Human Numbers

Guest Post by Dr. Dorothy Bennett

One particular number will undoubtedly be of the utmost importance for the future of humankind. That is: the maximum number of humans that our planet can support – meaning continue to support for hundreds and thousands of years. If we want the human species to survive in the long term, this may be the single most important issue in the world, for reasons now to be discussed. Obviously, the number of people on the earth cannot grow forever: on a finite planet the number of people must stop increasing eventually and become stable (unless it oscillates). There seems to be no broad, international consensus on what that upper supportable limit is. However, whether we choose the highest or the lowest available estimates, it is clear enough that in this 21st century this number will be reached, or perhaps already has been reached. The consequences for our children are potentially so far-reaching that all of us should be concerned.

Some serious estimates place the sustainable world population at only 0.5-2 billion people [see references 1,2], compared to the 6.9 billion alive now and the 9.1 billion forecast by 2050 [3,4]. For example, one detailed estimate, based on a “modest” average lifestyle something like that in Eastern Europe today, is around 3 billion [5], including about 23 million in the UK and 254 million in the USA. But even this total estimate seems not to take into account the substantial loss of fertility of land and sea in the tropics, which is forecast as a result of further global warming [6], and which may reduce the world sustainable figure to 2 billion or less [2]. If the true figure is much below 9 billion, then the unpleasant truth is that billions of human deaths can be expected, as a direct result of this overpopulation, and most likely within the lifetime of our children.

An obvious response is: how could it be only 0.5-3 billion, when the earth seems to be supporting 6.9 billion people already? But while life may seem fine in 2010 for a privileged few of us, many people are already starving, food and fuel prices are already rising steadily. The era of such human numbers has depended on using up certain resources, especially oil, that are not renewable and that will soon be gone.

Biologists are familiar with the fact that any species that overpopulates its habitat will die back, usually to a level well below the sustainable number. The extra deaths may be through disease, fighting for territory or food, lack of food or water, or poisoning by accumulation of waste products. Humans have no magical way to escape this consequence of overpopulation. Indeed, we humans add to the problem some damaging artificial waste products, especially extra carbon dioxide by the burning of fossil fuels and deforestation, identified by ever more evidence as the major cause of global warming [6]. (At least this activity is likely to decline shortly, as fossil fuels become exhausted [7].) We also add to the equation our awesome skills and technical aids in killing one another. The situation is unprecedented. We have never before approached the limits of our global habitat’s long-term capacity to support us, but now we are, or we have reached and overshot it already.

It is urgent for us to adjust our visions and plans to this new situation. Not only will human population growth stop within this century, but our numbers will almost certainly have exceeded the supportable number, and will have to fall back. Our best hope is that we may reach a stable population size after that. The requirement for this to happen is a daunting one: no less than changes in the behaviour of the entire human species. But perhaps, when it is realised that the survival of the species really is at stake, we can change.

Skipping for a moment to a possible future: if we can work intelligently to stabilize our numbers and our environment (the planet), how many humans might there finally be, say by the 23rd century? Ideally we would hope that populations will not only survive but also have a tolerable life, with enough food, shelter, medical and social support and so on. But we could cope with a more modest lifestyle than is currently the norm in say Western Europe. So perhaps there could be as many as 3 billion of us. Habitable land would be nearer the poles than now, following the overheating of the tropics, forecast to be irreversible [2]. A diversity of other animal and plant species would probably be maintained, including much forest to sustain the carbon balance. We will probably eat a lot less meat and dairy produce. Solar power may be much utilized, much perhaps transmitted from the baking tropical deserts. We may still be using nuclear fission, perhaps more geothermal energy, and perhaps there will be new sources of clean energy (although we should not put our hope in that as an excuse for neglecting to take other actions now). We will use a lot less energy per person than a European in 2010 – travelling less, transporting food and goods less, insulating buildings more, discarding much less of everything, designing for durability.

However, returning to the bad news, it should be stressed that a stable, tolerable world like this is likely only if a worldwide consensus is reached on stabilizing human numbers. Even then, it seems certain that there will be steadily increasing hardships to come in the mean time. Energy and food prices will rise implacably as resources become increasingly stretched by the increasing global population. This is already visibly happening. The cost of manufactured items will rise with the cost of the energy needed to make them, made worse by the exhaustion of fossil fuels. Global oil production (from a careful data-based method of estimation) appears to have peaked already in 2006 [7]. That is, production has now begun falling, as reserves are used up. Industry and national economies will come under increasing strain – also already happening. There will be many environmental refugees from the tropics to more temperate lands, further increasing the pressure on all resources. The new ability to colonize polar regions will not compensate for the major loss of habitable land near the equator.

