Fred Pearce's view of the latest United Nations population projections misrepresents our results (Nature 473, 125; 2011). The causes of the differences between the 2008 and the 2010 revisions are more complex and varied than he conveys.

The figures he cites to show that fertility estimates are lower than those made two years ago are the projected fertility figures for 2010–15 in the 2010 revision. The most recent estimates refer to 2005–10 and show an increase in fertility with respect to the 2008 revision in 84 countries, a decrease in 56 countries, and no change in the other 57 countries considered. Estimated fertility for 2005–10 is higher in the 2010 revision for both developed and least-developed countries.

In the 2008 revision, projections stopped in 2050. The differences in fertility between the 2010 revision and the 2008 revision in 2045–50 are generally small: 110 countries have higher fertility and 87 have lower fertility in the 2010 revision. In about half of those countries, the differences arise partly from revised higher or lower estimates, respectively, for 2005–10 (for a peer-reviewed description of projection methodology, see L. Alkema et al. Demography; in the press).

Contrary to Pearce's suggestion, no country maintains a fertility of 2.1 children per woman between 2010 and 2100 in the medium variant of the 2010 revision. In that variant, projected fertility is lower than 2.1 by 2095–2100 in 182 of the 197 countries. Furthermore, the populations of 118 countries — 55% of the world population — are projected to be declining by 2100, indicating that their fertility levels are projected to stay well below 2.1 for several decades.

The medium variant of the 2010 revision produces a 2050 world population that is, as Pearce notes, 156 million higher than that projected in the 2008 revision. This 1.7% difference is comparable to that between earlier revisions produced during this decade.