The scenarios in the database portray a weak relationship between population and economic growth; the correlation is slightly negative. Scenarios that lead to a very high gross world product are generally associated with central to low population projections, while high population projections do not lead to the highest gross world product scenarios. At extremely high levels of average global income the correlation is strongly negative. The highest per capita incomes in 2100 - in the range between US$30,000 and US$45,000 - are achieved with a low-to- medium population growth.
Figure 2-7 illustrates some of the relationships between population and gross world product in the scenarios. It compares only 39 scenarios as information about population and gross world product assumptions is available for only a few scenarios. In most of these, global population transition is achieved during the 21 st century and stabilization occurs at a population between 10 to 12 billion people in the year 2100. Generally, this is associated with relatively high levels of economic development, in the range US$200-500 trillion in the year 2100. Scenarios at the lower end of this scale are labeled collectively as the "mid-range cluster," which includes all IIASA-WEC scenarios (IIASA-WEC, 1995; Nakicenovic et al., 1998b), IS92a and b (Pepper et al., 1992), and AIM96 (Matsuoka et al., 1994). The two highest scenarios are labeled as the "extra high growth" cases, namely IS92e (Pepper et al., 1992) and IMAGE 2.1, Baseline-C (Alcamo and Kreileman, 1996).
One scenario, IS92f, shows high population growth (over 18 billion people by 2100) with comparatively low economic growth (about the same level as the mid-range cluster of scenarios, approximately US$300 trillion). At the other side of the scale are the two IS92 variants (c and d (Pepper et al., 1992)) with low population projections (about 6 billion people by 2100).
Primary energy consumption is another fundamental determinant of GHG emissions. Clearly, high energy consumption leads to high emissions. However, what is more important for emissions is the structure of future energy systems. High carbon intensities of energy - namely high shares of fossil energy sources, especially coal, in total energy consumption - lead to scenarios with the highest CO2 emissions. The primary energy paths of different scenarios are compared here, and the issue of energy carbon intensity is considered in the next section.
Figure 2-8 shows the primary energy consumption paths in the scenarios and its historical development since 1900. It gives the whole distribution of the 153 scenarios in the SRES database that report primary energy consumption, the median, and the 95th, 75th, 25th and 5th percentiles. As a result of the relatively large differences in the base-year values, the primary energy consumption paths are plotted as an index and spliced to the historical data in 1990. In 1990, primary energy was about 370 EJ, including non-commercial energy (Nakicenovic et al., 1996).
On average the global primary energy consumption has increased at more than 2% per year (fossil energy alone has risen at almost 3% per year) since 1900. Also, the short-term trend from 1975 to 1995 shows a similar increase. In the scenarios the average growth rates to 2100 range from 2.4% per year to -0.1% per year, with a median value of 1.3% per year.
For the full range of scenarios, the factor increase above the 1990 level is 0.9 to 10 by 2100. 10 However, Figure 2-8 indicates that this full range includes a few noticeable outliers, especially toward the high end of energy consumption levels. The rest of the scenarios are grouped more closely together, which compresses the range to a factor increase of about 1.5 to 7.5 times the 1990 level. The degree of clustering is discussed in greater detail below.
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