Peak Human Population and the Global Environment

David P. Turner / September 11, 2020

Introduction

A key pursuit in the field of Earth System Science is measuring and monitoring global scale structures and processes.  These measurements have led to the concept of the “Great Acceleration”, a name given to the period since around 1950 during which many global scale attributes related to the human enterprise (the technosphere) began rising in an exponential fashion.  The increase in global population is the iconic example.

Intuitively, it seems unlikely that this level of population increase and associated resource consumption could continue indefinitely on a finite planet.  Practically speaking, problems have begun to arise both with resource shortages and environmental degradation from excess waste production (e.g. global warming and ocean acidification from massive fossil fuel combustion).

Humanity clearly must transition to a more sustainable relationship with the rest of the Earth system.  The way forward lies in bending those exponentially rising Great Acceleration curves for population and resources use, hitting the peaks, and engineering declines.

As noted by ecologists long ago, total resource use (Impact) is a function of the number of people (Population), their per capita use (Affluence), and the efficiency with which raw resources are converted to useful products (Technology).

Resource use per person obviously varies tremendously, hinting at the special responsibility of the more developed countries to limit population growth (the net effect of births minus deaths and immigration minus emigration).  But all humans consume natural resources.  Thus, the high projected population growth rates in less developed countries must also be brought down.  The sooner global population peaks, the less natural capital (e.g. biodiversity) will be degraded, the less likely that competition for resources will lead to human conflict, and the less likely that climate change will trigger tipping points in the Earth system that precipitate extreme impacts on humans.

Past, Present, and Future Global Population

The global population size doubled between 1927 and 1974 and has nearly doubled again since 1979.  It is now 7.8 billion.

However, the rate of annual global population growth has fallen in recent decades (from > 2% per year to 1.05% per year), mostly associated with a decreasing trend in fertility (children born per woman during her reproductive lifetime).

Family planning programs by governmental and nongovernment organizations have significantly impacted the trend towards lower fertility rates.

Projections by demographers of peak global population range widely.  The median estimate from the United Nations Population Division is for a population of 10.9 billion in 2100.  Most of the increase from the present is in Africa.

However, recent research points toward lower values, possibly a peak of 9.7 billion around 2064 and a decline to 8.8 billion by 2100. 

Factors Influencing Demographic Projections

Projections of peak global population have significant policy implications.  Relatively low estimates may have the effect that national commitments to stabilize population are downgraded and that overhyped media accounts of depopulation sap political will to continue family planning programs.  Relatively high estimates for peak global population foster the impression that humanity it doomed to an overcrowded and overheated planet, hence favoring lifeboat ethics.

Despite the critical implications of their results, the models used to predict peak population are very sensitive to the assumptions made about trends in fertility. 

The recent lower estimates for peak global population rely on continued or increasing reductions in fertility in the high fertility countries.  But demographers in the past have sometimes overestimated declines in fertility, and may be doing so now as well.  Historic trends of declining fertility have stalled in some high fertility countries, possibly related to falling support for family planning.  The Catholic Church still formally prohibits artificial birth control. 

Nevertheless, several emerging trends may support lower projected peaks in global population. 

One is that efforts to shift cultural norms favoring large family size increasingly include family planning messaging in popular media (e.g. serial dramas), which are having significant success with both genders

Another is that the incidence of unplanned pregnancy is declining globally (1990-2014), probably as a function of improving access to family planning resources.

The Covid-19 pandemic could push birth rates down (at least in the more developed countries) because financial insecurity will dispose women in developed and developing countries to postpone or forgo having children.  

Mortality rates may also be higher than expected.  Life expectancy has generally increased in recent decades throughout the world.  Much of that increase is associated with reduced child mortality but increasing longevity is also a factor.  However, life expectancy in the U.S. went down from 2014 to 2017 because of increasing fatal drug overdoses and suicides.  Climate change is expected to bring an increase in extreme weather events causing mortality directly (as in flooding), and indirectly by way of impacts on agriculture and possibly the incidence of war.

Implications Beyond Absolute Population Size

A leading concern about a rapid peak and then decline in national populations is the associated increase in the ratio of older retired people to younger working people.  As the population ages, the number of active workers available to support each elderly person tends to decline.  Hence, taxes may have to be increased to provide income and health care to the elderly.  Various mitigating factors include the improving health of elderly people, significant intergenerational transfers of wealth, increases in labor force participation by the elderly, and volunteer efforts by the elderly.

Also, a larger proportion of elderly people generally means decreased per capita demand for resources and a more peaceful society.

