Revised 29 August 2021

All self-regulating systems use negative feedback processes to counteract the positive feedback processes which would otherwise drive the system towards self-destruction. This includes nature where ecosystems are prevented from exceeding their carrying capacities with the negative feedback process of succession, the gradual and orderly process of change in an ecosystem brought about by the progressive replacement of one community by another until a stable climax is established. (see Section *** for more detail). 

In the distant past, continued growth of human populations was limited and restricted by human intervention by way of the practice of abortions, infanticide, war, and the casting out of the elderly from the community. Nature also intervened by way of disease, pestilence, and famine.  If humankind is to continue to survive for many more millennia, and if humans are to live a peaceful and joyful life and die a natural death, then humankind has no choice but to monitor and regulate its reproduction cycle so that the number of births each year do not exceed the number of deaths. 

Self-regulation of our reproduction cycle can be voluntary or by way of coercion. Methods of birth control include abstinence and contraception. Putting morality aside, if contraception fails, then abortion is a backup. The ultimate contraception is sterilisation. Although sterilisation is a less invasive procedure for a male partner than a female partner, sterilisation of only a male partner does not necessarily prevent further births.  

Responsible family planning and  voluntary self-regulation is preferable. With regards to curbing and monitoring population growth, the catch-phrase of think globally, act locally translates into a local community  being a group of responsible parents who restrict their average family sizes to replacement level of about 2.11 children per family. In order to maintain a ZPG population there is no reason why some families cannot and should not have more than two children. After all, some couples will choose to have no children and some females and males will be infertile. The mechanics of ensuring that the average family size does not rise above replacement level is the issue and not the number of children born into any particular family. 

The decline in total fertility in the high-income countries below replacement level has not been due to coercion by government, social pressure, or any conscious majority awareness of the need to curb population growth. There is a strong relationship between the level of income of a country and its total fertility. A high level of national income tends to be accompanied by a low total fertility. Middle-income and low-income countries have a long way to go before they achieve the same high levels of income and low levels of total fertility. Financial support from high-income countries would accelerate the closing of the gap. 

A number of organisations have estimated projections of the world’s population. All projections agree that the world population in the future will be bigger though increasing in size less rapidly, will be more urban, and will be an older population (Bongaarts & Bulatao, 2000). The United Nations World Population Prospects 2017 Revision states:

“… the analysis has concluded that, with a certainty of 95 per cent, the size of the global population will stand between 8.4 and 8.7 billion in 2030, between 9.4 and 10.2 billion in 2050, and between 9.6 and 13.2 billion in 2100.  … there is roughly a 27 per cent chance that the world’s population could stabilize or even begin to fall sometime before 2100.”

Projections are not predictions and are based on many assumptions (Cohen, 1995). Some projections are less likely than others depending on the extent that the population of a country surpasses the carrying capacity of the local environment and its dependence on imported food, water, and other critical resources. Political strife between countries is also highly unpredictable and control of fossil fuel supply chains and the peaking of fossil fuels and minerals will become major issues. Whatever the future holds in store for us, continued growth in the world population before 2100 is inevitable unless dire consequences of climate change, inadequate access to energy be it fossil fuels or renewable energy, political strife, and over-reaching the carrying capacity of a local environment take over. All that current and future generations can do during a transition to ZPG is to be better prepared and become more resilient to change.