In my population lecture I said that world population growth would likely stop this century (e.g., Lutz et al 2001 put the stabilization odds at 85%), and that this would be a watershed event in the history of humanity because it would be the first time that populaiton growth stopped due to voluntary declines in fertility (the last time global human population stopped growing was in the 1300s, but this was due to the black death plague). The new analysis by Gerland et al doesn't change the expectation of eventual population stabilization, but does suggest that a 21st century arrival date for this world historical event is probably too optimistic, putting odds of 21st century population stabilization at only 30%.
The projected "failure to stabilize", as seen in this key figure (below) from Gerland et al, arises because of demographic trends in Africa. Unlike all other regions of the world, where population is expected to stabilize or decline, Africa continues to grow. This is because African fertillity declines, which were expected to follow the rapid trajectory previously seen in Asia and Latin America, have instead slowed or in some cases actually stalled. African population is now expected to nearly quadruple from a current level of near 1 billion to over 4 billion in 2100.
A key underlying reason that fertility in Africa has not declined is not so much that people want to have large families, but that women are unable to realize their desire for smaller families because they don't have access to birth control. As stated in the article, "the unmet need for contraception (the difference between the demand for contraception and its use) has re-mained substantial at about 25%, with no systematic decline over the past 20 years."
This appalling state of affairs is both discouraging and encouraging. It is discouraging for the obvious reason that this unmet demand is simultaneously both an unjust violation of the rights of women to control their own fertility, as well as an environmental degradation that will undermine the possiblity of sustainable human development in one of the places that needs it most. But it is also encouraging in that it suggests that, unlike many of the problems we cover in Global Change, the solutions don't rely on getting people to change their habits, but in providing families (and particularly women) with the means to do what they already want to do.
The study of Gerland et al thus reminds us of one of the most robust findings of the sociological and demographic literature on development: that the key to slowing populaiton growth is the education of girls and women, paired with greater voluntary access to contraception (see this recent essay by development expert Wolfgang Lutz on Population Policy for Sustainable Development). (and Andy Revkin, in his NYTimes blog Dot Earth, uses Gerland et al to call for more "cross talk" between the UN sessions on population and global climate change.)
