow, at the rate of 0.3% per year, the trend has persisted throughout the past two centuries (Nakicenovic, 1996). The overall tendency toward lower carbon intensities results from the continuous replacement of fuels with high carbon content by those with low carbon content.

The carbon intensities of the scenarios are shown in Figure 2-11 as an index spliced in the base year 1990 to the historical development. The median of all the scenarios indicates a continuation of the historical trend, with a decarbonization rate of about 0.4% per year, which is similar to the trend in the IS92a scenario (Pepper et al., 1992).

Figure 2-12: Global decarbonization and deintensification of energy in the scenarios, 1990 to 2100; energy and carbon intensities shown as an index (1990 = 1). Some of the scenarios are identified in the figure. Data source: Morita and Lee, 1998.

The scenarios that are most intensive in the use of fossil fuels lead to practically no reduction in carbon intensity. The highest rates of decarbonization (up to 3.3% per year) are from scenarios that envision a complete transition to non-fossil sources of energy.

Figure 2-12 illustrates the relationships between energy intensities of gross world product and carbon intensities of energy across the scenarios in the database. Both intensities are shown on logarithmic scales. The starting point is the base year 1990 normalized to an index (1990 = 100) for both intensities. Scenarios that unfold horizontally are pure decarbonization cases with little structural change in the economy; scenarios that unfold vertically indicate reduction in the energy intensity of economic activities with little change in the energy system. Most scenarios stay away from these extremes and develop a fan-shaped pattern - marked by both decarbonization and declining energy intensity - across the graph in Figure 2-12.

The fan-shaped graph illustrates the notable differences in policies and structures of the global energy system among the scenarios. For example, in a number of scenarios decarbonization is achieved largely through energy efficiency improvements, while in others it is mainly the result of lower carbon intensity because of the vigorous substitution of fuels. A few scenarios follow a path opposite to the other scenarios: decarbonization of primary energy with decreasing energy efficiency until 2040. After 2040 the ratio of CO2 per unit of primary energy increases - in other words, recarbonization.

2.4.12. Comparison of Indicators

Figure 2-13 illustrates the database distributions of CO2 , population, gross world product, primary energy, and carbon intensity of energy. The circles closest to the center denote the minimum value of the distribution; the solid circles denote the median value; and the shaded circles represent the maximum database value for each variable. While the values are connected in the form of a snowflake, it is important to note that those of a given range (e.g., minimum, median, and maximum), taken together, do not necessarily yield a consistent or logically possible scenario. As is shown in Chapter 4 (Figure 4-4), actual scenarios may fall into a median range on some axes and into a higher or lower range on others. Snowflake diagrams are useful because they allow the reader to see at a glance the full range of values encompassed by the database. Subsequent snowflake diagrams plot SRES scenario values on the various axes to illustrate where the scenarios fall relative to the database minimum, median, and maximum values. Snowflake diagrams should be used only for purposes of scenario classification and interpretation and not for scenario design, since the latter could lead to logical inconsistencies.

Figure 2-13: Global emissions scenarios in the database and their main driving forces. The minimum, maximum and median (50th percentile) values for 2100 are shown on seven axes of the heptagon based on the scenarios in the database. The seven axes show the ranges across the scenarios in 2100 of CO2 emissions in GtC, population in billions, gross world product in trillion US dollars at 1990 prices, gross world product growth rates in percent per year, energy intensity in MJ per US dollar at 1990 prices, primary energy in ZJ (1000 EJ) and carbon intensity in tC per TJ.

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