Peak Human Population and the Global Environment

David P. Turner / September 11, 2020


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.


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.

The Biodiversity Bottleneck

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Figure 1. The biodiversity bottleneck displays the ongoing reduction in biodiversity caused by human actions. The fate of biodiversity after the bottleneck is uncertain, but some degree of recovery is possible if humanity self-regulates. Image Credit: Monica G. Whipple and David P. Turner

David P. Turner / May 29, 2020

Earth’s biodiversity is under siege by the global human enterprise (the technosphere).  Most species will survive into the distant future, possibly a future in which the human population has shrunk, and the value of biodiversity is more widely appreciated.  But many species will go extinct along the way. 

Biologist E.O. Wilson and others have evoked the image of a bottleneck in this context (Figure 1).  A bottleneck implies a tightening of constraints on flow.  In the case of the biodiversity bottleneck, flow refers to the survival of species through time.

As the future unfolds and the technosphere continues to grow, the possibilities for species to pass through the biodiversity bottleneck diminish.  But there is a lot of room for maneuver here.  A worthy project for humanity – especially over the next few decades − is to keep that bottleneck as wide as possible.  After passing through it, global biodiversity may recover to some degree as the technosphere begins to weigh less heavily on the biosphere.  It all depends on us.


Evidence of human impacts on biodiversity surrounds us.  Comparisons of current rates of extinction and those in the fossil record indicate that vertebrate species are now going extinct at a rate 100 or more times faster than is observed in most previous geologic periods.  The human-driven Sixth Extinction began perhaps 50,000 years ago when primitive humans arrived on Australia and wiped out many prey species that were unfamiliar with the new bipedal super predator.  Anthropologists refer to the “Pleistocene Overkill” to describe the wave of mammal extinctions that occurred when humans first crossed from Asia to North America about 15,000 years ago.

Modern humans continue to exterminate species directly by overhunting for food (e.g. the passenger pigeon) but also by widespread trafficking in wildlife and animal parts for food, as well as for medicinal and prestige purposes.  Various plant species are also endangered, notably several tropical hardwoods known as rosewood.  Sustained pressure on wildlife habitat from land use change and disruptions in geographic ranges caused by climate change adds further stress on top of overexploitation.  Genetic variation within many species is shrinking as their populations and geographic ranges contract, hence reducing their capacity to survive environmental change (formally termed a population bottleneck).

In essence, the expansion of technosphere capital (the mass of human made objects) is consuming biosphere capital (the biodiversity and biomass of the biosphere).  This loss of biodiversity − usually defined in terms of diversity of species and ecosystems − will likely continue over the coming decades.  As noted, though, the magnitude of the loss will depend heavily on human decisions. 

There are pragmatic, aesthetic, and ethical rationales for conserving Earth’s biodiversity.  Conservationists argue that retaining biodiversity maintains the functional integrity of ecosystems, and hence the full array of their ecosystem services.  Each species has a unique niche and contributes to ecosystem processes like nutrient cycling and recovery from disturbances.  With respect to aesthetics, the earlier mentioned Professor Wilson has suggested that humans have genetically determined biophilia − we get spontaneous pleasure from interacting with diverse forms of life.  The ethical argument is made strongly by the Deep Ecology movement.  For supporters, there is no human exceptionalism – all species have an equal right to survive and prosper on this planet.

Given the multiple rationales for wishing to widen the biodiversity bottleneck, what collective actions (besides the overriding one of limiting climate change) can help?  Scientists and policy experts have identified biodiversity-friendly practices such as reducing pesticide use, buying certified products, reducing invasive species, and reducing water pollution.  But here are four others that have high relevance.

1.  Stop Trafficking in Wildlife and Wild Animal Parts

The global trade in wild animals and wild animal parts puts tremendous downward pressure on the populations of many species.  Wild animals are commonly sold in Southeast Asian food markets despite laws against it.  Tigers, rhinos, and pangolins continue to be poached for dubious medicinal purposes.  Wild caught animals are sold as “bush meat” in parts of Africa and South America.  

A global trade in live animals intended as pets also flourishes.  Millions of  songbirds are collected every year in the primary forests of Indonesia and sold as pets or trained as contestants in bird song competitions.  Tropical fish are collected in the wild for marketing to aquarium owners.  

