Overpopulation combined with the inefficient resource consumption of modern society. If our resource usage per person reduced at the same rate that population increased, it wouldn’t be a big deal.

Also that graph is ridiculous. If there was one less child born per person alive, there would be zero children being born in most developed countries (it takes two people to have a child, which would mean two less children per couple). Of COURSE that would result in a drastically reduced carbon footprint, because we’d die out.