On 15 November, the world will pass a major milestone, as the human population hits 8 billion for the first time. Of course, it is impossible to know exactly when we will reach this threshold, but the United Nations has chosen this date to mark the occasion, based on its modelling.
Coming just 11 years after the human population hit 7聽billion, it聽might seem as if the number of聽people in the world is growing faster than ever. But, in fact, the聽growth rate is plummeting, with聽fertility rates now below replacement levels聽鈥� the amount聽required to maintain a population聽鈥� in most of the world.
In 2019, the UN forecast that the population would keep rising to 11 billion by 2100, but the medium scenario in its latest forecast is that it will peak in the 2080s. Two forecasts by the European Union鈥檚 Joint Research Centre and the Institute for Health Metrics and Evaluation (IHME) in Seattle, Washington, predict the peak will聽come earlier, by 2070 (see diagram, below). This will be good news for efforts to limit climate change and the accelerating mass聽extinction of species.
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But declining populations in聽many regions will bring new problems, and any environmental benefits will depend hugely on people鈥檚 wealth and what they spend their money on.
While the link between the number of people alive on the planet and their impact on it is complex, there is no doubt that the growing human population is leaving ever less room for the rest of life on Earth. Three-quarters of聽all land and two-thirds of the oceans have already been significantly altered by people.
Humans now account for a third of the biomass of all land mammals, measured in terms of聽carbon content. Our livestock makes up almost all of the rest, with wild land mammals just 2聽per聽cent. Similarly, the biomass of farmed birds is 30 times bigger聽than that of wild birds.
How many people can live sustainably on the planet? Estimates vary widely, but one 2020 study concluded our current food system can only feed 3 billion without breaching key planetary limits. Surprisingly, though, simply changing what we grow, and where, could raise this to nearly 8聽billion. Reducing meat consumption and food waste could increase it to 10聽billion.
Will we exceed this upper limit?聽There is little doubt that the聽population will grow to near 10聽billion by 2050. 鈥淎 lot of the growth is already baked in,鈥� says demographer at聽the Wilson Center, a think tank in Washington DC. 鈥淭he future mothers are already born.鈥�
It is after 2050 that the uncertainty grows. Researchers agree that fertility rates will keep falling all around the world, but the UN sees them falling more slowly than others, says , who led the IHME forecast.
Two-thirds of the global population now live in places where the fertility rate 鈥� the average number of children per woman 鈥� has fallen below the replacement level according to the UN. Populations have already been declining in a number of countries with low fertility, including Japan, Italy, Greece and Portugal.
In places with a high proportion聽of young people, such聽as India, fertility falling below the replacement rate won鈥檛 immediately lead to a declining population 鈥� there is a lag that can last many decades. But over the course of this century, more and more populations will decline. According to Vollset鈥檚 forecast, for instance, the population of India will peak at 1.6 billion in 2049 and聽decline to 1.1 billion by 2100.
It is in parts of Africa, the Middle East and Asia where fertility rates remain well above replacement levels. Most population growth up聽to 2050 will happen in just eight countries, according to the UN鈥檚 forecast: the Democratic Republic of the Congo, Egypt, Ethiopia, India, Nigeria, Pakistan, the Philippines and Tanzania.
Access to contraceptives is the fundamental reason behind falling fertility levels. Education and women鈥檚 rights, including bans on child marriage, are also key. It is in places where education and rights for women and girls are lacking that fertility rates are highest.
Where women can choose how聽many children to have, many other factors also come into play. 鈥淐ost is a huge one,鈥� says Sciubba. In the UK, for instance, .
Many people aim to have things like a good job, a stable relationship and a suitable home before having children, but may struggle to achieve this or by the time they do. Others don鈥檛 want to have children and no longer feel compelled by social pressure to do so. Yet some are put off by pessimism about the future, says Sciubba.
For individual countries, rather than the world, another major factor is migration. Migration to聽wealthy nations , but Sciubba doesn鈥檛 think it will be allowed to increase enough to prevent these populations falling in the future. Less than 4聽per cent of people worldwide move to other countries, she says, and that figure聽hasn鈥檛 changed in decades.
While falling populations may be good from an environmental perspective, some economists and聽governments view them as聽a聽disaster. In most Western countries, working-age people pay聽for the pensions and care for those who are retired, so a rising proportion of older people causes serious financial strains. In other countries where relatives care for older people, the strain will be felt at聽the family level, says Sciubba.
But ageing populations don鈥檛聽necessarily spell economic disaster. Take Japan. 鈥淚t鈥檚 the oldest country on the planet, with a median age of 48, which has never happened before in all human history,鈥� says Sciubba. 鈥淎nd it still has a very strong economy.鈥�
The age level of populations is generally measured in terms of the ratio of people over 65 to those aged between 20 and 64, known as the dependency ratio. Japan has the highest dependency ratio in the world. But focusing on this metric alone is misleading, says at Columbia University in New York, author of聽Decline and Prosper!.
The impact of ageing also depends on the health of the population, he says. In the , Japan is average and it is mostly eastern European countries that score worst. 鈥淭he solution is not to聽raise fertility, but to invest in health,鈥� says Skirbekk.
Some countries are, however, trying to raise fertility. For instance, China is facing a dramatic decline in its population, which is on course to halve to around 730 million by 2100, according to Vollset鈥檚 forecast. This is why China is taking measures such as ending its one-child policy, but its efforts haven鈥檛 been successful, says Vollset.
Raising fertility is a lot harder than lowering it, he says. 鈥淔rom what we know about governments who try to influence fertility, they have been relatively successful when it comes to bringing fertility down, but it has proven to be much more difficult to increase fertility.鈥� What鈥檚 more, even when policies do boost fertility, the effect tends to be short-lived, says Vollset.
The reasons for this are complex, but where, say, costs deter people from having bigger families, it requires substantial subsidies to make a difference. By contrast, inexpensive measures such as providing access to family planning and contraceptives, or promoting the benefits of smaller families, can have a big impact.
The enormous inertia in population growth also means it聽takes generations for policies to聽have an effect. 鈥淚t鈥檚 not something you can change quickly,鈥� says Skirbekk.
If efforts to boost fertility fail and the world鈥檚 population does peak before 2100, any green benefits will depend on breaking the current correlation between people鈥檚 wealth and their environmental footprints. .
But there is a glimmer of hope here: carbon dioxide emissions per person have declined since 2012 and . Unfortunately, . If nothing else changes, this will lead to continued habitat destruction and deforestation even after the world population peaks.
Or will the future be much wilder than we imagine? None of the population forecasts take climate change into account, and its impacts will become ever-more severe over the century. But given that future population growth is largely determined by those of us alive today, the big picture is unlikely to change much, says Sciubba. 鈥淚t鈥檚 not going to be radically different 鈥� unless you鈥檙e talking apocalyptic.鈥�
