Approaching 10 billion — Can our planet cope with population growth?
The physical capabilities of humans are lacklustre compared to most animals — we are not the strongest, nor the fastest and our senses are not the sharpest. In a bare-handed confrontation with a lion, for instance, a human would not fare well. Yet through our cunning, cooperation and resourcefulness, we have become unrivalled.
In the early stages of human history, our ability to create stone tools and control fire gave us a significant survival advantage. These skills enabled us to become effective hunter-gatherers and to prosper.
Over time, our technology improved and our influence grew. The advent of farming around 10,000 BC enabled us to better control our food supply and to settle in communities, thereby increasing our population.
The Industrial Revolution, which started around the middle of the 18th century, was a watershed in human history. By harnessing the power of machines and unleashing the condensed energy within fossil fuels, we supercharged our advancement and the population exploded, growing from 1 billion to 7 billion over a period of just 200 years.
The unprecedented growth in our population and consumption over the last two centuries has had a devastating impact on our planet. It has been a key driver of a number of environmental issues, including: climate change, deforestation, loss of biodiversity, depletion of fish stocks and widespread plastic pollution.
At the World Economic Forum in Davos in January, Sir David Attenborough remarked:
“We are now so numerous, so powerful, so all pervasive, the mechanisms we have for destruction are so wholesale and so frightening that we can exterminate whole ecosystems without even noticing it.”
It seems obvious that our environmental problems will become more difficult to solve as the human population continues to grow to 10 billion by 2050, yet the link is rarely mentioned. Why?
For one thing, all human life is sacred; everyone deserves the chance to live a long and happy life, to love and to raise a family. The concept that the simple state of being has an adverse effect on the planet is difficult to come to terms with. It is comparatively easier to look at ways to reduce our impact or to focus on specific causes of environmental problems, such as burning fossil fuels.
If we are to have any hope of tackling environmental issues and feeding the world, however, we must at least consider whether there are any ethical and feasible solutions to curtail population growth — we must address the elephant in the room. There is no escaping the fact that our planet has finite resources and hence there is a natural limit to how many people it can sustain.
The link between the human population and the environment
Rapid growth in the human population has had a pernicious effect on the natural world. Since 1970, the human population has almost doubled, while over the same period, the population of the world’s vertebrate animals has dropped by 60%. The effect has been more pronounced in South and Central America, where wildlife has suffered an 89% decrease.
At this rate, humans are on track to cause the sixth mass-extinction event. It is difficult to see how the natural world will survive the triple threat of climate change, a further doubling of the human population and the snowball effect of collapsing ecosystems.
While humans are not physically large — standing shoulder to shoulder, the entire population could fit into an area the size of Los Angeles — we cast a long shadow. We need a lot of space and resources to sustain our lifestyles (e.g. for housing and land for agriculture to feed us), especially in the developed world. So, it unsurprising then that as the human population has grown, deforestation and habitat loss has increased, decimating the populations of other species in the process. We have effectively taken space and resources away from the natural world.
The charts below show how growth the human population and our consumption is linked to the deterioration of a number of key environmental indicators, including: degradation of ecosystems, increased loss of tropical forests and increased acidification of the ocean.
One way to assess the sustainability of our combined impact is to compare the level at which we consume resources and generate waste with the natural capacity of our planet. This concept is referred to as our ‘ecological footprint’.
On this metric, we are running a large ecological budget deficit: we would need 1.7 Earths to sustain our current consumption patterns. In other words, we are living well beyond our means and are only supporting this by raiding the planetary savings account that has been built up over billions of years, which clearly is not sustainable — you cannot live off garage sales forever.
We have depleted fish stocks, cleared forests for pastures, drained inland lakes and acquirers, melted glaciers and have irrigated rivers until they run dry. Even if we get back into the black, it will take these reserves centuries, or even millennia, to recover.
The Global Footprint Network produces a metric called ‘earth overshoot day’, which measures the point in the year when we exceed our annual budget, i.e. when our consumption exceeds the natural capacity of the planet. In 1970, when the global population was around 3.7 bn, we were just about breaking even. Now, as our population draws closer to 7.7 bn, we are blowing our annual budget by the 1st of August.
The relationship between our ecological impact and our population is a simple one: our total ecological footprint is the product of the global population and the average footprint per capital. Despite this, we tend to avoid linking population growth with environmental issues; we typically focus on measures to change our consumption and reduce our impact.
While measures to reduce our individual impact are beneficial and absolutely necessary, it is also important to consider that if our population were to double again, we would need to halve our average individual impact just to tread water. We would then need to halve our impact again to balance the ecological books.
It is unclear exactly how long we can continue to live beyond the means of the planet. The longer we do so and as the gap widens, the more difficult it will be for all life on earth, including humans. And it is the poorest who will be disproportionately impacted.
