Some countries have taken active steps to promote contraceptive use, delayed motherhood and female education, which has been shown to reduce family size. China, the most populous nation, introduced a controversial one-child policy in 1978, which has prevented hundreds of million potential births. However, this kind of social engineering is still not as effective as economic growth in reducing family size. Taiwan, which moved from a developing to developed nation in the past few decades, has seen a slightly bigger reduction in fertility than China over the same period. Interestingly, Taiwan is one of a handful of countries whose governments now have policies in place to boost fertility by creating incentives for larger families.
The problem is that growth is now almost entirely occurring in regions of the world like sub-Saharan Africa, where people are the poorest, infrastructure is worst and environmental degradation is already problematic. And the current demographic shift, in which developed nations have an older average population, whereas those of developed nations are younger, will continue. It means that the native workforce will be insufficient to support rich-nation societies, which will increasingly rely on migrants from the developing world.
That said, it appears that we are already regulating our population to some degree, with some countries, such as Japan, expected to have a smaller population by 2050. As well as improvements in nutrition and healthcare that have seen more people living for longer, the general global trend is towards reduced fertility (fewer people) and economic growth (greater per-person resource use) – as people get richer, they generally have fewer children.
The decline in population growth is not fast enough for many environmentalists, though. They see reducing human numbers as vital for a safe future for humanity.
However, there are plenty of agronomists and economists who believe there is enough to go around. We can feed nine and even ten billion, they insist, so long as we improve agricultural efficiency in places where outdated practices are used.
But with over half of the world's land already used for agriculture, do we really want to plough the rest? In the next four decades we're going to have to produce more food than we have during the last 10,000 years in soils that are degraded, marginal lands and under the difficult conditions of climate change.
While it may be true that we could feed a couple of billion more people, it is also true that people around the world are starving right now – and not from a shortage in global food supply, but because of wars, or crops already hit by climate change, or being too poor to afford food whose price has been artificially raised by commodities trading. And as people become richer and switch from rice or cassava to milk and meats, the problem will only escalate. Resource use is not just determined by population size, but also by how rich people are.
Ultimately, if everyone on Earth were to eat as the average American does, or to live their lifestyle, there is not enough to go around. The only way we can make this work as a species is by being realistic about resource availability – from cropland to water – and tailor our expectations of what constitutes a good life to the limitations imposed by our population.
We will surely have to become less numerous over time and limit our consumption to sustainable levels. It will mean improving efficiency and reducing waste, but also being less greedy.
In that way, we should be able to accommodate not just an extra two billion people, but as many of the other animals and plants we share our planet with.