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| Table 3.1: Model drivers | | Table 3.1: Model drivers |
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| <h2>Drivers and types of scenarios </h2>
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| Under baseline conditions scenario drivers are typically assumed to develop either along a pathway that is considered a best guess translation of current trends into the future. Alternatively, multiple contrasting scenarios can be considered to explore the range of plausible future trends. The former approach is chosen in many studies, such as the OECD Environmental Outlook (OECD, 2012), with the specific aim to serve as starting point for policy interventions aiming to improve the baseline outcomes within and across sectors and domains. The latter approach recognizes structural uncertainties in how the world might unfold, and aims to explore how such uncertainties would play out in future ranges of outcomes. They also serve to investigate how robust policy interventions play out under different future conditions. Examples of multiple baseline studies are (IPCC, 2000), (Millennium_Ecosystem_Assessment, 2006). See Figures 3.2 and 3.3 for illustrative results from the so-called Shared Socio-economic Pathways (SSPs), recently developed to support climate change research across different research communities. (Moss et al.; Vuuren et al., 2012). An important element of the SSPs are the qualitative narratives or storylines characterizing alternative futures. From there, assumptions are made about internally coherent sets of scenario drivers, and finally key model drivers such as population and GDP growth factors.
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| Figure 3.2 illustrates the wide range in the population size projected for the long term: by 2100 the world could see around the same number of people as today, or about twice as many. The projections are made by IIASA, using a population (Millennium_Ecosystem_Assessment, 2006)modeling approach that links aggregate education levels to fertility and mortality rates per country, and together with migration flows these determine the future population size.
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| Taking the population projections and the underlying educational attainment per age cohort, long term economic growth models are used to project the associated economic development expressed as GDP per capita. Together with the population the total GDP is calculated. This was done by three different teams at OECD, IIASA and PIK, each using their own models. Figure 3.3a shows the GDP projections until 2100 from the OECD model ENV-Growth, differing by a factor 3.7 in 2100. As the most populous scenarios are at the lower end of the GDP range, the differences in GDP per capita will be even bigger than that. Differences in population and economic growth rates between different countries and regions make that the distribution of total economic assets over different parts of the world are bound to shift, with Asia and later Africa and to a lesser extent Latin America becoming dominant in the world; see Figure 3.3b.
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| As mentioned, different models were used by teams at different institutes, allowing to investigate how different model structures and assumptions, but also different interpretations of the qualitative scenario story lines in model parameters, lead to quite different results (Figure 3.3c). The projections for the SSP3 scenario not only differ with respect to the level in 2100, but also in the pathway over the century.
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| For more information on how Baseline scenarios play out in more detail in economic, social and ecological terms, see the results per module of the IMAGE 3.0 framework in chapters 4 through 8.
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| Once Baseline scenarios are implemented, modifications at various levels can be made that reflect policy interventions aiming to diverge from the trends emerging under baseline conditions. This can take many different shapes and forms, depending on the subject, scale, timeframe and policy levers under consideration. Reducing climate change impacts is one obvious example, but also reducing nutrient loading of coastal sees, slowing down the rate of biodiversity loss, reducing water stress, and many other options to alleviate anticipated future problems have been explored with IMAGE during the last decade. Again, for more information on policies, instruments and goals pursued, see Chapters 4 through 8 in this book.
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| <h2>References</h2> | | <h2>References</h2> |
