On November 15, the world will reach a milestone, when the human population will reach 8 billion for the first time. Of course, it is impossible to know exactly when we will reach this threshold, but the UN has chosen this date to mark the occasion, based on its modelling.
Just 11 years after the human population hit 7 billion, it may seem like the number of people in the world is growing faster than ever. But, in fact, the rate of growth is plummeting, with fertility rates now below replacement levels – the amount needed to sustain a population – in most of the world.
In 2019, the UN predicted that the population would continue to increase to reach 11 billion by 2100, but the medium scenario of its latest forecast is that it will peak in the 2080s. The European Union and the Institute for Health Metrics and Evaluation (IHME) in Seattle, Washington, predict that the peak will come sooner, by 2070 (see diagram below). This will be good news for efforts to limit climate change and the acceleration of mass extinction of species.
But declining populations in many areas will bring new problems, and any environmental benefits will depend heavily on how rich people are and what they spend their money on.
While the connection between the number of people living on the planet and their impact on it is complex, there is no doubt that the growing human population is leaving less and less room for the rest of life on Earth. Three-quarters of all land and two-thirds of oceans have already been significantly modified by humans.
Humans now make up a third of the biomass of all terrestrial mammals, measured in terms of carbon content. Our livestock make up almost all of the rest, with wild land mammals making up only 2%. Similarly, the biomass of farmed birds is 30 times greater than that of wild birds.
How many people can live sustainably on the planet? Estimates vary widely, but a 2020 study concluded that our current food system can only feed 3 billion people without crossing key planetary boundaries. Surprisingly, however, simply changing what and where we grow could bring that figure up to almost 8 billion. Reducing meat consumption and food waste could bring it to 10 billion.
Are we going to exceed this ceiling? There is no doubt that the population will reach nearly 10 billion by 2050. “Much of the growth is already priced in,” says demographer Jennifer Sciubba of the Wilson Center, a think tank in Washington DC. “The future mothers are already born.”
It is after 2050 that uncertainty grows. Researchers agree that fertility rates will continue to fall around the world, but the UN sees them falling more slowly than others, says Stein Emil Vollset, who led the IHME forecast.
Two-thirds of the world’s population now live in places where the fertility rate – the average number of children per woman – has fallen below replacement level according to the UN. Populations have already declined in a number of low-fertility countries, including Japan, Italy, Greece and Portugal.
In places where the proportion of young people is high, such as India, a drop in fertility below replacement level will not immediately lead to a decline in population – there is a lag that can last for decades. But during this century, more and more populations will decline. According to Vollset forecasts, for example, India’s population will peak at 1.6 billion in 2049 and decline to 1.1 billion in 2100.
It is in parts of Africa, the Middle East and Asia that fertility rates remain well above replacement levels. Most of the population growth through 2050 will occur in just eight countries, according to UN forecasts: Democratic Republic of Congo, Egypt, Ethiopia, India, Nigeria, Pakistan, the Philippines and Tanzania.
Access to contraceptives is the fundamental reason for falling fertility rates. Education and women’s rights, including the prohibition of child marriage, are also essential. It is where education and rights for women and girls are lacking that fertility rates are highest.
When women can choose how many children to have, many other factors also come into play. “The cost is huge,” says Sciubba. In the UK, for example, the cost of raising a child to 18 is around £160,000, according to the Child Poverty Action Group.
Many people wish to have a good job, a stable relationship and a decent home before having children, but may find it difficult to achieve this or find themselves unable to have as many children as they wish by the time they they do. Others don’t want to have children and no longer feel pressured by society to do so. Still, 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 wealthy countries has prevented population declines in many such countries, but Sciubba does not believe it will be allowed to increase enough to prevent such populations from falling in the future. Less than 4% of the world’s people move to other countries, she says, and that figure hasn’t changed in decades.
While declining populations may be good from an environmental perspective, some economists and governments view it as a disaster. In most Western countries, people of working age pay pensions and care for pensioners, so a growing proportion of older people is causing serious financial stress. In other countries where parents are caring for the elderly, the pressure will be felt at the family level, says Sciubba.
But the aging of populations is not necessarily synonymous with economic catastrophe. Take Japan. “It’s the oldest country on the planet, with a median age of 48, which has never happened before in all of human history,” says Sciubba. “And it still has a very strong economy.”
The age level of populations is usually measured in terms of the ratio of people aged over 65 to those aged 20-64, known as the dependency ratio. Japan has the highest dependency ratio in the world. But focusing only on this metric is misleading, says Vegard Skirbekk of Columbia University in New York, author of Decline and prosperity!.
The impact of aging also depends on the health of the population, he says. In the health-adjusted dependency ratios that his team calculated, Japan is in the middle and it is especially the countries of Eastern Europe that score the worst. “The solution is not to increase fertility, but to invest in health,” says Skirbekk.
However, some countries are trying to increase fertility. For example, China is facing a dramatic decline in its population, which is on track to halve to around 730 million by 2100, according to Vollset forecasts. This is why China is taking steps such as ending its one-child policy, but its efforts have not been successful, Vollset says.
Increasing fertility is much more difficult than decreasing it, he says. “From what we know of governments trying to influence fertility, they have been relatively successful in lowering fertility, but raising fertility has proven much more difficult.” Moreover, even when policies boost fertility, the effect tends to be short-lived, Vollset says.
The reasons for this are complex, but where, for example, costs deter people from having larger families, substantial subsidies are needed to make a difference. In contrast, low-cost measures such as providing access to family planning and contraceptives, or promoting the benefits of smaller families, can have a large impact.
The enormous inertia of population growth also means that it takes generations for policies to have an effect. “It’s not something you can change quickly,” says Skirbekk.
If efforts to boost fertility fail and world population peaks before 2100, any green benefit will depend on breaking the current correlation between people’s wealth and their environmental footprint. The richest 10% of people are responsible for about half of all carbon emissions.
But there is a silver lining here: carbon dioxide emissions per person have fallen since 2012 and could keep falling. Unfortunately, people always eat more meat as they get richer. If nothing else changes, it will lead to continued habitat destruction and deforestation even after the world’s 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 increasingly severe as the century progresses. But given that future population growth is largely determined by those of us living today, the bigger picture is unlikely to change much, Sciubba says. “It won’t be radically different – unless you’re talking about apocalyptic.”
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