Multiple international agreements aim to stem wild animal trafficking, especially the Convention on International Trade in Endangered Species of Wild Fauna and Flora.  Under its auspices, national law enforcement agencies regularly confiscate illegal shipments of wild animals, animal parts, and wood from endangered tree species.  However, these efforts face deep resistance for cultural and economic reasons. 

A new brake on wild animal trafficking is fear of zoonotic pathogens.  The SARS-CoV-2 virus that is causing the Covid-19 global pandemic likely jumped from a wild animal host to a human in a market where wild animals are sold illegally.  New legislation passed in China limits sales of wild animal meat.  Unfortunately, enforcement is spotty, and the new law still allows sales of wild animal parts for medicinal purposes.  Sustained international pressure on wildlife traffickers is needed.

The Covid-19 pandemic is apparently impacting wildlife protection in other ways.  Conservationists fear that the loss of the tourist ‘halo’ or proximity effect, because of Covid-19 shutdowns, will increase the incidence of poaching in nature reserves.  Resumption of tourism would help in that regard.

2.  Expand the Size and Number of Protected Areas

A key driver of declining biodiversity is habitat loss.  To bring as many species as possible through the biodiversity bottleneck will require protection of representative areas for all unique types of ecosystems.

The Rewilding Movement has argued for creating protected areas that are large enough to support the whole complement of native species characteristic of each ecosystem type, along with the entire range of abiotic processes such as floods and fires that help maintain it.

Presently, about 15% of global land plus inland waters is protected to some degree.  For the oceans within national jurisdictions, the figure is about 13%.  Not all ecosystem types are represented.  For many types of ecosystems, the current protected area is quite small relative to its original geographic distribution (e.g. the Atlantic rain forest in Brazil).

Aspirations for expanding the protected areas of land and ocean range from 17% of land to half of Earth as a whole (the latter courtesy of the illustrious Professor Wilson).

Protected area plans can be developed over large domains, e.g. the entire United States.  These plans rely on integration of different managed lands such as wilderness areas, national parks, national forests, urban areas, and private reserves.

An international scientific advisory body (IPBES, Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services) is dedicated to biodiversity assessment and conservation.  Like the Intergovernmental panel on Climate Change, IPBES produces periodic assessments and examinations of policy options.  IPBES is supported by member countries, including the U.S., and sustained national contributions are warranted.

In the private sector, land trust organizations such as The Nature Conservancy have as their key strategy the purchase of lands for conservation purposes; contributions are encouraged.  Public/private conservation partnerships are proliferating; participants are welcomed.

3.  Design Sustainable Cities

The proportion of the global population that lives in an urban setting recently passed the 50% mark and is expected to keep climbing in the coming decades.  A potential benefit to biodiversity conservation lies in the land that is abandoned as people supported by subsistence agriculture and nomadic herding move to towns and cities.  The freed-up land can potentially be repurposed in part or whole for wildlife conservation.  An underlying assumption is that agricultural intensification can take up any slack in food production and keep everyone fed. 

Urban greenbelt areas, parks, and gardens may serve directly as another assist to biodiversity conservation.  They support native and alien species and could serve as refuges for plant and animal species that are extirpated regionally by climate change and land use change.  Urban rivers and streams can likewise be managed to protect and support wildlife.

4.  Strategically Increase Ecotourism

In theory, the local economic benefits of nature-based tourism inspire local conservation efforts.  However, high tourist demand produces pressure to increase supply.  More local economic development (e.g. hotels and restaurants) plus more intensive visitor utilization of natural resources may end up degrading local ecosystems.  The research literature contains ecotourism case studies of successes, as well as failures.

Ecotourism narrowly defined refers to tourism that allows visitors to experience local wildlife and landscapes, creates incentives to protect those organisms and landscapes, and supports local communities.  More ecotourism is probably not appropriate in many places where it already exists because capacity is limited.  Rather, it is needed where wildlife is under threat and conservation incentives might be effective.      

Building socio-ecological systems is an emerging route to local sustainability.  These stakeholder groups optimally self-regulate to conserve the economic health and ecological health of the local environment.  Nobel Prize winner Elinor Ostrom has developed principles for structuring and operating these groups.  Monitoring the social, economic, and ecological dimensions of sustainability is a key requirement for successful ecotourism management.

