Human population reduction is not a quick fix for environmental problems Corey J. A. Bradshaw1 and Barry W. Brook The Environment Institute and School of Earth and Environmental Sciences, The University of Adelaide, Adelaide, SA 5005, Australia Edited by Paul R. Ehrlich, Stanford University, Stanford, CA, and approved September 15, 2014 (received for review June 5, 2014) The inexorable demographic momentum of the global human of Earth’s human population because resource demands and population is rapidly eroding Earth’s life-support system. There are living space increase with population size, and proportional consequently more frequent calls to address environmental prob- ecological damage increases even when consumption patterns lems by advocating further reductions in human fertility. To exam- stabilize (22, 23); it is therefore essential that scenarios for future ine how quickly this could lead to a smaller human population, we human population dynamics are explored critically if we are to used scenario-based matrix modeling to project the global popula- plan for a healthy future society (24). tion to the year 2100. Assuming a continuation of current trends in There have been repeated calls for rapid action to reduce the mortality reduction, even a rapid transition to a worldwide one- world population humanely over the coming decades to centuries child policy leads to a population similar to today’s by 2100. Even (1, 3), with lay proponents complaining that sustainability a catastrophic mass mortality event of 2 billion deaths over a hypo- advocates ignore the “elephant in the room” of human over- st thetical 5-y window in the mid-21 century would still yield around population (25, 26). Amoral wars and global pandemics aside, 8.5 billion people by 2100. In the absence of catastrophe or large the only humane way to reduce the size of the human population fertility reductions (to fewer than two children per female world- is to encourage lower per capita fertility. This lowering has been — wide), the greatest threats to ecosystems as measured by regional happening in general for decades (27, 28), a result mainly of — projections within the 35 global Biodiversity Hotspots indicate higher levels of education and empowerment of women in the that Africa and South Asia will experience the greatest human pres- developed world, the rising affluence of developing nations, and ’ sures on future ecosystems. Humanity s large demographic momen- the one-child policy of China (29–32). Despite this change, en- tum means that there are no easy policy levers to change the size of vironmental conditions have worsened globally because of the the human population substantially over coming decades, short of overcompensating effects of rising affluence-linked population extreme and rapid reductions in female fertility; it will take centu- and consumption rates (3, 18). One of the problems is that there ries, and the long-term target remains unclear. However, some re- is still a large unmet need for more expansive and effective duction could be achieved by midcentury and lead to hundreds of family-planning assistance, which has been previously hindered millions fewer people to feed. More immediate results for sustain- by conservative religious and political opposition, premature ability would emerge from policies and technologies that reverse claims that rapid population growth has ended, and the reallo- rising consumption of natural resources. cation of resources toward other health issues (33). Effective contraception has also been delayed because of poor education demography | fertility | catastrophe | war | mortality regarding its availability, supply, cost, and safety, as well as op- position from family members (33). Notwithstanding, some ar- he size of the global human population is often considered gue that if we could facilitate the transition to lower fertility Tunsustainable in terms of its current and future impact on the Earth’s climate, its ability to distribute food production equita- Significance bly, population and species extinctions, the provision of adequate ecosystem services, and economic, sociological, and epidemio- ’ logical well-being (1–8). Others argue that technology, ingenuity, The planet s large, growing, and overconsuming human pop- and organization are stronger mediators of the environmental ulation, especially the increasing affluent component, is rapidly ’ impact of human activities (9–11). Regardless, Homo sapiens is eroding many of the Earth s natural ecosystems. However, ’ now numerically the dominant large organism on the planet. society s only real policy lever to reduce the human population According to the United Nations, the world human population humanely is to encourage lower per capita fertility. How long reached nearly 7.1 billion in 2013, with median projections of 9.6 might fertility reduction take to make a meaningful impact? billion (range: 8.3–11.0 billion) by 2050 and 10.9 billion (range: We examined various scenarios for global human population 