On a finite planet, nothing physical can grow indefinitely. The more of us there are, the fewer resources there are for each of us and for members of other species with which we share the planet.
The challenge of sustainability
The question of human population size is fundamentally one of sustainability. Only resources that can be sustained indefinitely can support humanity in the long term. Given this limitation, we must consider carefully our consumption, what living standards are acceptable, what technologies make best use of the resources available and how to maintain the ecosystems on which we depend. We are already eating into our capital: according to the World Wildlife Fund / Global Footprint Network Living Planet Report, we are collectively consuming the renewable resources of approximately 1.5 Earths, although people in the developed world consume much more than people in developing countries.
There are no magic numbers — only trade-offs. Any given area of land can sustain many more very low-consuming poor people at bare subsistence level than high-consuming rich people living like millionaires. Population will certainly stop growing at some point. This will either be sooner by fewer births — the humane way of informed individual decision-making on family size — or later by more deaths — the natural, inhumane way of famine, disease and predation or war. There is no third alternative of indefinite growth.
The sustainability equation
For activities to be genuinely sustainable it must be possible for them to continue indefinitely. This was theorised in 1968 by American biologist and educator Paul R. Ehrlich in his book, The Population Bomb.
The impact that human beings have on the environment and the numerous demands that people place on the resources that are available on the planet can be summarised by what is known as the Ehrlich or IPAT equation, I=PAT. I = impact on the environment or demand for resources, P = population size, A = affluence and T = technology.
The two most important conclusions deriving from this relationship are that Earth can support only a limited number of people in a sustainable manner and humanity has a clear choice between more people who have poorer lifestyles and fewer people who have a better quality of life.
Read more about sustainability and the Ehrlich equation.
A stable and healthy environment
Living sustainably means maintaining a stable and healthy environment for both humanity and biodiversity. The implications are radical. A sustainable society, i.e. one that could physically be sustained indefinitely, will need as a minimum a stable or reducing population, very high levels of reuse and recycling, 100 per cent renewable energy and no net loss of soil or biodiversity. No country is yet near it.
This requires a radical change in outlook and our consumption, our technology choices and our population numbers in order to live within the means of the planet. That is something we simply are not currently doing. We must tackle all three if our children and grandchildren are to have decent lives.
We need to move towards sustainable consumption. This means matching our individual and collective consumption to the resources available. Sustainable business and governmental policies would ensure the take-up of renewable energy and material resources while phasing out those with adverse side effects. We need to prioritise the development and use of sustainable technologies to limit the collective impact of our activities on the planet. But in a finite world even renewable resources are only available in limited quantities, so it will still be essential to minimise waste of energy, water, food and other commodities.
And we need to stabilise our numbers at a sustainable population level that consumes resources at a rate the Earth can support. Halting population growth and, in many countries, reversing it is a vital part of living sustainably. In some societies, population growth has already slowed or stopped. Typically, the empowerment of women and improved availability of contraception have played major roles.
A stable and ethically acceptable transition to sustainable numbers can only occur gradually. For densely populated, high-consuming countries, this may require several generations to complete. If our children and grandchildren are to inherit a world worth living in, the process must begin without delay.