What's The Ideal Number Of Humans On Earth?

Earth might be looking a little worse for wear, after the last four-hundred years of reckless wide-scale resource extraction, but to its credit it hasn't collapsed entirely. Despite our best efforts, it continues to gamely welcome our rapidly expanding population, barring the occasional earthquake. Whether the planet might be a little better off with fewer of us is a different question, a freighted one.

We are not advocating for reducing the population. We like the population. We wish it well. What we are curious about, in this week's Giz Asks, is what the planet's population size would be in an ideal world. Have we blown past that number, or can we pack a few billion more in here before the whole thing falls apart?

The economists, geographers, conservationists and population experts we spoke to were, with some exceptions, a bit more optimistic about the Earth's capacity than you might expect — we might've blown past the "ideal" number some time ago, but if we can just stop or at least spend less time trying to murder the planet, we might have a while yet, in this place.

Joe Bish

Director of Issue Advocacy, Population Media Center (PMC)

Just arriving at an educated guess at ideal human population oversimplifies the puzzle: because while population size sets the scale of human impact, it is not the sole determinant of the human relationship with Earth. Unlike other animals whose impact is generally limited by their appetites, humans have discretionary environmental impacts far and above our basic biological needs. This behaviour is often lumped under the umbrella-phrase of "consumption", but really includes an enormous number of resource extraction, production and consumptive decisions that occur around the clock, at a worldwide scale of 7.5+ billion people.

Given all of the factors that go into humanity's interactions with the natural world, we won't assert that there is a magic number to which we should hue. Rather, we would choose to improve the dynamics that lead to a mind-boggling net addition 1.5 million people per week — 9,000 more people each hour, and every hour, which the Earth must support. These factors include a lack of gender equality, poor or non-existent reproductive health services, lack of family planning information and services, and catastrophic lack of girls' education in many parts of the world. No honestly informed person will dispute that biodiversity is already suffering horribly and in danger of systemic collapse. Improving the health, rights, and education of people already on the planet — and thereby working to slow down and stop population growth — is something we fully support for the health of every plant and animal species on the planet. Including our own.

Carissa Véliz

Research Fellow at the Uehiro Centre for Practical Ethics, University of Oxford

Bent Flyvbjerg

Professor at Säid Business School, University of Oxford

From an anthropocentric perspective, to calculate the Earth's ideal population size, one would first need to establish an ideal benchmark for what we think is a good life, and calculate the resources it takes to sustain that lifestyle.

As a first approximation, let's take the French lifestyle as a benchmark. According to the Global Footprint Network's calculator, if all of humanity were to live like the French, we would need about 2.5 Earths to sustain that lifestyle. Any lifestyle that cannot be universalized to the rest of humanity cannot be just. Every newborn should be able to enjoy their fair share of the world's resources.

Thus, to ensure the ideal population size so that everyone could enjoy a comparable way of life, taking the French lifestyle as our benchmark, we would need to reduce the world population to about 3 billion people (4.6 billion less than today's population). If, instead, we chose the lifestyle of people in the USA as the benchmark, then the world population would need to be reduced to 1.9 billion. It is unclear, however, that the American lifestyle is any better than the French, in terms of well-being. On the contrary: it looks like Europeans are faring better than Americans. In other words, it seems that Americans are less efficient in living well ecologically. What this shows is that comparable or superior levels of wellbeing may be achieved spending less resources. Presumably, however, there will be a minimum ecological footprint (resource expenditure) to achieve a level of wellbeing we could consider ideal.

Apart from reducing the world's population, there is a second way to bring about a situation in which all human beings could enjoy an equal share of the world's resources: if we wanted to keep the world's human inhabitants at the current 7.6 billion, then we would need to reduce our ecological footprints, and live like people in India, by either adopting a similar lifestyle or developing more ecological methods of sustaining our ways of life.

As a species humans have come late — maybe too late — to the realisation that the Earth's ideal population size depends on lifestyle and ecological footprint, and that all three need to be managed wisely to avoid environmental Armageddon. That should not keep us from trying to get the balance right between population size and ecological footprint, needless to say. If we do this, we may just be lucky enough to avoid the fate of becoming the species that, from an ecological perspective, made its "ideal" population size zero.

Bruce Newbold, PhD

Professor of Geography at McMaster University and author of Population Geography: Tools and Issues.

As the Earth's population has grown, there has been a long-running debate and commentary on what the Earth's optimal population size should be. Starting with Thomas Malthus in the 18th century, who wrote that population growth far outpaced the growth of the food supply, academics and commentators have debated whether the Earth can continue to support a growing population. In fact, Malthus was concerned that the world's population — less than 1 billion at the time — was already too large. Despite such dire predictions, we have found new ways to feed and support its population.

More than likely, we have already surpassed some optimal population size or threshold. The question, however, is bedeviled by other related questions. Should all of the world's population have the same access in terms of quality and quantity to food? Should there be a more equitable standard of living? Will everyone have equal access to resources and equal opportunities? This is certainly not the case now, but determining the answers to these questions will determine how we answer whether there is an optimal population size.

Erle C. Ellis

Professor, Geography & Environmental Systems, University of Maryland

The question, "how many people can Earth support?" has been asked and answered many times. Antoni van Leeuwenhoek computed 13.4 billion in 1679. Cesare Marchetti proposed 10^12 (1,000 billion) in 1979. In 1994, Paul Ehrlich stated that "the present population of 5.5 billion ... has clearly exceeded the capacity of Earth to sustain it." Now, more than 7 billion people live on Earth and are generally better fed, healthier, and living longer than at any time in human history. Rates of population growth peaked in the 1970s as the result of the "demographic transition", as more urbanized, better educated populations tend to have smaller families. Populations will most likely peak in 2100 at about 11 billion. Yet per capita human demands for food, water, energy and other environmental resources continue to grow, as wealthier populations make greater demands on Earth's resources.

