Indeed, growing populations won’t be the only demographic challenge the world faces in the decades to come. Remember how I said that population was growing globally but unevenly? Even as a country like India will likely need to cope with 500 million new people between now and mid-century, most rich nations—where the fertility rate is below replacement level and people live longer and longer—will need to cope with extreme aging. As the 2010 World Population fact sheet shows (download a PDF here), in Japan, Italy and Germany there are already only 3 working people to support every 1 pensioner. Compare that to sub-Saharan Africa and the Middle East, where the ratio is closer to 25 to 1. That disparity will only grow in the future, exacerbating budget headaches and skewing economies. In the U.S., for example, spending on Social Security and Medicare is expected to rise from 8.4% of GDP today to 14.5% by mid-century.

So here’s the planet we could have in 2050: an overpopulated, overstressed developing world and an aging, economically stagnant developed world, with inequality even larger than it is today. Is there any way to escape that fate? While development and education will be incredibly important (especially for women—literacy is one of the best ways to reduce fertility), the answer may end up being immigration. Think about it—in the future the developed world will lack young workers, and the developing world will have an excess of that resource. Immigration could be a way to balance demographics and economics—alleviating population pressure in the poorer parts of the world while jump starting aging developed nations. The U.S. already does this—immigration will provide most of American population growth. It would be a radical solution, given the political resistance to increased immigration in much of the rich world. (If you think it’s a hot topic in the U.S., try Japan, which steadfastly resists assimilating foreigners, despite the dire threat posed by an aging population.) But it might be the only way to save our overpopulated planet.