Emissions Scenarios - IPCC

An Overview of Scenarios 199
Box 4-6: Possible IVansitions between Scenario Families: The A2-Al-MiniCAM Scenario
The four scenario storyhnes have been stylized as global socio-economic developments that evolve in different directions, but
globally and continuously. In reality, different regions may follow the developments pictured in the scenarios in different time
periods. For example, the world may in reality develop according to one of the storylines and after some time move toward
another. As an illustration of this, one scenario (A2-A1-MiniCAM) was elaborated by a member of the writing team. This
scenario is described here even though some members of the writing team considered its inclusion undesirable and possibly
confusing as it was submitted too late to have the team thoroughly discuss its consistency-^'* and to clarify its relationship to the
four storylines. However, the point that in reality transitions between scenarios are possible is a valid one and the contrasting
viewpoint is presented here for consideration of the reader, following IPCC practice.
The A2-A1-MiniCAM transition scenario explores a world m which the prerequisites for development, such as education,
effective institutions, and high saving rates, take some time to develop, so that rapid development does not begin to occur until
between 2020 and 2050 depending on the region. In this scenario, total GDP by 2100 is below the median of the historical
scenario data base and, with the population 20% higher than the current median UN forecast, average per capita income is at
the lower end of the historical data base range. In such a relatively poor world the economic structure is more sensitive to
environmental change than in the marker scenario, and the institutional stmcture is less capable. Thus, the impacts of climate
change are larger and the ability to adapt less than those in the Al world. The primary driving forces for the A2-Al-MiniCAM
transition scenario follow the logic of this story line, as detailed below.
Population is lower in the A2-A1-MiniCAM scenario variant than in the A2 marker, since its total completed fertility early in
the 2P' century is lower. This reflects the continued rapid historical declines and leads to slower population growth rates than
in the population scenario adopted for the A2 scenario family. Total completed fertility declines slowly in the forecast period
with a long-term asymptotic level of 2.25 for all regions, which results in a global population that is still growing by over 100
million per year m 2100. The values for migration and death rates used to generate the population trajectory follow those of the
UN median forecast. The population scenario of A2-AI-MiniCAM is quite close to the UN medium population projection (see
the discussion of the B2 scenario family below) until about 2060. Thereafter, however, A2-Al-MiniCAM's population scenario
continues to grow luiearly to about 12 billion, whereas the UN median projection stabilizes at about 10 biUion by 2080 and
slightly declines thereafter.
The regionally heterogeneous ("delayed") pattem of development of the precursors to rapid economic growth (e.g. education)
means that some developing regions experience stagnant or very slowly growth in per capita incomes well into the 2U' century.
As regions begin rapid development they approach and follow the average OECD labor productivity pattem, so GDP growth
rates accelerate post-2050 and average nearly 2.5% per year over the second half of the 21" century. GDP thus rises more slowly
eariy m the 21st cenmry in this scenario, reaching just over US$50 trillion by 2050. After 2050, GDP rises more rapidly to reach
just under US$200 trillion in 2100.
Per capita final energy demands are lünited by per capita income, and rise only as economic growth occurs. After growmg
slowly until 2050, per capita energy demands grow by more than 1.5% annually to reach 120 GJ per capita by 2100. With
increases in the efficiency with which services are provided, this results in a global average level of energy services similar to
that currently seen in Western Europe. Global per capita income and energy use by 2100 approach that of Westem Europe in
1990. Therefore global energy intensities m 2100 approach those of Westem Europe in 1990, a value lower than in other
scenarios but in line with current observations.
Natural gas and oil dominate the primary energy system, and contribute slightly more than half of the primary energy. Non-fossil
sources contribute about 30% of total primary energy, with coal providing the remainder. Total fossil energy carbon emissions reach
22 GtC by 2100. Sulfur controls are delayed in this implementation until economic growth takes off after 2050. With high levels of
fossil fuel use, and relatively low rates of control, sulfur emissions are about ten million tons higher m 2100 than they are today.
The relatively slow growth m output and productivity in the economy in general is mirrored in the agricultural sector, with lower
growth in agricultural productivity until post 2050. The large population complicates this problem, leadmg to large-scale
expansion of agricultural lands and a resultant decrease in forested and unmanaged lands, especially in developing regions.
Land-use changes do not offset any of these emissions, since the relatively high population, the rapid growth in income, and the
growth in modem biomass result in essentially zero carbon emissions from land use and agriculture, rather than the substantial
uptake seen in many other scenarios.
In particular, the consistency of a continued fast demographic
transition to 2050 combined with a scenario of stagnating per capita
income growth for as much as five decades is questioned by a number
of members from the writing team.