The Socio-Enviro-Economic System
National Science Review 2016

Modeling sustainability: population, inequality, consumption, and bidirectional coupling of the Earth and Human Systems

Burgess COMMENTARY

Peter Burgess

1 Planet Earth has been the habitat of humans for hundreds of thousands of years. Human life depends on the resources provided by the Earth System: air from the Earth's atmosphere; water from the atmosphere and rivers, lakes, and aquifers; fruits from trees; meat and other products from animals; and over the past 10,000 years, land for agriculture, and metals and other minerals from the Earth's crust. Until about 200 years ago, we used renewable biomass as the major source of materials and energy, but over the course of the past two centuries, we have instead become heavily dependent on fossil fuels (coal, oil, and natural gas) and other minerals for both materials and energy. These nonrenewable resources made possible both of the revolutions which drove the the growth in consumption per capita and population: the Industrial Revolution and the Green Revolution. Our relationship with our planet is not limited to consuming its resources. Waste is an inevitable outcome of any production process; what is produced must return to the Earth System in some form. Waste water goes back to the streams, rivers, lakes, oceans, or into the ground; greenhouse and toxic gases go into the atmosphere, land, and oceans; and trash goes into landfills and almost everywhere else.

2 Using gross domestic product (GDP) per capita as a rough measure of consumption per capita, the extent of the impact of the Human System on the Earth System can be estimated from the total population and the average consumption per capita. This can be also seen from the defining equation for GDP per capita, i.e., GDP per capita = GDP/Population. One may rewrite this equation as GDP = Population × GDP per capita. By taking variations, we get:

δGDPGDP=δPopulationPopulation+δGDP per capitaGDP per capita

 This equation simply means that the relative change in the total GDP is comprised of two components, i.e., the relative changes in population and GDP per capita. A graphical demonstration of this decomposition can be seen in the inset of Fig. 1. Data from [229], with updates from the Maddison Project, 2013 (for the underlying methodology of the updates see [228]). Population data for the inset from [230].

3 In fact, the UN's 2015 Population Revision has already raised the global total in 2100 by 360 million to 11.2 billion just from the last estimate published in 2013 [1].

4 While there has been some reduction of the energy intensity and emissions intensity of economic growth in wealthy countries, one has to be cautious about extrapolating recent improvements, as small as they may be, because these improvements have been at least partly due to the outsourcing of energy-intensive sectors to poorer countries [122,123,124,125,126,127,128,129,130,131,132,233], and because there are basic physical limits to further efficiency improvements, especially in the use of water, energy, food, and other natural resources [113,114,234,235,236].

5 For example, between 1950 and 1984, the production of grains increased by 250% due to the use of fossil fuels for fertilization, mechanization, irrigation, herbicides, and pesticides [237]. These technological advances, together with the development of new seed varieties, are referred to as the ‘Green Revolution’ that allowed global population to double in that period [238].

6 Thus, while the rate of materials intensity of GDP growth has declined (very slowly: 2.5 kg/$ in 1950, 1.4 kg/$ in 2010), the per capita rate continues to increase. The only materials category whose per capita use has remained relatively stable is biomass [4,132], probably reflecting the physical limits of the planet's regenerating natural resources to continue to provide humans with ever-growing quantities of biomass [239,240]. (See [134] for a conceptual model of regenerating natural resources.)

7 In some estimates, as much as 10–15 million years for carbon [241].

8 Rates of deforestation and agricultural expansion have accelerated in recent years with extensive new infrastructure providing conduits for settlement, exploitation, and development. Even within the remaining habitat, fragmentation is causing rapid species loss or alteration, and is producing major impacts on biodiversity, regional hydrology, and global climate, in particular in tropical forests, which contain over half of Earth's biodiversity and are an important driver in the climate system. Ongoing worldwide habitat fragmentation, together with anthropogenic climate change and other human pressures, may severely degrade or destroy any remaining ecosystems and their wildlife [242,243,244,245,246,247,248,249,250,251]. For example, the Living Planet Index, which measures biodiversity based on 14,151 monitored populations of 3,706 vertebrate species, shows a staggering 58% decline in populations monitored between 1970 and 2012 [252]. Continuation of these trends could result in the loss of two-thirds of species populations by 2020 (only 4 years from now) compared to 1970 levels (i.e., in just half a century).

9 The US, one of the highest resource use per capita countries in the world (e.g., with an energy consumption per capita 4 times that of China and 16 times that of India in 2010 [87]), is projected to grow in population by ∼50% from both natural increase and immigration, generating a very large increase in total resource use and waste generation for the US alone.

10 For example, while Africa's population is projected to rise over 6-fold from 366 million in 1970 to 2.4 billion in 2050, the UN's projection of annual net emigration from Africa remains about constant until 2050, at ∼500,000, similar to the average from 1970 to 2000, thus the projected percentage emigrating declines sharply. (For comparison, between 1898 and 1914, ∼500,000 people emigrated each year just from Italy alone, when its population was only 30–35 million.) Then, from 2050 to 2100, the annual net emigration is arbitrarily projected to smoothly decline to zero, even as Africa's projected population continues rising to over 4 billion [3]. However, recent international migration has increased on average with global population [253] and net migration to the developed countries has increased steadily from 1960 to 2010 [3] and more explosively recently. Thus, the UN projection of emigration seems unrealistically low, both relative to its increasing population and in the context of a rapidly aging, and supposedly shrinking, population in the developed countries, as well as recent migration pattern alterations following conflicts and associated social disruptions. The United Nations High Commissioner on Refugees estimates that by the end of 2015, a total of 65.3 million people in the world were forcibly displaced, increasing at a rate of ∼34,000 people per day. There are 21.3 million refugees worldwide, with more than half from Afghanistan, Syria, and Somalia [254]. Yet, this figure only reflects refugees due to persecution and conflict but does not include refugees as a result of climate change, famines, and sea level rise. Net migration in 2015 to Germany alone was 1.1 million [255]. The UN's 2012 projections of migration for other regions are similarly arbitrary and unrealistic. Net annual emigration from Latin America is projected to decline from nearly 1.2 million in 2000–2010 to ∼500,000 by 2050, and then decline to zero by 2100. Net annual immigration to Europe rose from 41,000 in 1960–1970 to almost 2 million in 2000–2010, and explosively today due to increasing social strife, and yet, the UN's projection for 2010–2050 is a continuous decline in net immigration down to only 900,000 by 2050, and then a decline to zero by 2100 [3]. Even the UN Population Division itself admits, ‘We realize that this assumption is very unlikely to be realized but it is quite impossible to predict the levels of immigration or emigration within each country of the world for such a far horizon’ [3].