As our population and ecological footprint increases, we are more likely to face critical shortages of fresh water, widespread famine, epidemics and more frequent wars and civil unrest over dwindling resources.
Further, the capacity of the earth to support life is likely to decrease over time with the effects of climate change, biodiversity loss and the depletion of natural resources — think of this as the effect of credit card debt catching up with us. The effect of climate change only increases the magnitude of the challenge of reaching a sustainable level of consumption.
In his book, ‘An Essay on the Principle of Population’, Thomas Malthus argued that humans have a tendency to utilise abundance of resources for population growth rather than maintain a higher standard of living. He wrote:
“That population does invariably increase where there are the means of subsistence, the history of every people that have ever existed will abundantly prove.
And that the superior power of population cannot be checked without producing misery or vice…”
In other words, Malthus was not optimistic that humankind could prevent itself from reaching its natural limit, which he believed would inevitably result in misery (e.g. famine, disease and war).
Malthus’ theory reminds us that we cannot ignore the impact of population growth if we are to avoid a scenario where ‘misery’ limits our population for us. Such a scenario would be devastating for both humans and the natural world.
Historical population growth
It took 200,000 years for the human population to reach 1 billion and only 200 more years for it to increase to 7 billion.
Modern humans evolved in Africa around 200,000 years ago and began migrating across the earth. To start with, we were hunter-gatherers and our population was low, less than 1 million.
There were two significant events in human history that led to rapid population growth:
a) The agricultural revolution — the advent of farming around 10,000 BC provided a more stable and controllable source of food. It allowed humans to settle in communities increasing the supportable density of populations.
Over several thousand years, agricultural practices improved and spread from the Middle East across the globe and the population increased to around 700m by the mid-1700s
b) The industrial revolution — the industrial revolution, which started around 1760 led to exponential growth in the human population, through improvements in medicine and food production, transportation and storage, which reduced mortality rates, particularly for infants.
Fast forward to 2019 and there are now 7.7 billion people on this planet.
Projected future population growth
Future population growth is inherently uncertain and so all projections must make assumptions regarding changes in the key drivers of the population. The three main drivers are:
i) Mortality rates (i.e. percentage of deaths in the population per annum)
Improvements in mortality rates increase the average length of time that a person is expected to live (“life expectancy”) and therefore cause the population to grow.
Life expectancies have improved dramatically over the last few centuries, mostly driven by advances in medicine and technology. This has been a significant contributor to the increase in the steady-state population. For example, 200 years ago, it would be common for a child and their parents to walk the earth together. Now, a child is likely to be able to share a meal with their parents, their grandparents and even their great grandparents.
Mortality rates are expected to continue to improve, albeit at a slower rate. Hence, mortality improvements are likely to have relatively smaller impact on future population growth.
ii) Fertility rates
If the number of live births per woman (Total Fertility Rate, or “TFR”) is greater than the rate needed to sustain the population (the “Replacement Rate”), the population will grow. The Replacement Rate is linked to mortality rates — if there was no female mortality between birth and childbearing age, the rate would be 2.0. In developed nations, the replacement rate is close to 2.1, while the global average replacement rate is 2.3.
The global average total fertility rate is currently 2.5 and hence the steady-state population is increasing. However, the TFR varies significantly by country and region, leading to vast differences in population growth across the planet.
In the developed world, the TFR is less than 2, but in the least developed nations it is closer to 4. In Niger, the TFR is over 7 live births per woman.
iii) Population momentum — The population changes as it moves from the current level to the steady-state (or equilibrium). This effect is referred to as population momentum and explains why the population can continue to grow even if mortality rates have stabilised and the total fertility rate has settled at the replacement rate. Momentum effects can continue for decades where there is a significant demographic transition and where the difference between the current and steady-state is large.
The UN performs biennial projections of the world population, which take into account estimated changes in all three key drivers of growth (i.e. mortality rates, total fertility rates and population momentum). The chart below shows the projections from the latest revision in 2017.
In the best estimate scenario (i.e. 50/50 chance of being too high or low), the population is projected to reach around 10 billion people by 2050 and peak at just over 11 billion people in 2100. This scenario assumes that the TFR drops from 2.5 live births per woman to 2.2 in 2050 and then to 2.0 by 2100.
If the estimated TFR is underestimated by just 0.5 live births per woman, then the population will reach 16.5 billion by 2100. And if the total fertility rate remains at the current level, the population will continue to increase and will reach 26 billion by 2100.
All of the UN scenarios assume the population continues to grow unencumbered, i.e. that the growth can be supported by the planet’s existing limited resources. However, as discussed previously, this seems unlikely, particularly in the constant fertility scenario (with 26 billion people by 2100) — if growth continues at this rate, then Malthusian misery is likely to impose a natural limit on the population.
