HANOI ― As the new year dawns, the world is rapidly growing old, and I’m not talking about the earth beneath our feet. No, birth rates for nearly all of humankind are plummeting.

In fact, fewer than 20 percent of the world’s nations are now experiencing growing population rates, the CIA reports. All the rest have either stable or declining populations. The reasons for this are varied ― as are the likely consequences. But first the facts.

The nations with the world’s lowest birth rates are here in Asia. The so-called “average replacement rate” that allows population levels to remain stable is at least 2.1 children per couple; that accounts for young women who die before their child-rearing years. Well, in Singapore, Macao, Hong Kong and Taiwan, the average birth rates range from .78 to 1.1. (Here in Vietnam it’s 1.89.)

Thirty-four countries are bearing fewer than 1.5 children per couple, including South Korea, Greece, Japan and Germany. Overall, only 116 of 224 nations have fertility rates of 2.1 or higher.

Extremely poor sub-Saharan African states have the highest birth rates, including Niger, Mali, Somalia and Uganda, where, on average, women bear 6 to 7 children each. Families in underdeveloped countries have so many babies primarily because birth control is not readily available. At the same time, childhood mortality rates are quite high ― 10.4 percent in Somalia, for example ― prompting mothers to continue having babies to be sure some, at least, survive.

Where does the United States sit? Just under the replacement rate ― at 2.06 births per couple. The U.S. also takes in more immigrants than any other nation, bolstering the population. Still, the overall population growth rate for Americans, taking both births and deaths into account, is now only 0.9 ― and falling.

One result: America’s milk industry is crisis. Milk consumption has fallen by one-third since 1975. The Department of Agriculture says the U.S. now has too few children to support it.

Well-off nations like the U.S. are having fewer children in part because more and more women are unwilling to take time off work to have babies. Also, the cost of children, from the child-care years all the way through college and beyond, can be terrifying ― particularly now that Western economies have been stagnant or mired in recession for so long.

The most obvious consequence is: The elderly population is ballooning in most every developed nation, leading to higher and higher pension, health care and related costs ― with a smaller working population to pay for all of that.

Exacerbating the problem, average life expectancy worldwide is rising. The Lancet, a British medical journal, published an expansive study in December showing that average life spans worldwide have increased by 10 years since 1970. That’s a great leap forward, but it also leaves the dwindling younger generation with more years of elder care to pay for.

Japan is the bellwether state for all of this. In an elementary school outside Tokyo, only one child showed up for first grade this year, the New York Times reported. At its peak in the 1960s, that school served 1,250 elementary students. This year’s school-wide enrollment is only 37 students. In that town, Nanmoku, 56 percent of residents are 65 or older. Analysts are projecting that sales of adult diapers in Japan will soon surpass those for babies.

Japan’s economy has remained stagnant for decades now, discouraging adults from having children. The nation’s population growth rate is in negative territory: -0.08.

Demographers say the world’s overall population will continue growing in the decades ahead but eventually will begin to stagnate and drop ― perhaps later this century. That does offer some benefits. The World Bank’s dire prediction ― food production worldwide will have to increase 66 percent by 2055 ― may not turn out to be as problematic as predicted.

Over time, greenhouse gas along with other damaging emissions and pollutants may decline. At the same time, however, people will begin abandoning some small towns, as is already happening in Japan and China (where the population growth rate is only 0.48).

All this foretells huge economic challenges in the decades ahead as most countries produce fewer young workers so that industrial production declines, and the ratio of workers to retirees tips heavily toward the elderly ― “disproportionally more old people depending upon a smaller generation behind them,” as the Economist put it.

Housing markets will collapse, and economies will suffer as businesses contract ― just like the milk industry in America today.

All of this tells us: We’re on our way toward a very different world.

By Joel Brinkley

Joel Brinkley, a professor of journalism at Stanford University, is a Pulitzer Prize-winning former foreign correspondent for the New York Times. ― Ed.