The population sector of the model drives the evolution of every other parameter of the model, and is in turn affected by other sectors via their aggregate effects on birth rates and life expectancies.
The world population is divided by age into three categories: 0 to 14 years, 15 through 64 years, and 65 plus years. The global workforce is assumed to be 75% of the middle-aged group. Population development is affected by factors including GDP, education, food security, and environmental factors including pollution levels, water quality, and biodiversity. The FeliX global population projection, shown below in red, is calibrated to historical data from the FAO (shown including FAO extrapolations to future population in grey).
In the BAU scenario, the world population is expected to peak just above 10.6 billion people near the end of this century. For comparison, all five SSP projections are also shown through 2100. The specific factors informing the FeliX population projections will be detailed in a subsequent post.
Population growth in this model is validated by comparison to figures from the World Population Prospects: The 2012 Revision, and the associated Science paper "World population stabilization unlikely this century", both publications of the UN Department of Economic and Social Affairs (UNDESA). This research identifies a median expectation of 10.9 billion, adding:
There is an 80% probability that world population, now 7.2 billion people, will increase to between 9.6 billion and 12.3 billion in 2100.
Because global population numbers are central to every other sector in the FeliX model, we use the high and low projections from UNDESA probabilistic projections (representing the 80% confidence interval) to examine the sensitivity of other figures of merit to the full range of likely fertility rates.