Ethical options for curtailing population growth
There are no easy options for limiting our population to a sustainable level. Immediately ruling out a Soylent Green type scenario of state-encouraged euthanasia, the only ethical way to arrest population growth is to transition to a global average total fertility rate of fewer than 2 live births per woman.
Some countries have implemented policies to restrict fertility rates, most notably China with its one-child policy (1978–2014), which was subsequently increased to a two-child policy. The one-child policy has been successful in curbing China’s population growth — government officials estimate that China’s population would be 400 million higher today without the policy. However, the policy has been contentious and is not without critics.
It is doubtful whether strict population controls could be applied in democratic countries. These policies ultimately take away a woman’s right to choose, are questionable from an ethical standpoint and are unpopular — people would rather governments stay out of their bedrooms.
A potential method to influence the total fertility rate in a more positive and ethical way is through investing in education and the empowerment of women. Historical data across regions clearly shows a strong negative correlation between years of education and total fertility rate, as seen in the chart below. That is, women who attain a higher level of education are more likely to have smaller families. For example, in Ghana, the total fertility rate of women with a high school education is 2–3 compared to 6 for women with no education.
While there is some debate as to whether the relationship between education and fertility rates is a causal one (i.e. whether higher education drives lower fertility rates), there are a number of qualitative reasons as to why this is likely to be the case. Women who attain a higher level of education:
- Are better able to support themselves and are more likely to feel empowered to assert their views on important topics such as their preferred family size;
- Have greater knowledge of modern birth control methods and are more likely to utilise, these;
- Have better employment prospects and hence a higher opportunity cost of bearing children (e.g. educated women are more likely to delay having children until they establish their careers);
- Are better able to support their children (through higher incomes and better knowledge of prenatal and child care) and are therefore more confident that their children will survive; and
- Have a wider and more informed view of their ideal family size, through greater exposure to their community and global social networks.
Austrian based research centres, the International Institute for Applied Systems (IIASA) and the Wittgenstein Centre (WC), include the link between education level and fertility rates in their population projections. Their projections consider global trends in education and how these are likely to affect total fertility rates. They define a number of scenarios around future education:
- SSP1 Accelerated educational development — An optimistic scenario where developing countries accelerate their investment in education and achieve a similar rate of improvement to that achieved by standout countries, such as South Korea, in recent decades.
- SSP2 Best estimate (or Global Education Trend) — Developing countries continue to improve education levels at the historical average rates observed for more advanced countries.
- SSP3 Stalled educational development — The improvement in education levels for developing countries is slower than the historical average and fertility rates remain high.
The chart below shows the population projections for the three different scenarios alongside the UN best estimate case, for reference.
The WC and IIASA best estimate projection indicates a slower population growth profile than the UN best estimate, with the global population peaking at 9.4 bn in 2070. Under a more optimistic scenario, where education levels improve at a higher rate, the population peaks at 8.5 bn in 2055 before falling below current levels by 2100. However, if improvements in education stagnate, then the fertility rate is estimated to remain above 2 and the population continues to grow beyond 12.8bn after 2100.
These projections and the historical correlation data demonstrate the crucial role that investing in education and empowering women could play in limiting the human population to a sustainable level. In the accelerated education scenario, the population peaks at 8.5bn, just 0.8bn higher than today and solving our environmental issues and eradicating poverty seem just that bit more achievable.
Even if this solution is less effective at reducing population growth than expected, it would lead to a better educated population and would improve gender equality — significant benefits in their own right.
 Sir David Attenborough in an interview with Prince William at the World Economic Forum, Davos, 22 January 2019
 UN World Population Projections, 2017 revision
 “WWF Living Planet Report — 2018: Aiming Higher” (2018) Grooten, M. and Almond, R.E.A.(Eds).
 Kolbert, E. ‘The Sixth Extinction: An Unnatural History’ (2014), Henry Holt and Company
 Global footprint network, https://www.footprintnetwork.org/
 Malthus, T. “An Essay on the Principle of Population” (1798)
 World Population Clock, January 2019, (http://www.worldometers.info/world-population/)
 For example, male life expectancy at birth in England increased from around 40 years in 1850 to 79 years in 2017.
 Espenshade TJ, Guzman JC, Westoff CF “The surprising global variation in replacement fertility” (2003). Population Research and Policy Review
 UN Population Division (2017 revision)
 Soylent Green is a 1973 dystopian film centred on overpopulation and a suffering planet. It includes the famous quote “Soylent Green is people!”.
 Pradhan, E. “Female Education and Childbearing: A Closer Look at the Data” (2015), The World Bank
 Jungho Kim (2016) “Female education and its impact on fertility” IZA World of Labour