(linked here )
is one of the key sources of global population projections for the DBIS Foresight Project on Global Food and Farming. That project forecasts that, probably, global population will peak at between 9 and 10 billion in the second half of the 21st century, and will then decline somewhat from that point onwards. One might be lulled into a false sense of security that, because of this, the sustainability challenges facing mankind will somehow sort themselves out or be in some way self-limiting.
When I look at the population projections and the methods used to calculate them, however, it occurs to me to apply some healthy scepticism. There is potentially an element of self-fulfilling prophecy in some of the projections. Sometimes, the methods make assumptions about long-run fertility rates settling on a number, say, slightly above 2 or (more often) slightly below 2. Consider this - a rate of 2 would (theoretically) produce a static total population (all other things being equal, for example mortality rates equalling birth rates). A rate of above 2 would result in an ever-increasing population and a rate of below 2 would result in an ever declining population. Therefore, the 'accuracy' of the projected rate, and changes in this rate over time, are of critical importance in determining the population projections in the longer-term. Sensitivity analysis on this key variable should therefore be a main subject of focus when critically evaluating such projections. Some such analysis has already been done, and is referenced in the report.
The main chart from the analysis is shown below: