The United Nations projects that over the coming century the global balance of population will radically shift, with Asia losing its longtime status as home to a majority of the world's population and Africa rising quickly to account for somewhere around a third of humanity, an increase from about 15 percent today. Individual countries show the trend in detail fairly well:
As is well known, China's fertility rates are being artificially depressed, while India's remain high. Consequently, it is likely that India will surpass China's total population, which is likely to peak sometime in the next twenty or thirty years, and will become the most populous country on earth by mid-century. What is unknown is just how populous India will be in 2100--the UN gives a range between about 900 million and 2.6 billion, which is to say that the UN's demographers have no idea.
What is less well known is how quickly many African countries are growing. Nigeria and Tanzania are forecast to be among the five most populous countries by the end of the century, and the UN's estimates for Nigeria (pictured) are, again, almost no help: the UN's mean forecast is that Nigeria's population will be about 650 million in 2100, but the range is somewhere between 275 million and 1.2 billion -- a margin of error roughly the size of Africa's total population today. (Even if we restrict ourselves to the 95 percent confidence intervals, we are still left with a range between 425 million and 1.1 billion.)
How does international relations respond to challenges such as divergent population growth? Over the past century, such complications have been ignorable, because poor countries were normally also relatively small (Europe long accounted for about a quarter to a third of the world's population) while also fairly weak. But for the next several decades many such countries will be both growing their mass population while also growing their per-capita GDPs at rates that vastly outpace the Industrial Revolution.
I've put together another figure which shows changes in GDP per capita for selected countries and regions over the past millenium to illuminate the scale and speed of the changes we're seeing in global economics.
This figure demonstrates that the past two centuries have been a period of exceptionally strong divergence between the "West" and the "Rest." It also suggests that this period is drawing to a close--for the first time since World War II, Western Europe's lead in economic development over the rest of the world is eroding, while for the first time since the early Qing Dynasty, China's GDP per capita is approaching the global mean.
The notion of the "developed" world is one that has also proven malleable over time. Portugal, for instance, was a charter member of the OECD, but its economic development looks downright stagnant compared to South Korea. And, of course, Japan famously became the first non-Western nation to achieve Western levels of technological and economic development.
So what does a world in which Europe is a rounding error look like? What happens when the Atlantic Community is less a security community and more a gated community--a small neighborhood of wealthy and aging people whose borders are patrolled by heavily armed guards? Do states that are growing so rapidly and yet have comparatively few resources behave in the same way that realism or liberalism expect? UPDATE: A better way of asking this is: Are our theories of IR as scalable as they purport to be?
I'm not sure that our theories of international relations have properly specified their operative assumptions in full.