For many years overpopulation was the ur-worry. The prospect of too many people on a finite planet stood behind common environmental worries from pollution to global warming. Significant numbers of educated couples skipped having children at all, or no more than one, so they would do their part in preventing overpopulation. In China, having a single child was a forced decision.

While the global population of humans will continue to rise for at least another forty years, demographic trends in full force today make it clear that a much bigger existential threat lies in global underpopulation.

That worry seems preposterous at first. We've all seen the official graph of expected human population growth. A steady rising curve swells past us now at 7 billion and peaks out about 2050. The tally at the expected peak continues to be downgraded by experts; currently U.N. demographers predict 9.2 billion at the top. The peak may be off by a billion or so, but in broad sweep the chart is correct.

But curiously, the charts never show what happens on the other side of the peak. The second half is so often missing that no one even asks for it any longer. It may be because it's pretty scary news. The hidden half of the chart is that it projects a steady downward plunge toward fewer and fewer people on the planet each year—and no agreement on how close to zero it can go. In fact, there's much more agreement about the peak than about how few people there will be on the planet in a 100 years.

A lower global population is something that many folks would celebrate. The reason it's scary is that the low will keep getting lower. All around the world, the fertility rate is dropping below replacement level, country by country, so that globally there will soon be an unsustaining population. With negative population growth, each generation produces fewer offspring, who produce fewer still, until there are none. Right now, Japan's population is way below replacement level, as is most of Europe, Eastern Europe, Russia, the Former Soviet Republics, and some Asian countries. It goes further: Japan, Germany and Ukraine have absolute population decline; they are already experiencing the underpopulation bomb.

The shocking news is that the developing world is not far behind. While they are above replacement level, their birthrates are dropping fast. Much of Africa, South America, the Mideast, and Iran have rapidly dropping fertility rates. The drop in fertility has recently stalled in some sub-Saharan African nations but, that's because development there has stalled. When development resumes, fertility will drop again—because fertility rates are linked to urbanity. There is a deep feedback cycle: The more technologically developed a society becomes, the fewer offspring couples will have, the easier it is for them to raise their living standards, the more that progress lowers their desire for large families. The result is the spiral of modern technological population decline—a new but now universal pattern.

All that it would take to break this downward spiral is that many women living in cities all around the world decide to have more than two children in order to raise the average fertility level to 2.1 children. That means substantial numbers of couples would have to have three or four children in urban areas to make up for those with none or only one. It possibly could become fashionable to have four kids in the city. The problem is that these larger families are not happening anywhere where the population has become urban, and urbanity is now the majority mode of the population and becoming more so. Every developed country on the planet is experiencing falling birth rates. The one exception has been the U.S. because of its heavy immigration, primarily because of Catholic Hispanic immigrants, but even that is changing. The most recent report shows that the birth rates of Hispanic immigrants in the U.S. is dropping faster than ever. Soon the U.S. will be on par with the rest of the world, with plunging birth rates.

To counter this scary population implosion, Japan, Russia, and Australia pay bonuses for newborns. Singapore (with the lowest fertility rate in the world) will pay couples $5,000 for a first child and up to $18,000 for a third child—but to no avail; Singapore's rate is less than one child per woman. In the past, drastic remedies for reducing fertility rates were difficult, but they worked. Drastic remedies for increasing fertility don't seem to work, so far.

Our global population is aging. The moment of peak youth on this planet was in 1972. Ever since then the average age on Earth has been increasing each year, and there is no end in sight for the aging of the world for the next several hundred years! The world will need the young to work and pay for medical care of the previous generation, but the young will be in short supply. Mexico is aging faster than the U.S., so all those young migrant workers who seem to be a problem now will soon be in demand back at home. In fact, after the peak, individual countries will race against each other to import workers, modifying immigration policies, but these individual successes and failures cancel out and won't affect the global picture.

The picture for the latter half of this century will look like this: Increasing technology, cool stuff that extends human life, more older people who live longer, millions of robots, but few young people. Another way to look at the human population in 100 years from now is that we'll have the same number of over-sixty-year olds, but several billion fewer youth.

We have no experience throughout human history with declining population and rising progress (including during the Black Plague years). Some modern countries with recent population decline have experienced an initial rise in GDP because there are fewer "capitas" in the per-capita calculation, but this masks long-term diminishment. But there can always be a first time!

Here is the challenge: This is a world where every year there is a smaller audience than the year before, a smaller market for your goods or services, fewer workers to choose from, and a ballooning elder population that must be cared for. We've never seen this in modern times; our progress has always paralleled rising populations, bigger audiences, larger markets and bigger pools of workers. It's hard to see how a declining yet aging population functions as an engine for increasing the standard of living every year. To do so would require a completely different economic system, one that we are not prepared for at all right now. The challenges of a peak human population are real, but we know what we have to do; the challenges of a dwindling human population tending toward zero in a developed world are scarier because we've never been there before. It's something to worry about.