As birth rates fall, who will keep the economy running? Immigrants.

The ticking sound in capitals around the world is a demographic time bomb – of a sort unimaginable only a couple of generations ago.

The 20th century concern, dramatized in the title of a bestselling book in 1968, was a “population bomb.” The fear? That accelerating birth rates, especially in the developing world, were outstripping the Earth’s resources and hurtling the planet toward widespread famine.

Now, however, we’re facing the opposite: a depopulation bomb.

Birth rates are plummeting around the globe.

And one effect – perhaps just as unthinkable at the moment – could well be to turn inside out the most incendiary political issue in the developed world.

Immigration.

U.S. Republican Party candidate Donald Trump and other nationalist-populist politicians across the developed world have built their support by denouncing immigration as a dire threat.

But as the depopulation bomb explodes, nations that embrace immigration as a vital necessity will likely be the ones best able to navigate its economic, social, and political fallout.

The sheer scale of the changes underway was laid out last week in a powerful essay by political economist Nicholas Eberstadt in the journal Foreign Affairs.

Not since the 1300s, when the plague was ravaging Europe and Asia, has the world’s population shrunk. And in the centuries after the black death, there was a spectacular rebound.

Now, for the first time in history, we’re headed for a period of sustained reduction in the number of people on Earth. Women worldwide have been opting to have far fewer children.

Martin Meissner/AP

As European birth rates drop, fewer and fewer young people like this little girl at a farm in Castrop-Rauxel, Germany, will join the workforce. To keep their economies humming, governments will likely turn to immigrants.

Births everywhere except sub-Saharan Africa have fallen below what demographic experts call the “replacement level” necessary to sustain populations at current levels.

The political implications are stark. More deaths than births mean that an ever smaller workforce will have to ensure continued economic productivity. It will also need to sustain a far larger number of older people, born in the era of population growth and living longer.

Thus the reason why immigrants might be valuable.

And while Mr. Eberstadt maintains that “few yet see [depopulation] coming,” there are growing signs that politicians in a number of countries are already looking for ways to respond to it.

China acted on its 20th century fear of overpopulation by mandating a “one-child policy” in the 1980s. Now, the ruling Communist Party is alarmed by depopulation. It shelved the one-child policy nearly a decade ago, announcing first a “two-child policy” and then a “three-child policy,” but they have had little impact on birth rates.

Some advanced democracies recognize the importance of immigration in sustaining and growing their economies – and ensuring support for their graying populations.

Canada and Australia have been seeking to attract immigrants through a system designed to match their skills with areas of economic need. Competing with them is the 27-nation European Union – a region already in demographic decline – which has launched similar incentives.

Still, in EU countries and the United States, the politics of immigration seem headed in a very different direction.

The issue is embroiled in angry debate over what to do with the large number of refugees arriving from areas of conflict, poverty, or environmental blight in search of a new life.

Mr. Trump and far-right politicians in Europe have played on grassroots fears of what the newcomers might mean for their communities. This strategy has been so successful that even centrist and left-of-center governments have begun prioritizing policies to curb the tide of refugees rather than making a positive case for immigration.

Still, the economic repercussions of the “depopulation bomb” could force politicians of all stripes eventually to find common ground on accepting immigrants.

In the U.S., the birth rate has been declining, although less steeply than in other advanced democracies. Its population, at current rates, will peak only decades from now.

But that’s thanks in substantial part to its large number of immigrants.

And while Mr. Trump has vowed to begin mass deportations if elected, there are signs that key allies, including running mate JD Vance, recognize the difficulty of charting a successful future without immigrants.

Mr. Vance, like leading anti-immigrant politicians in Europe, has been strongly critical of women who choose not to bear children. In the absence of immigrants, the only way to prevent depopulation is to have more babies.

But there are no signs of any reversal of the worldwide trend among women to have fewer children, for a range of economic, social, and personal reasons.

Far more likely, as the demographic effects take hold, is emerging worldwide competition to attract and integrate immigrants who can keep national economies growing.

That’s where immigrants have given America an edge – boosting growth and helping to fuel the innovation and entrepreneurship that underpin the country’s global advantage.

That’s an argument no major U.S. politician – including the Democratic presidential candidate, Kamala Harris – seems ready to make in the current political climate.

But as I noted in an earlier Patterns column, it was a point that Republican President Ronald Reagan made eloquently in his final remarks in office.

America’s strength rested on welcoming newcomers from “every corner of the world,” he said. “If we ever closed the door to new Americans, our leadership in the world would soon be lost.”