It may not be the first way we think of ourselves, but almost all of us alive today happen to be children of the "world population explosion" -- the momentous demographic surge that overtook the planet during the course of the 20th century. Thanks to sweeping mortality declines, human numbers nearly quadrupled in just 100 years, leaping from about 1.6 or 1.7 billion in 1900 to about 6 billion in 2000.
This unprecedented demographic expansion came to be regarded as a "population problem," and in our modern era problems demand solutions. By century's end, a worldwide administrative apparatus -- comprised of Western foundations and aid agencies, multilateral institutions, and Third World "population" ministries -- had been erected for the express purpose of "stabilizing" world population and was vigorously pursuing an international antinatal policy, focusing on low-income areas where fertility levels remained relatively high.
To some of us, the wisdom of this crusade to depress birthrates around the world (and especially among the world's poorest) has always been elusive. But entirely apart from its arguable merit, the continuing preoccupation with high fertility and rapid population growth has left the international population policy community poorly prepared to comprehend (much less respond to) the demographic trends emerging around the world today -- trends that are likely to transform the global population profile significantly over the coming generation. Simply put, the era of the worldwide "population explosion," the only demographic era within living memory, is coming to a close.
Continued global population growth, to be sure, is in the offing as far as the demographer's eye can see. It would take a cataclysm of biblical proportions to prevent an increase in human numbers between now and the year 2025. Yet global population growth can no longer be accurately described as "unprecedented." Despite the imprecision of up-to-the-minute estimates, both the pace and absolute magnitude of increases in human numbers are markedly lower today than they were just a few years ago. Even more substantial decelerations of global population growth all but surely await us in the decades immediately ahead.
In place of the population explosion, a new set of demographic trends -- each historically unprecedented in its own right -- is poised to reshape, and recast, the world's population profile over the coming quarter century. Three of these emerging tendencies deserve special mention. The first is the spread of "subreplacement" fertility regimens, that is, patterns of childbearing that would eventually result, all else being equal, in indefinite population decline. The second is the aging of the world's population, a process that will be both rapid and extreme for many societies over the coming quarter century. The final tendency, perhaps the least appreciated of the three, is the eruption of intense and prolonged mortality crises, including brutal peacetime reversals in health conditions for countries that have already achieved relatively high levels of life expectancy.
For all the anxiety that the population explosion has engendered, it is hardly clear that humanity will be better served by the dominant demographic forces of the post-population-explosion era. Nobody in the world will be untouched by these trends, which will have a profound impact on employment rates, social safety nets, migration patterns, language, and education policies. In particular, the impact of acute and extended mortality setbacks is ominous. Universal and progressive peacetime improvements in health conditions were all but taken for granted in the demographic era that is now concluding; they no longer can be today, or in the era that lies ahead.
THE GLOBAL BABY BUST
In arithmetic terms, the 20th-century population explosion was the result of improvements in health and the expansion of life expectancy. Human life expectancy at birth is estimated to have doubled or more between 1900 and 2000, shooting up from approximately 30 years to nearly 65 years. Population growth rates accelerated radically thanks to the concomitant plunge in death rates. Despite tremendous population growth, rough calculations suggest that the world's population would be over 50 percent larger today in the absence of any other demographic changes.
The world's population currently totals about 6 billion, rather than 9 billion or more, because fertility patterns also changed over the course of the 20th century. And of all those diverse changes, without question the most significant was secular fertility decline: sustained and progressive reductions in family size due to deliberate birth control practices by prospective parents.