6.8–16.6 billion) by 2100 (12), with more recent refinements change to the year 2100 by adjusting fertility and mortality placing the range at 9.6 to 12.3 billion by 2100 (13). So rapid has rates (both chronic and short-term interventions) to determine been the recent rise in the human population (i.e., from 1.6 the plausible range of outcomes. Even one-child policies im- posed worldwide and catastrophic mortality events would still billion in 1900), that roughly 14% of all of the human beings that – have ever existed are still alive today (14). likely result in 5 10 billion people by 2100. Because of this Worldwide, environmental conditions are threatened primar- demographic momentum, there are no easy ways to change the broad trends of human population size this century. ily because of human-driven processes in the form of land con- version (agriculture, logging, urbanization), direct exploitation Author contributions: C.J.A.B. and B.W.B. designed research; C.J.A.B. and B.W.B. per- (fishing, bushmeat), species introductions, pollution, climate formed research; B.W.B. contributed new reagents/analytic tools; C.J.A.B. analyzed data; change (emissions), and their synergistic interactions (15). Al- and C.J.A.B. and B.W.B. wrote the paper. though it is axiomatic that a smaller human population would The authors declare no conflict of interest. reduce most of these threatening processes (16), separating This article is a PNAS Direct Submission. consumption rates and population size per se is difficult (17) Data deposition: Data available from the Aekos Data Portal, www.aekos.org.au (dx.doi. because of their combined effects on the loss of biodiversity and org/10.4227/05/53869A9434A46). nonprovisioning natural capital (3, 18, 19), as well as the varia- 1To whom correspondence should be addressed. Email: [email protected]. tion in consumption patterns among regions and socio-economic This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. classes (20, 21). Sustainability requires an eventual stabilization 1073/pnas.1410465111/-/DCSupplemental. 16610–16615 | PNAS | November 18, 2014 | vol. 111 | no. 46 www.pnas.org/cgi/doi/10.1073/pnas.1410465111 Downloaded by guest on September 29, 2021 rates, most of the sustainability problems associated with the We accessed 2013 fertility data by 5-y age groups from the US Census large human population would be greatly alleviated (3, 34–36). Bureau International Database. We converted the births per 1,000 women Even in an ideal socio-political setting for lower birth-rate pol- into age-specific fertilities (mx) by dividing the 5-y classes equally among icies and the commitment to global-scale family planning, however, their constituent years and accounting for breeding female mortality within i each of the 5-y classes. All age-specific population size, mortality, and fer- several questions remain: ( ) How quickly could we achieve tility data we derived from these sources are available online at dx.doi.org/ a smaller human population by adjusting such sociological levers 10.4227/05/5386F14C65D34. (or via unexpected, large-scale stressors), and (ii)whereinthe world are human populations likely to do the most damage to their Leslie Matrix. We defined a prebreeding 100 (i) × 100 (j)element,Lesliematrix supporting environment over the coming century? To address (M) for females only, multiplying the subsequent projected population vector the first of these questions on population trajectories, we built by the overall sex ratio to estimate total population size at each time step. Fertilities (mx) occupied the first row of the matrix (ages 15–49), survival deterministic population models for humans, based on broad, – multiregion geographical data drawn from the World Health probabilities (1 Mx) were applied to the subdiagonal, and the final diagonal transition probability (Mi,j) represented survival of the 100+ age class. Complete Organization (WHO) and the United States Census Bureau. R code (43) for the scenario projections is provided in Datasets S1 and S2. Using a Leslie-matrix approach, we projected the 2013 world population through to the year 2100 with several adjustments to Global Scenarios. For each projection, we multiplied the Nx vector by M for 87 fertility, mortality, and age at first childbirth (primiparity) to in- yearly time steps (2013–2100, except for one fertility-reduction scenario that vestigate the relative importance of different vital rates (repre- was extended to 2300). All projections were deterministic. Scenario 1 was senting possible policy interventions or stressors) on the trajectory a business-as-usual (BAU) “control” projection, with all matrix elements kept “ ” and population size at the end of this century, and on the ratio of constant at 2013 values. Scenario 2a was a realistic projection with a linear the “dependent” component of the population (<15 and >65 y) decline in Mx, starting in 2013, to 50% of their initial values by 2100 (i.e., via improving diet, affluence, medicine, female empowerment, and so forth).