The question of how many people Earth can support will continue to fascinate. But the most important question, to paraphrase demographer Joel Cohen, is "how do we humans want to live on planet Earth?" Do we want to transform the entire planet into engineered ecosystems designed to support only human beings- to live in towers, eating vat meat powered by massive nuclear energy systems? If so, 1012 [1,000 billion people] might actually be possible. If it were somehow possible to go back to living as hunter-gatherers, perhaps only a few 10's of millions, or in pre-industrial agrarian societies, maybe a few billion could live on this planet. But these are just fantasies at this point in history.

The real question for the Anthropocene is how to move our societies towards creating and sustaining thriving societies that sustain both healthy, happy people and a healthy non-human nature, leaving plenty of room for wild creatures to live and thrive in habitats free of human interference. Though no small change in direction would be needed, such a planet is achievable now, and can be even with 11 billion people. Human societies have emerged as a global force of nature. If, and only if, our societies make better futures their goal, and cooperate together in achieving this, will we create and sustain the societies and nonhuman natures that we and future generations will desire to live in.

Paul R. Ehrlich

Bing Professor of Population Studies and President of Center for Conservation Biology, Stanford University. Fellow, National Academy of Sciences and Royal Society; Crafoord Prize Laureate

Our research group has concluded two decades ago that with foreseeable technologies an "optimum" population might be 1.5 to 2 billion people. The idea was that would be enough people to have big cities for those who like fine restaurants and opera, and few enough people to permit lots of wildlands for hunters and hermits. Overall, we thought that number might be long-term sustainable.

Scientists have learned a lot more in the past decades about things like toxification of the planet, climate disruption, nuclear war, and so on that makes those numbers seem optimistic as the population races past 7.5 billion, economists still fail to realise that growth is the disease, not the cure for human problems, and the most powerful nation in the world is now governed by moronic thugs — working in any way they can to speed the coming collapse.

John Hawks

Vilas-Borghesi Distinguished Achievement Professor of Anthropology at the University of Wisconsin — Madison

From an anthropological point of view, there really is no ideal population size. If we look at the world 20,000 years ago, our best estimates suggest fewer than 10 million people lived in the whole world at that time. For every one of those people 20,000 years ago, there are 1000 people today. The Earth 20,000 years ago could not support 7 billion people with Paleolithic lifeways. People invented new ways to get more food from the same space. People invented new ways to live in spaces where they couldn't live before, and spread into every corner of the planet. All those changes were culture — without our culture, the world's population today would be totally unsustainable.

Now the question is whether we can continue to adapt our culture to accommodate larger populations. We didn't always manage this. During most of the last 2 million years, there were several human species. The fact that they all survived for hundreds of thousands of years suggests that no one of them was pushing against the others very hard. Each of them must have been limited by resources, and probably their populations crashed when resources became limited. It's only during the past hundred thousand years that our own species, Homo sapiens, started to grow in population size, and that may have been tied to cultural innovations in hunting or social organisation. So today we're still living the consequences of those changes 100,000 years ago, and we haven't stopped yet.

David Lam

Director, Institute for Social Research, Professor of Economics, and Research Professor, Population Studies Center, University of Michigan

For centuries people have tried to calculate some version of the Earth's ideal population size, or to calculate the "carrying capacity" of the world. Like most economists, I have never been a big fan of the question itself or the ways in which it is answered. One reason for scepticism about both the question and the answers is that we are constantly developing technologies that drastically change our ability to turn resources into the things we consume. When the world reached 3 billion people in 1960 and was growing at over 2% per year (doubling in less than 40 years), there was widespread fear that we would have mass starvation in the following decades. We did not. World food production today is 3.5 times greater than it was in 1960, a 50% increase in food per person. Even in Africa, where population growth has been the fastest, food production per person is 14% higher today than in 1961. The proportion of the population in poverty has declined in every developing region in recent decades. Very few people would have predicted that the world could support 7 billion people at a higher standard of living in every region of the world than was the case with 3 billion people in 1960. So trying to estimate the ideal size or the maximum sustainable population from our current vantage point is unlikely to be a fruitful exercise.

The world is projected to add another 4 billion people by 2100, with population probably levelling off soon after that at 11 or 12 billion people. Rapid fertility decline has caused population growth rates to decline everywhere. Africa will be the only region with positive population growth by around 2050, and even Africa's population growth will probably be at low levels by 2100. Given the world's ability to absorb the extra 4 billion people that were added between 1960 and 2011, with rising standards of living all over the world, I am optimistic that the world can add another 4 billion between 2011 and 2100, with standards of living continuing to improve. One of many caveats to this is that we must solve the problem of climate change, a challenge that cannot be minimized. But assuming that we do come up with solutions to climate change, a reasonable forecast is that world population will stabilise at about 12 billion people in a bit over 100 years, say in 2125. The demographic projections, tricky as they are, are much easier than economic projections. But given past experience, my forecast is that living standards in 2125 will be at least as high as they are today, and they will be sustainable over time. All of this leads me to conclude that 12 billion people sounds like a good number for the world. If 12 billion people is where the world is headed, and if those 12 billion are likely to have a reasonable and sustainable standard of living, I can't think of an ethical or philosophical reason for thinking that the ideal population is anything less than 12 billion.

Do you have a question for Giz Asks? Email us at [email protected]

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