• Authors: Dean Spears, Sangita Vyas, Michael Geruso, Gage Weston
  • Published in: PLOS ONE
  • Download paper

Abstract

The size of the human population is projected to peak in the 21st century. But quantitative projections past 2100 are rare, and none quantify the possibility of a rebound from low fertility to replacement-level fertility. Moreover, the most recent long-term deterministic projections were published a decade ago; since then there has been further global fertility decline. Here we provide updated long-term cohort-component population projections and extend the set of scenarios in the literature to include scenarios in which future fertility (a) stays below replacement or (b) recovers and increases. We also characterize old-age dependency ratios. We show that any stable, long-run size of the world population would persistently depend on when an increase towards replacement fertility begins. Without such an increase, the 400-year span when more than 2 billion people were alive would be a brief spike in history. Indeed, four-fifths of all births—past, present, and future—would have already happened.

Erratum 23 July 2025: "The novel contribution of our paper is long-term cohort-component population projections beyond 2100. For motivation and context, we plot these, in Figure 1, alongside historical estimates of the world population size that we did not produce. In our paper, we reported “Historical data are taken from Kaneda and Haub (2022)”, which are retrospective estimates published by the Population Reference Bureau. This citation is incorrect. We thank Professor Philip N. Cohen for bringing this error to our attention. In fact, the historical estimates plotted alongside our projections in Figure 1 track estimates from Our World in Data, which itself is based on further sources. A 2023 New York Times article by coauthor Spears referencing this article correctly reported Our World in Data as the source for the historical data. This citation error does not impact the main contents or understanding of the article. Nevertheless, we regret it."