If any or several of the answers to the questions above is less than the current population, the question becomes: What are the most appropriate and important measures to reach a long-term sustainable global population, according to the best available scientific evidence? In this report we summarise the results of our research on population. [...] Overview of main demographic approaches In this section, we provide a general overview of the currently more-influential approaches to the question of global demographic trends simulation and an analysis of the observable methodological differences, in order to provide the foundations required to understand our Earth4All assumptions and methodological choices, and their relation to the aims we int. [...] At the other end of the spectrum, we find World3, a system dynamics model of the interactions between population, industrial growth, food production and limits in the ecosystems of the Earth, originally produced and used by the seminal Club of Rome study providing the foundations for the book The Limits to Growth (1972). [...] While estimates based on different behavioural/policy assumptions can be used in the model, and the different results compared, the use of an exogenous demographic component is equivalent to the assumption that none of the mechanisms included in the IAM interact in any way with the demographic process. [...] The choice of modelling approach ultimately depends on the assumptions regarding (1) the main drivers of demographic trends; (2) the primary policy levers that affect demographic developments; and (3) the level and complexity of interactions between demographic mechanisms and the socio-economic–natural nexus.
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