More and more people posed greater dangers to animal habitats, water supplies, green spaces and, ultimately, the food chain, while advanced nations would be overwhelmed with suburban sprawl, toxic emissions, clogged highways, over-crowded schools and congested living space. Both in this country and around the globe, overpopulation was a threat to the environment. That would occur when the number of births each year equalled the number of deaths, and America's population growth, and the world's, came to a standstill. We don't hear much talk about overpopulation these days. From a regional standpoint, the Pittsburgh area has already achieved ZPG, at least when you factor movement out of the area.
Nobody seems to think that's a good thing.
Of course, the world's population hasn't stopped growing. Twelve years after we hit 4 billion people, the world grew to 5 billion people in 1986 and then hit 6 billion in 1999. All in all, the 20th century could easily be renamed the fertile millennium, as the world had less than 2 billion souls when the century began. Surprisingly, some demographers think it may never happen. One report suggests that the world's population may level at 6.9 billion sometime between 2020 and 2029, while United Nations analysts think that ZPG may not occur until 2038 with 7.5 billion people.