What accounts for such improvement? Zlotnik said it has a lot to do with greater availability of data. “The UN has been…mandated by governments to produce population estimates since the 1950s,” she stated, and they have refined their projections process over the years based on an increasing record of data and identification of some “patterns that can inform the future.”
Former UN Population Division Director Hania Zlotnik spoke recently at the Wilson Center discussing last year’s highly-publicized UN world population projections and explaining the methodology behind the figures. “The latest projections are very special,” she said, adding that “we think that this methodology is a lot better than what we had before.”
What accounts for such improvement? Zlotnik said it has a lot to do with greater availability of data. “The UN has been…mandated by governments to produce population estimates since the 1950s,” she stated, and they have refined their projections process over the years based on an increasing record of data and identification of some “patterns that can inform the future.”
The increasingly large dataset accumulated during this process is particularly important because of what Zlotnik called “inertia” in population change. “There’s something called ‘population momentum,’ where the population changes on the basis of how many people have already been accumulated on the planet,” she explained, pointing out that many of the people included in the UN’s population projections for 2050 have already been born.
This inertia helps make population projections fairly accurate, but last year’s projections differed from previous efforts in two significant ways.
First, the UN Population Division provided projections not just to 2050 but also to 2100. These more long-term projections estimated an end-of-century population of more than 10 billion – notably higher than previous reports, which had predicted that population would stabilize mid-century at around nine billion.
Second, the underlying methodology used shifted from a deterministic to a probabilistic approach that, according to Zlotnik, does a better job capturing variability in each country’s fertility rate over the next century. “Essentially,” she explained, “a model was developed for every country that takes into account the past path of fertility change [and] also takes into account changes that have happened in other countries…Then [it] does a simulation for the future for every country, in which 100,000 paths are projected, essentially by throwing dice, and then the central path of those 100,000 is used to project the future.”
Zlotnik cautioned that projecting so far into the future is an inexact – albeit necessary – science: “Of course, as we go further into the future, the numbers are more subject to uncertainty. But [the long-term projections] help us give people a better feeling of how important it is to change trends from here to 2050, so that we ensure that, at the end of the century, the number of people on the planet is sustainable.”
“In order to make sure that the population projections made by the United Nations [are realized],” Zlotnik said, “it’s very important that fertility continues to decline, and especially that fertility decline happens in the countries that still have very high fertility.”
“Fertility decline happens in many ways,” she continued, “but the immediate reason why fertility can be reduced is that more people use modern methods of contraception.”
A Publication of the Stimson Center.
