According to the UN's estimations, the world's population reached an initial billion in approximately 1800. We didn't reach the second billion for about another 130 years. But then the baby train left the station, and before the end of the 21st century we amassed six million, with each billion coming about every 11 years. Doomsayers and environmentalists heralded the end of days, predicting runaway population growth spelling the end of global resources and ushering the destruction of the environment. And while I don't discount the dangers of explosive population growth, the true threat is population decline, which we're already seeing in many parts of the developed world.
As the graph to the left depicts, global population growth has already begun to slow, and according to UN projections will plateau approximately in the middle to latter part of this century. But take a look at Europe, where the population has been in decline since the 1990s. And we in the United States are not far behind. Declining birth rates typically lead to a disastrous effect on the economy, as labor shortages ultimately reduce economic output. Technology can offset these losses, but as George Friedman outlines in The Next 100 Years, the true population struggle of this century will be competition among developed countries to secure additional labor in the form of immigration.
So for any non-believers out there, I'll outline just why this phenomenon is occurring. Generally, population growth rates start out slow in any nascent society, as dangers abound (disease, war, famine, etc) and poor infrastructure and health care lead to high infant mortality rates and low life expectancies. As a society develops and begins to prosper, we see a corresponding decrease in infant mortality and increase in life expectancy, resulting in a population "boom." But ultimately in its path toward development, the society becomes rich and prosperous enough that childbirth is no longer seen as attractive a proposition. Typically this corresponds with women's rights--education about the risks/rewards of childbirth and an entrance into the workforce--and the advent of effective birth control. As women begin to have fewer children, the population growth enters a transitional phase toward an ultimate plateau. You can see it in America--not just in the data, but in conventional pop culture. How often have you heard about the "ideal" two children and a dog with the white picket fence? Well, it takes an average of 2.1 births per woman just to maintain a population. And as the above graph depicts, this phenomenon is occurring across the globe and all demographics, not just in America and Europe.
So forget about overcrowding. The real challenge for future policymakers is how to combat this population decline. And once you really start to think about it, suddenly immigration reform becomes a much more pressing (and holistic) issue than the laughable policy option of building some fence.