A decline in the number of children per family can have many beneficial side effects including: 1) more resources (parental attention and ability to finance education) per child, 2) improved quality of life for parents (less stress and more free time), and 3) rising per capita income.

Conclusion

The sooner global population peaks and begins to decline, the greater the possibilities for achieving global sustainability.  Since about 40% of pregnancies globally are still unplanned, a primary tool for insuring children are born into a welcoming and opportunity-rich environment is continued and improved provision of family planning support in both the developing and developed world.  More political will and contributions to NGOs are needed.  At this point in human history, the local and global challenges (environmental, economic, and social) that arise from a stable or declining population are likely more manageable than those arising from high rates of population growth.

A Positive Narrative for the Anthropocene

David P. Turner / July 16, 2020

Humans are story-telling animals.  Our brains are wired to assimilate information in terms of temporal sequences of significant events.  We are likewise cultural animals.  Within a society, we share images, words, rituals, and stories.  Indigenous societies often have myths about their origin and history.  Religious mythologies remain prevalent in contemporary societies.

The discipline of Earth System Science has revealed the necessity for a global society that can address emerging planetary scale environmental change issues – notably climate change.  A shared narrative about the relationship of humanity to the biosphere, and more broadly to the Earth system, is highly desirable in that context. 

The most prevalent narrative about humanity’s relationship to the Earth system emphasizes the growing magnitude of our deleterious impacts on the global environment (think ozone hole, climate change, biodiversity loss).  The future of humanity is then portrayed as more of the same, unless radical changes are made in fossil fuel emissions and natural resource management.

In the process of writing a book for use in Global Environmental Change courses, I developed an elaborated narrative for humanity − still based on an Earth system science perspective but somewhat more upbeat.  I used the designation Anthropocene Narrative to describe it because Earth system scientists have begun to broadly adopt the term Anthropocene to evoke humanity’s collective impact on the environment. 

There are of course many possible narratives evoked by the Anthropocene concept (e.g. the historical role of capitalism in degrading the environment), all worthy of study.  But for the purposes of integrating the wide range of material covered in global environmental change classes, I identified a six stage sequence in the relationship of humanity to the rest of the Earth system that serves to link geologic history with human history, and with a speculative vision of humanity’s future (Figure 1).  The stages are essentially chapters in the story of humanity’s origin, current challenges, and future.  The tone is more hopeful than dystopian because our emerging global society needs a positive model of the future.  

Figure 1.  An Earth system science inspired Anthropocene narrative with six stages.  Image credits below.

The chapters in this Anthropocene narrative are as follows.

Chapter 1.  The Pre-human Biosphere

The biosphere (i.e. the sum of all living organisms) self-organized relatively quickly after the coalescence of Earth as a planet.  It is fueled mostly by solar energy.  The biosphere drives the global biogeochemical cycles of carbon, nitrogen, and other elements essential to life, and plays a significant role in regulating Earth’s climate, as well as the chemistry of the atmosphere and oceans. The biosphere augments a key geochemical feedback in the Earth system (the rock weathering thermostat) that has helped keep the planet’s climate in the habitable range for 4 billion years.  By way of collisions with comets or asteroids, or because of its own internal dynamics, the Earth system occasionally reverts to conditions that are harsh for many life forms (i.e. mass extinction events).  Nevertheless, the biosphere has always recovered − by way of biological evolution − and a mammalian primate species recently evolved that is qualitatively different from any previous species. 

Figure 2.  The pre-human biosphere was a precondition for the biological evolution of humans.  Image Credit: NASA image by Robert Simmon and Reto Stöckli.

Chapter 2.  The Primal Separation

Nervous systems in animals have obvious adaptive significance in term of sensing the environment and coordinating behavior.  The brain of a human being appears to be a rather hypertrophied organ of the nervous system that has evolved in support of a capacity for language and self-awareness.  These capabilities are quite distinctive among animal species, and they set the stage for human conquest of the planet.  The most recent ice age receded about 12,000 year ago and a favorable Holocene climate supported the discovery and expansion of agriculture.  With agriculture, and gradual elaboration of toolmaking, humanity ceased waiting for Nature to provide it sustenance.  Rather, Nature became an object to be managed.  This change is captured in the Christian myth of Adam and Eve’s expulsion from the Garden of Eden (Figure 3).  They lived like all other animals in the biosphere until they became self-aware and began to consciously organize their environment.

Figure 3.  The story of Adam and Eve symbolizes the separation of early humans from the background natural world.  Image Credit: Adam and Eve expelled from Eden by an angel with a flaming sword. Line engraving by R. Sadeler after M. de Vos, 1583. Wellcome Trust.

