The issue of human overpopulation has fallen out of favour among most contemporary demographers, economists, and epidemiologists. Discussing population control has become a taboo topic.

The silence around overpopulation prevents us from making the necessary link between the planet’s limited ability to support its people (its carrying capacity) and health and development crises.

Human carrying capacity is the maximum population that can be supported at a given living standard by the interaction of any given human-ecological system. This apparently simple concept has many nuances and is rarely used by population scientists. However, in rejecting this term, purists risk making a terrible conceptual flaw, that of thinking that environmental and human resources are largely irrelevant to human population size.

It is irrefutable that human ingenuity and cooperation can increase human carrying capacity. But even so, human welfare will continue to depend on the external world, including for resources such as food and water. Humans are neither computer ciphers nor caged mice.

That is to say, while a given area might tolerate a theoretically higher density of human population than it does, the reality of human evolution in distinct groups, separated by culture, religion, and language, means that this theoretical maximum will rarely be attained. A degree of underused carrying capacity can be viewed as a desirable buffer around disparate groups, vital for reducing tension and preventing conflict.

The concept of human carrying capacity is contested. Many academics reject it altogether, but not all. I think of it as emerging from the interaction of five forms of “capital”:

human (knowledge, skill, health, ingenuity)

natural (fossil fuels, uranium, metals, climate, soil, phosphate, food, forests, fish)

social (inequality, inter-and intra-group rivalry, including based on ethnicity, language, religion, class, caste and “claste”)

physical (technology, infrastructure)



financial (tradeable currency).

Substitution between these forms of “capital” is imperfect (they are not all capitals in a strict economic sense, hence the inverted commas). Economically, capital is something that produces income, without necessarily being itself consumed, such as land used to grow an orchard, or money in a term deposit. Human “capital” can be accumulated by investment in education and by providing good nutrition, but can be destroyed, not only by ageing or illness, but also by violence and even genocide.

Diminishing phosphate supplies (essential for life) in one place can be compensated by its import (say, from guano or mines), or of food grown with phosphate elsewhere. Ingenuity can seek new forms of phosphate (from a new mine) but the process is not infinite; complete substitution eventually fails. Human and physical capital partially substitute fossil or nuclear fuels, for example by the construction of water turbines, or using silicon or “power tower” technology to capture some solar energy.

Social capital is also crucial. Consider Ireland in 1845 and Rwanda in 1993. In both countries food supplies per person were tight but starvation was rare. A population crash in each country within five years was not inevitable, but in each case occurred. In Ireland the potato blight devastated crop yields over several harvests. English relief (social capital) was scarce. Human capital to combat the blight, or cure the typhus was insufficient. But social capital among most of the Irish - especially within each of the two main forms of Christianity practised - was fairly high; direct violence among the starving was rare. Starving people are too weak to kill, but semi-starving people can, as can well-fed people who foresee impending scarcity of food or other resources.

The 1994 Rwandan genocide typifies an emergent social phenomena, triggered by a population resource imbalance, catalysed by social-political events including ethnic rivalry. A more harmonious Rwandan population would not have become genocidal in 1994, even with the same population. But had its population continued to rise, with no compensatory increase in other elements of carrying capacity - say, ecotourist - calamity is still likely to have occurred, but later.

Carrying capacity can be analysed at any scale. I live in a small flat with plenty of space for two. Twenty could fit in, but not 200. Twenty people would need very high co-operation to survive for long. If Canberra was cut off from the rest of the world for all time we could survive for many years, even without fossil fuel, phosphate and metal. We could live off potatoes and mutton, recycle our nightsoil for fertiliser and so on, but only if we had enough co-operation and sufficient human capital to resurrect a hybrid pioneer-Aboriginal lifestyle. Poverty and hardship would increase; life expectancy would fall; as would population size. Firewood and draft animals would be our main energy source. Human carrying capacity of this region would decline.

But though Canberra could subsist in this scenario many cities could not; their populations are too big. And in many settings riots - which waste physical capital and are costly in energy terms - are more realistic scenarios than cooperation. In some places, forms of ethnic cleansing are likely to become far more attractive “solutions” than everyone more-or-less equally sharing poverty and hardship for the foreseeable future.

Global factors are also important. The carrying capacity of Hong Kong is enhanced by its links with the world; its trade. The rising cost of energy is an under-recognised factor in the European economic crisis: they spend about $1 billion per day importing energy. But diminishing fossil fuel supply is not necessarily calamitous – if our collective ingenuity is sufficient to create adequate substitutes. So far it is not.

We face a race between problems and solutions. As the joke goes, “An economist and an ecologist have fallen from a tall building. The ecologist is panicking but the economist remains calm. Don’t worry, she says: "demand will create a parachute”. But to open the parachute we need to see the ground.

At the 1992 Rio Earth Summit, US President Bush made it clear that the US lifestyle was non-negotiable. Most over-consumers are effectively “locked in”, if not truly addicted to their consumption. They will support collective appropriation of resources to maintain their lifestyles for as long as possible, even if at the expense of other human beings. If these people are far away and “invisibilised”, and if the guilt is borne collectively rather than individually, so much the better.

Our world produces food for about 9 billion people, were it fairly distributed. But as energy costs rise, as extreme weather events continue and probably increase, as sea level rises, and as both poverty and global unfairness seem likely to remain so resilient, I can only conclude that major problems lie ahead. We need that parachute. In fact, we need lots of them.