The Population Division of the United Nations Department of Economic and Social Affairs released the 21st round of its official global population projection, the 2008 Revision, on March 11, 2009. The 2008 Revision suggests that under a medium variant assumption, in which the total fertility rate (TFR) will decline from 2.56 children per women in 2005-2010 to 2.02 in 2045-2050, the world population will likely increase from 6.83 billion in 2009 to 9.15 billion in 2050. If TFR were 0.5 higher than in the medium variant – as in a high variant assumption – world population would reach 10.5 billion. If TFR were 0.5 lower than in the medium variant – as in a low variant assumption – world population would still increase to 8 billion. Therefore, global population growth seems inevitable even if fertility decline accelerates. The trend that all additional global population growth will occur exclusively in the developing world has not changed.
However, compared with the projection results in the 2006 Revision, the medium variant global population size in 2050 is 40 million lower in the 2008 Revision . To a certain extent this result might be a surprise to those who have been tracking the UN population projection records during the past six decades. Since the end of the last century, the UN Population Division’s population projections had continuously adjusted the global population size in 2050 upward from 8.91 billion in the 1998 Revision to 9.19 billion in the 2006 Revision.
In fact, there are a number of factors in the new projection that should contribute to a larger rather than smaller population size. Global TFR in the baseline period (2005-2010) increases from 2.55 in the 2006 Revision to 2.56 in the 2008 Revision. The TFRs are then assumed to decline to the same level (2.02) in 2045-2050 in both of the revisions. Moreover, the assumptions of life expectancy at birth have also been adjusted upward from 67 years to 68 years for the baseline, and from 75 years to 76 years for 2045-2050. The success of HIV/AIDS prevention and treatment has reduced the number of countries considered to be highly affected by the disease from 62 countries in the 2006 Revision to 58 in the 2008 Revision. The assumed continuous effort of HIV prevention and treatment will lead to an estimated 30 million fewer deaths from HIV/AIDS during 2005-2020 in the 2008 Revision than were projected in the 2006 Revision. Based on these facts, one would expect that the newly revised projection should result in a larger population size than the previous Revision.
What causes the smaller population size by year 2050 in the new projection? Analysis of variations in regional fertility levels tells us that the projected 40 million decline in population in the 2008 Revision is almost completely due to an assumption of significantly declining fertility rates in the 49 least developed countries.
For the baseline of 2005-2010, the estimated fertility rate for developed countries increases from 1.60 in the 2006 Revision to 1.64 in the 2008 Revision. This increase might be based on the recently observed growth in fertility levels in Western Europe. The estimated baseline fertility level for the developing world, excluding the 49 least developed countries, increases from 2.45 in the 2006 Revision to 2.46 in the 2008 Revision. Moreover, the assumptions on TFRs for the middle of the century in the 2008 Revision also increase from 1.79 to 1.80 and from 1.91 to 1.93 for developed countries and developing countries (excluding the least developed countries), respectively. These changes cause a slightly higher global fertility rate in both the baseline and future years in the new Revision.
However, TFR from the least developed countries is assumed to be significantly lower in the 2008 Revision than in the previous Revision: decreasing from 4.63 to 4.39 in the baseline year, and from 2.50 to 2.41 in 2045-2050. This is the key factor contributing to the 40 million reduction in global total population by year 2050. Another cause of the lower population projection could be the higher estimated net migration flow from high fertility areas in developing nations to low fertility regions in the developed world – the annual average net migrants increases from 2.3 million in the 2006 Revision to 2.4 million in the 2008 Revision.
The changes in the assumptions of TFR in various world regions reflects the fact that UN population projections, like most other population forecasts, maintain a strong faith in the theory of demographic transition and assumes that the world will soon converge demographically. In the 2008 Revision, the assumption of demographic convergence becomes even stronger – the TFRs of different world regions are expected to converge to replacement level (around 2.05 children per woman) more quickly than in the previous Revisions. However, this projected demographic trend is contingent upon and sensitive to the speed of fertility declines in the least developed nations. The 2008 Revision report indicates that “if fertility were to remain constant at the levels estimated for 2005-2010 … the world population could increase by nearly twice as much as currently expected”; and “to achieve such (TFR) reductions, it is essential that access to family planning expands, particularly in the least developed countries.”