Inevitably, presidential campaigns focus on immediate domestic difficulties or foreign dangers. Momentous developments - inexorably gathering storms - are unnoticed, until social upheavals upend governments’ assumptions. But Nicholas Eberstadt has noticed.

For the first time since the Black Death in the 1300s, he writes in Foreign Affairs, Earth’s population is going to decline. A lot. This will create social hazards that will challenge political ingenuity. Still, it will be, primarily, a protracted reverberation of a relatively recent, and excellent, event in humanity’s story: the emancipation of women.

Eberstadt, who is incapable of writing an uninteresting paragraph, is an economist and demography-is-destiny savant at the American Enterprise Institute. He says a large excess of deaths over births will be driven not by a brute calamity like the bubonic plague but by choices: those regarding fertility, family structures and living arrangements, all reflecting “a worldwide reduction in the desire for children.”

Today, two-thirds of the world’s population lives in countries with below-replacement levels (2.1 births per woman) of fertility. Since the Soviet Union collapsed, Russia has had 17 million more deaths than births. The 27 European Union countries are, collectively, 30 percent below replacement. Last year, France had fewer births than in 1806, when Napoleon won the Battle of Jena. Italy had the fewest since its 1861 unification. America’s “demographic exceptionalism” is despite its fertility rate (1.62 last year), thanks to immigration.

Periodically, including recently, alarmists have warned about a “population explosion” producing “overpopulation.” Such Cassandras do not notice the correlation between population increases and abundance produced by increased numbers of workers, innovators and entrepreneurs. In the past century, billions have risen from poverty as global population has quadrupled.

But, Eberstadt says, as the world has become richer, healthier, more educated and more urbanized, “the most powerful national fertility predictor” has been something related to these changes: changes in “what women want.” Volition shapes birth rates because now people everywhere are “aware of the possibility of very different ways of life from the ones that confined their parents.”

The waning of religious belief, which has generally encouraged fecundity, has coincided with increased valuing of “autonomy, self-actualization, and convenience.” Soon, when global population passes its apogee, there will emerge a worldwide wave of “top-heavy population pyramids, in which the old begin to outnumber the young,” Eberstadt says. The number of “super-old” (those 80-plus, already the world’s fastest-growing age cohort) will almost triple, to 425 million.

It is possible that “the pervasive graying of the population and protracted population decline will hobble economic growth and cripple social welfare systems in rich countries,” Eberstadt writes. Also: “A coming wave of senescence,” smaller family units, fewer people getting married, “high levels of voluntary childlessness,” “dwindling workforces, reduced savings and investment, unsustainable social outlays, and budget deficits” are the fate of developed nations - unless they make “sweeping changes.”

Eberstadt is, however, tentatively cheerful: “Steadily improving living standards and material and technological advances will still be possible.” The Earth “is richer and better fed than ever before - and natural resources are more plentiful and less expensive (after adjusting for inflation), than ever before,” and the global population is more “extensively schooled” than ever. What is required is “a favorable business climate,” which is Eberstadt’s shorthand for allowing market forces to wring maximum efficiency from fewer people: “Prosperity in a depopulating world will also depend on open economies: free trade in goods, services, and finance to counter the constraints that declining populations otherwise engender.”

The “demographic tides” are, Eberstadt writes, running against the quartet of nations (China, Russia, Iran, North Korea) that, oblivious of demography, are exaggerating their future powers. China’s next generation “is on track to be only half as large as the preceding one.”

Although the United States is “a sub-replacement society, it has higher fertility levels than any East Asian country and almost all European states,” Eberstadt says. Even more important, thanks to immigration, “the United States is on track to account for a growing share of the rich world’s labor force, youth, and highly educated talent.”

One issue in this year’s presidential campaign is germane to the convulsive demographic changes that are coming: immigration. Concerning this, Donald Trump is obtuse, and Kamala Harris has, as about most things, vagueness born of timidity.