Ecotourism, and tourism more generally, cannot be discussed in the context of biodiversity conservation without considering their global scale impacts.  As noted, climate change is a threat to biodiversity, and the carbon footprint of tourism is estimated to be 8% of total greenhouse gas emissions

Air travel is the foundation for much tourism, but it is especially difficult to decarbonize.  Short hop electric airplanes and long-haul flights powered by renewable energy based liquid fuels are technically feasible.  They could replace fossil fuel powered flights, but more government supported research is needed, and air travelers must be willing to pay an increased fare as these fuels are brought online.  Airline-associated carbon offset programs − although varying in effectiveness and not a permanent solution − contribute significantly to biodiversity conservation.  They help expand protected area and, by sequestering carbon, help slow climate change.


The human capacity to extinguish other species on this planet, and to pervasively alter wildlife habitat, means that we are in many ways responsible for which species and ecosystems will survive.  As we move through peak human population this century and begin to more purposefully manage our impacts on Earth’s biota, let’s keep the biodiversity bottleneck of our own making as wide open as possible.  Progress towards that goal would be both pragmatic and gratifying.

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Earth Day 2020

Earth Day 2020 and Global Solidarity

David P. Turner / April 19, 2020

Earth Day in 2020 is the 50th Anniversary for this annual gathering of our global tribe.  Historically, it has been an opportunity to note declines in environmental quality and to envision a sustainable relationship of humanity to the rest of the Earth system.

This year, in addition to the usual concerns about issues like climate change and ocean acidification, Earth Day is accompanied by concern about the specter of the COVID-19 pandemic.  A glance at the geographic distribution of this virus is the latest reminder that interactions with the biosphere, in this case the microbial component, can link all humans in powerful ways. 

Environmental issues that were on the front burner when Senator Gaylord Nelson initiated Earth Day in 1970 were mostly local − polluted rivers, polluted air, and degraded land cover.  These issues were addressed to a significant degree in the U.S. by passage of the Clean Water Act (1972), the Clean Air Act (1970), and the Endangered Species Act (1973).  These were national level successes inspired by environmental activism.

Awareness of global environmental change in 1970 was only dimly informed by geophysical observations such as the slow rise in the atmospheric CO2 concentration.  But by the 1980s, climate scientists began a drumbeat of testimony to governments and the media that the environmental pollution issue extended to the global scale and might eventually threaten all of humanity. 

The United Nations has functioned as a forum for international deliberations about global environmental change issues, and the signing of the Montreal Protocol on Substances that Deplete the Ozone Layer in 1987 hinted at the possibilities for global solidarity with respect to the environment.

To help matters, economic globalization in the 1990s began uniting the world in new ways.  Huge flows in goods and services across borders fueled a truly global economy.  The level of communication required to support the global economy was based on the rapidly evolving Internet.  It provided the foundation for a global transportation/telecommunications infrastructure that now envelops the planet.

A political backlash to economic and cultural globalization has recently brought to power leaders like Donald Trump (U.S.) and Jair Bolsonaro (Brazil).  Their inclination is much more towards nationalism than towards global solidarity on environmental issues.

However, humanity is indeed united – in fear of climate change and coronavirus pandemics if nothing else.

Each year, the growing incidence of extreme weather events associated with anthropogenic climate change negatively impinges on the quality of life of a vast number of people around the planet.  This year, billions of us are locked down in one form or another to slow the spread of a virus that likely emerged from trafficking in wild animals.  In a mythopoetic sense, it is as if Earth was responding to the depredations imposed upon it by our species.

Philosopher Isabelle Stengers refers to the “intrusion” of Gaia (the Earth system) upon human history.  The message from Gaia is that she is no longer just a background for the infinite expansion of the human enterprise (the technosphere). 

Humanity can reply to Gaia with ad hoc measures like building sea walls for protection from sea level rise.  Or we can get organized and develop a framework for global environmental governance.

There are many impediments to becoming a global “we” that will work collectively on global environmental change issues.  Nevertheless, the incentives for doing so are arriving hard and fast.  The diminishment of the wild animal trade in China in response to COVID-19, and the unintended reduction of greenhouse gas emissions globally associated with efforts to slow the spread of COVID-19, signal that radical change is possible. 

Fitting testaments to an emerging global solidarity about environmental issues would be eradication of commercial exploitation of wild land animals everywhere in the world, and stronger national commitments to reduce greenhouse gas emissions relative to current obligations under the Paris Climate Agreement. 

Both initiatives of course face strong cultural and political headwinds.  But Earth Day, as one of the largest recurring secular celebrations in the world, is an opportunity to think anew.

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