This lowering has been happening in general for decades ( 27, 28), a result mainly of higher levels of education and empowerment of women in the developed world, the rising affluence of developing nations, and the one-child policy of China ( 29– 32). Amoral wars and global pandemics aside, the only humane way to reduce the size of the human population is to encourage lower per capita fertility. There have been repeated calls for rapid action to reduce the world population humanely over the coming decades to centuries ( 1, 3), with lay proponents complaining that sustainability advocates ignore the “elephant in the room” of human overpopulation ( 25, 26). Sustainability requires an eventual stabilization of Earth’s human population because resource demands and living space increase with population size, and proportional ecological damage increases even when consumption patterns stabilize ( 22, 23) it is therefore essential that scenarios for future human population dynamics are explored critically if we are to plan for a healthy future society ( 24). Although it is axiomatic that a smaller human population would reduce most of these threatening processes ( 16), separating consumption rates and population size per se is difficult ( 17) because of their combined effects on the loss of biodiversity and nonprovisioning natural capital ( 3, 18, 19), as well as the variation in consumption patterns among regions and socio-economic classes ( 20, 21). Worldwide, environmental conditions are threatened primarily because of human-driven processes in the form of land conversion (agriculture, logging, urbanization), direct exploitation (fishing, bushmeat), species introductions, pollution, climate change (emissions), and their synergistic interactions ( 15). So rapid has been the recent rise in the human population (i.e., from 1.6 billion in 1900), that roughly 14% of all of the human beings that have ever existed are still alive today ( 14). According to the United Nations, the world human population reached nearly 7.1 billion in 2013, with median projections of 9.6 billion (range: 8.3–11.0 billion) by 2050 and 10.9 billion (range: 6.8–16.6 billion) by 2100 ( 12), with more recent refinements placing the range at 9.6 to 12.3 billion by 2100 ( 13). Regardless, Homo sapiens is now numerically the dominant large organism on the planet. Others argue that technology, ingenuity, and organization are stronger mediators of the environmental impact of human activities ( 9– 11). The size of the global human population is often considered unsustainable in terms of its current and future impact on the Earth’s climate, its ability to distribute food production equitably, population and species extinctions, the provision of adequate ecosystem services, and economic, sociological, and epidemiological well-being ( 1– 8). More immediate results for sustainability would emerge from policies and technologies that reverse rising consumption of natural resources. However, some reduction could be achieved by midcentury and lead to hundreds of millions fewer people to feed. Humanity’s large demographic momentum means that there are no easy policy levers to change the size of the human population substantially over coming decades, short of extreme and rapid reductions in female fertility it will take centuries, and the long-term target remains unclear. 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. Even a catastrophic mass mortality event of 2 billion deaths over a hypothetical 5-y window in the mid-21 st century would still yield around 8.5 billion people by 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. 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. There are consequently more frequent calls to address environmental problems by advocating further reductions in human fertility. The inexorable demographic momentum of the global human population is rapidly eroding Earth’s life-support system.
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