Those who ponder humanity’s future in the twenty-first century generally take at face value demographic projections suggesting that the world population will reach something like 9 billion around 2050 and will then stabilize at about that level. The widespread belief that this 30 percent increase from today’s 6.9 billion people is inevitable undermines consideration of the role of population size in climate change, water scarcity, biodiversity loss, rising energy prices, and food security. Contributing to this is the related view that efforts to prevent population growth would require coercive government policies that constrain couples from having the children and the family sizes they want. While some analysts are confident that the world can feed, house, and otherwise support 9 billion or more people, others are less certain, and voices of caution about population growth are heard more often than in the past. A logical application of the precautionary principle in the face of current environmental problems would suggest that humanity could more easily accomplish these feats in an environmentally sustainable manner with a smaller population.
In a joint statement in 1993, representatives of 58 national scientific academies stressed the complexities of the population-environment relationship but nonetheless concluded, “As human numbers increase, the potential for irreversible changes of far-reaching magnitude also increases. … In our judgment, humanity’s ability to deal successfully with its social, economic, and environmental problems will require the achievement of zero population growth within the lifetime of our children.” In 2005, the United Nations’ Millennium Ecosystem Assessment identified population growth as a principal indirect driver of environmental change, along with economic growth and technological evolution.
In October 2010, a group of US and European climate and demographic researchers published findings from an integrated assessment model calculating the impact of various population scenarios on fossil-fuel carbon dioxide emissions over the coming century. If world population peaked at close to 8 billion rather than 9 billion, along the lines described in a low-fertility demographic projection published by the UN Population Division, the model predicted there would be a significant emissions savings: about 5.1 billion tons of carbon dioxide by 2050 and 18.7 billion tons by century’s end.