AT SOME POINT IN THE LAST FEW YEARS, the population of the planet topped seven billion, with more being born every millisecond. Population growth is one of the five “mega-trends” identified by the United Nations that make displacement and decline in humanitarian actions run rampant (the others, in case you were wondering, are urbanization, climate change, migration, and resource insecurity).

But over the past few years, a new trend is coming to light: More than half of the world’s population lives in countries where childbearing couples are having fewer than two children on average. To be clear, this means for every two people, they are having only one baby…a rate lower than the replacement value.

Across more-developed countries and regions — Europe, North America, Australia, East Asia — people are not having enough children to keep their populations stable. Aside from this steady decline in growth, the populations of these countries are getting older. What does this mean?

As an example, Singapore is one of the most crowded places on Earth at the moment, with a population of over 5 million crammed into 20,000 square miles. They are notoriously a low-fertility country, though, with a reproduction rate of less than 1. This means that, assuming a low rate of immigration, Singapore’s population could drop 90% in just three generations, and that population would be mostly elderly.

This rapid decline in global fertility means that our total population might soon be as high as it’s ever going to get. Scientists are saying now that the global population could peak as soon as the next 20 years, and then start rapidly falling. Countries that currently make up many of the world’s most prosperous nations may start to have economic difficulties as their aging populations retire and there are not enough young people to replace them (or care for them). A fall in global population might also mean less competition for valuable resources, so an average decline in poverty or starvation might be expected.

Averages doesn’t translate so well on the ground, however. With most of the population growth coming from less-developed countries, that means the global ethnicity makeup will strongly tilt away from Europeans and more towards Southeast Asians and Africans. As more developed countries steadily shrink, those countries are dangerously overcrowding, leading to increases in political instability, the spread of disease (like the African AIDS epidemic), and economic failures. These conditions, aside from draining national resources, are perfect for producing extremists — the “demographics of terrorism” state that the more poor, bored, young men you have in a country, the more likely they are to engage in volatile activities. The 10 countries with the highest fertility rates include South Sudan, the Republic of Congo, Uganda, and Afghanistan.

An increase in population in poorer countries also means an increase in immigration (legal or otherwise) to more-developed nations…who may have to rely on immigrants to keep their rapidly aging and unsupported populations afloat. However, we’ve already seen how the influx of new faces has led to racist anti-immigrant policies from countries struggling with national identity.

Australia’s national anthem refers, for example, to “Australia fair,” and the current government’s policies on immigration seem determined to keep it as light-skinned as possible. Interestingly, Australia is notoriously a country built by immigrants, with over 27% of its population foreign-born. As it became more developed and its population growth started to fall, it was increasingly reliant on immigrants to fill up the vast dusty interior, and power its industry. And yet, it’s not uncommon to hear views of shocking xenophobia espoused on national television, and to see bumper stickers proclaiming “F**k off, we’re full” on cars in grocery store parking lots.

What does this mean for the future? Well, many countries don’t have the infrastructure to support a large enough influx of immigrants to make up for their decline in population…which means that, no matter how many people migrate, populations in the world’s most developed nations will inexorably reduce. And as developing countries get freer access to birth control, and the empowerment of women (long a factor in controlling birth rates) becomes more prevalent, their population growth will also start to decrease.

We will see fewer resources being consumed, which will mean massive alterations in economic infrastructure as exports and imports reduce. No matter how you look at it, this dramatic shift in global fertility means huge changes that we are currently completely unprepared to handle.