Chapter 3.  The Build-out of the Technosphere

The next phase in this narrative is characterized by the gradual evolution and spread of technology.  An important driving force was likely cultural group selection, especially with respect to weapons technology and hierarchical social structure.  The ascent of the scientific worldview and the global establishment of the market system were key features.  Human population rose to the range of billions, and the technosphere began to cloak Earth (Figure 4).  The Industrial Revolution vastly increased the rate of energy flow and materials cycling by the human enterprise.  Telecommunications and transportation infrastructures expanded, and humanity began to get a sense of itself as a global entity.  Evidence that humans could locally overexploit natural resources (e.g. the runs of anadromous salmon in the Pacific Northwest U.S.) began to accumulate.

Figure 4.  The Earth at night based on satellite imagery displays the global distribution of technology dependent humans.  Image Credit: NASA/GSFC/Visualization Analysis Laboratory.

Chapter 4.  The Great Acceleration

Between World War II and the present, the global population grew from 2.5 billion to 7.8 billion people.  Scientific advances in the medical field reduced human mortality rates and technical advances in agriculture, forestry, and fish harvesting largely kept pace with the growing need for food and fiber.  The extent and density of the technosphere increased rapidly.  At the same time, we began to see evidence of technosphere impacts on the environment at the global scale – notably changes in atmospheric chemistry (Figure 5) and losses in global biodiversity.

Figure 5.  The impacts of the global human enterprise on various indicators of Earth system function take on an exponential trajectory after World War II.  Image Credit: Adapted from Steffen et al. 2015.

Chapter 5.  The Great Transition

This phase is just beginning.  Its dominant signal will be the bending of the exponentially rising curves for the Earth system and socio-economic indicators that define the Great Acceleration (Figure 5 above).  Global population will peak and decline, along with the atmospheric CO2 concentration.  Surviving the aftermath of the Great Acceleration with be challenging, but the Great Transition is envisioned to occur within the framework of a high technology infrastructure (Figure 6) and a healthy global economy.  To successfully accomplish this multigenerational task, humanity must begin to function as a global scale collective, capable of self-regulating.  Neither hyper-individualism nor populist tribal truth will get us there.  It will take psychologically mature global citizens, visionary political leaders, and new institutions for global governance.

Figure 6.  A critical feature of the Great Transition will be a renewable energy revolution.  Image Credit: Grunden Wind Farm

Chapter 6.  Equilibration

Human-induced global environmental change will continue for the foreseeable future.  The assumption for an Equilibration phase is that humanity will gain sufficient understanding of the Earth system – including the climate subsystem and the global biogeochemical cycles – and develop sufficiently advanced technology to begin using the technosphere and managing the biosphere to purposefully shape the biophysical environment from the scale of ecosystems and landscapes (Figure 7) to the scale of the entire planet.  Humanity is a part of the Earth system, meaning it must gain sufficient understanding of the social sciences to produce successive generations of global citizens who value environmental quality and will cooperate to manage and maintain it.  The challenges to education will be profound.

Figure 7.  An idealized landscape in which the biosphere and technosphere are sustainably integrated.  Image Credit: Paul Cézanne, Mont Sainte-Victoire, 1882–1885, Metropolitan Museum of Art.

As noted, this Anthropocene Narrative is largely from the perspective of Earth system science.  In the interests of coherence, humanity is viewed in aggregate form.  Humanities scholars reasonably argue that in the interests of understanding climate justice, “humanity” must be disaggregated (e.g. by geographic region or socioeconomic class).  This perspective helps highlight the disproportionate responsibility of the developed world for driving up concentrations of the greenhouse gases.  The aggregated and disaggregated perspectives on humanity are complimentary; both are needed to understand and address global environmental change issues.

The Anthropocene Narrative developed here is broadly consistent with scientific observations and theories, which gives it a chance for wide acceptance.  The forward-looking part is admittedly aspirational; other more dire pathways are possible if not probable.  However, this narrative provides a solid rationale for building a global community of all human beings.  We are all faced with the challenge of living together on a crowded and rapidly changing planet.  The unambiguous arrival of global pandemics and climate change serve as compelling reminders of that fact.  A narrative of hope helps frame the process of waking up to the perils and possibilities of our times.

Recommended Video:  Welcome to the Anthropocene (~ 3 minutes)

This blog post was featured as a guest blog at the web site for The Millennium Alliance for Humanity and the Biosphere (MAHB).

https://mahb.stanford.edu/blog/a-positive-narrative-for-the-anthropocene/