Corey Bradshaw and Barry Brook on 2 billion people by 2153 if only one child per woman on average by 2100 (2014) : https://www.pnas.org/content/early/2014/10/23/1410465111 The inexorable demographic momentum of the global human population is rapidly eroding Earth’s life-support system. There are consequently more frequent calls to address environmental problems by advocating further reductions in human fertility. To examine how quickly this could lead to a smaller human population, we used scenario-based matrix modeling to project the global population to the year 2100. Assuming a continuation of current trends in mortality reduction, even a rapid transition to a worldwide one-child policy leads to a population similar to today’s by 2100. Even a catastrophic mass mortality event of 2 billion deaths over a hypothetical 5-y window in the mid-21st century would still yield around 8.5 billion people by 2100. In the absence of catastrophe or large fertility reductions (to fewer than two children per female worldwide), the greatest threats to ecosystems—as measured by regional projections within the 35 global Biodiversity Hotspots—indicate that Africa and South Asia will experience the greatest human pressures on future ecosystems” (Corey Bradshaw and Barry Brook, “Human population is not a quick fix for environmental problems”, ”, PNAS, 27 October 2014: https://www.pnas.org/content/early/2014/10/23/1410465111 ).