Scenarios, uncertainty and conditional forecasts of the world population

Abstract

Current official population forecasts differ little from those that Whelpton made 50 years ago either in the cohort-component methodology used or in the arguments used to motivate the assumptions. However, Whelpton produced some of the most erroneous forecasts of this century. This suggests that current forecasters should ensure that they give users an assessment of the uncertainty of their forecasts. We show how simple statistical methods can be combined with expert judgment to arrive at an overall predictive distribution for the future population. We apply the methods to a world population forecast that was made in 1994. Accepting that point forecast, we find that the probability is only about 2% that the world population in the year 2030 will be less than the low scenario of 8317 million. The probability that the world population will exceed the high scenario of 10736 million is about 13%. Similarly, the probability is only about 51% that the high-low interval of a recent United Nations (UN) forecast will contain the true population in the year 2025. Even if we consider the UN high-low intervals as conditional on the possible future policies of its member states, they appear to have a relatively small probability of encompassing the future population.