So, is overpopulation a real threat? Not hardly. Though population density can threaten sustainability in some areas of the globe, the far greater danger for our future is what Longman calls "depopulation." On a global scale, we are seeing the population of older persons exploding and the numbers of young persons falling.
The United Nations now predicts that total world population may begin falling as early as 2040, and much of the surviving population will be very old indeed.