A huge change in vision and behaviour will be needed to address this new situation. One shock for many will be the outdating of current economic theory. Few people as yet seem to question the mantra that economic growth is desirable. Yet just a little thought indicates that growth is not invariably good, and anyway it cannot go on for ever. When population stops growing, then economies too will have to stop growing, because production and consumption by each person cannot rise forever.

This however need not be a problem. There is no need to produce or consume more per person than say Europeans typically do in 2010. Indeed, given the need to conserve resources and reduce carbon emissions, we should work towards producing and consuming quite a lot less than Europeans in 2010. This however apparently requires the very foundations of economics to be re-examined. The unquestioned assumptions regarding growth may prove to be a major cause of the excessive environmental impact of humanity. It is time for a new economic theory founded upon the goal of the long-term survival and stability of our species, and of our planetary ecosystem. I believe some economists are working on that; good for them.

Perhaps the most difficult issue to consider, in a vision for future survival, is what if anything we can do about the human population explosion itself, the root cause of all these problems. Though a difficult, controversial topic, we must address this. We could start by remembering that the rate of population change depends on just two quantities: the birth rate and the death rate. If there are more births than deaths today, then the population will increase. If the number of deaths is greater, it will decrease. We can change population growth only by changing one or both of these quantities. Aiming to increase the death rate would surely be against all humanitarian principles and conscience, even for the good of the species and the planet. So the only ethical action available to us on this is to seek ways to reduce the birth rate.

Birth rates are already below replacement level (below the death rate) in some (mostly European) countries, but overall the world population continues to increase [3,4,8]. In China of course, families have been limited to one child by law since 1980, but democratic nations generally see this law as inhumane (as do I). Alternative approaches include education and voluntary action, based on love of humanity and the wish for our descendents to have a planet on which they can survive. Given that many couples in many nations would apparently prefer to have smaller families but cannot get contraceptives [8], perhaps much can be achieved by such voluntary means.

From the foregoing arguments, it seems that this is the biggest of Big Issues facing us all, both in the future and now, though it is not yet widely discussed. It seems high time that we awake to this ticking population bomb – or perhaps reawake, since that phrase was coined by Paul Ehrlich in 1968 when the coming explosion was much less obvious. People, the media and governments should be discussing population pressures at least as much as CO2 emissions, or more, since human CO2 emissions depend on human numbers – more people make more CO2.
There is much resistance even to thinking about reducing family size. Many social customs and religious beliefs are opposed to the whole idea. And yet some ardent religious opponents of contraception may think again when they consider that it is the runaway success of human reproduction that is at the root of such problems as global warming and depletion of fuel and food resources. Others will raise the “demographic problem,” that reducing the birth rate would lead to many elderly and infirm people having to be supported by the work of too few young adults. But they are perhaps forgetting that the same young adults would have relatively more resources each, and fewer child dependents. The Chinese have seen no such problems, but are experiencing increasing success and prosperity. We will eventually have to cope with such an age distribution anyway, since population size cannot grow infinitely on a finite planet. If humanity does achieve stability and survival, the birth rate will finally equal the death rate (or it would not be stable). A third counter-argument is that people will stop having too many children if poverty is overcome. This may help, but women are still having more than two children on average, in the world’s most prosperous country, the USA [4]. Additional approaches are thus needed.

A strong political will may be needed to start thinking in these very different ways. Perhaps national governments will never be able to switch to thinking for the benefit of humanity – or perhaps they will. Perhaps at least our leaders will think of their children and grandchildren.

Is it true that action on these matters is urgently needed? The available information suggests that it is more than urgent – probably already too late to avoid a human die-back, which may be beginning already in some countries. As a speculation, our global numbers may peak by 2050 or sooner. But still, it is clearly urgent to act, for the sake of our families. The sooner we can begin, the fewer deaths may occur, and the sooner we may achieve the population stability and economic stability needed to get humanity through most of its future history.

Dot Bennett, Professor of Cell Biology
St George’s, University of London

Sources (chosen to be freely accessible)
[1] Living Planet Report. IUCN/ World Wildlife Fund (2008). [New edition due Oct 2010]
[2] The Revenge of Gaia. Lovelock J. (2007). Penguin Books, London, UK.
[3] World Population Prospects: The 2008 Revision. Population Database. United Nations Department of Economic and Social Affairs, Population division (2008).
[4] Global Statistics. Geohive (2008).
[5] Sustainable Populations by Country. Optimum Population Trust (2003).
[6] Climate Change 2007. Synthesis Report. Intergovernmental Panel on Climate Change (2007).
[7] Crude Oil – the Supply Outlook (Revised Edition). Executive Summary. Energy Watch Group (2008).
[8] Population Issues. United Nations Population Fu


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