11 In order to limit the total increase in average global temperatures within the context of current climate change negotiations over carbon budgets, there is a maximum total amount of carbon that can be emitted globally, thus carbon emissions must be apportioned across countries and across time. Lower estimates of migration to developed countries means lower estimates of emissions in the future in developed countries, which means the developed countries are not required to make as much effort today to lower their own emissions [104].

12 These dramatic declines appear highly unlikely given that countries like Japan and Germany have not yet declined by more than ∼1% [3], and already their governments have enacted a series of policies to encourage higher birth rates. In fact, there is evidence for the efficacy of various family policies in the recent fertility rebounds observed in several developed countries [97]. Similarly, Russia, which saw its fertility rate plunge after 1989 (reaching a low of 1.19 in 1999 from 2.13 in 1988), enacted pronatalist policies and liberalized immigration. The fertility rate has since rebounded, and the population decline reversed in 2009.

13 Despite widespread talk of population decline, it is important to emphasize that the only countries in the world to have experienced any declines beyond ∼1% have all been associated with the special circumstances of the collapse of the former Soviet Bloc (and some microstates, such as a few island nations), and even in these cases, migration has played a major role in population changes. This is even true within Germany, where the only Länder (provinces) which have declined significantly in population are all from the former East Germany.

14 For example, the 1999 UN projections significantly overestimated the time it would take to add the next billion people [256]. Worse still, the 2015 estimate for the world's total population in 2100 [1] has gone up by over 2 billion just since the 2004 estimate [257]. Even the 2010 UN projections had to be revised upwards in 2012 because previous estimates of total fertility rates in a number of countries were too low, and in some of the poorest countries the level of fertility appears to have actually risen in recent years [3,258,259], and the 2012 estimates have again been revised upwards in 2015 [1]. To put all this in perspective, the 2004 estimates projected a peak in world population of 9.22 billion, but in the 2015 projection, 9.22 billion will be reached as early as 2041, and will still be followed by at least another six decades of growth.

15 Poorer populations are expected to be more heavily impacted by climate change and other mounting environmental challenges despite having contributed much less to their causes [136,166,167,260]. International and internal migration is increasingly being seen as an important component of adaptation and resilience to climate change and a response to vulnerabilities from other environmental risks. Given the aging social structures in some parts of the world, policies that support increased migration could be important not only for environmental adaptation, but also for the realization of other socioeconomic and demographic goals. Thus, migration will help both the sending and receiving countries. Of course, given the scale of projected population growth, migration alone is unlikely to be able to balance the regional disparities.

16 The scientific community has urged limiting the global mean surface temperature increase relative to pre-industrial values to 2°C. The economic challenges of staying on a 2°C pathway are greater, the longer emission reductions are postponed [261]. Given the role that developed countries have played historically in the consumption of fossil fuels, and their much higher per capita carbon emissions today, it is imperative that the developed countries lead the way and establish a successful track record in achieving such reductions. Thus, it is positive that at the United Nations Framework Convention on Climate Change Conference of the Parties (COP21) in Paris in December 2015, governments crafted an international climate agreement indicating their commitments to emissions reduction for the near term (to 2025 or 2030). Most importantly, the US committed to reduce economy-wide GHG emissions below 2005 levels by 26%–28% in 2025, and the EU has committed to reduce 2030 GHG emissions relative to 1990 by 40% (excluding emissions from land-use changes). Assuming that the goals of the Intended Nationally Determined Contributions (INDCs) are fulfilled, the results in [262] and [261] show that a successful Paris agreement on near-term emissions reductions will be valuable in reducing the challenges of the dramatic long-term transformations required in the energy system to limit global warming to 2°C. The Paris framework will succeed even further if it enables development of subsequent pathways leading to the required additional global emissions reductions.

17 As another example, millions of metric tons of plastic enter the oceans every year and accumulate throughout the world's oceans, especially in all subtropical gyres [263,264].

18 The effect of technological change can be observed in the transition from agrarian to industrial society. This industrialization raised agricultural yields largely due to increasing inputs, thereby allowing rapid Human System expansion, but it also generated a societal regime shift that greatly increased per capita resource use and waste generation [131].

19 One can generalize the definition of Carrying Capacity to any subsystem with different types of natural resources coupled with sociodemographic variables. For example, the subsystems for water, energy, and agriculture—each coupled bidirectionally to human sociodemographic variables and to each other—result in Water CC, Energy CC, and Agriculture CC. Water CC can be defined as the level of population that can be sustained at a particular per capita consumption and a given level of water sources and supply in the area under study. In general, this level depends on both human and natural factors. For example, Water CC is determined by the natural flow rate of water into and out of the area, precipitation and evaporation, withdrawal rate from water sources, dispensing technology, recycling capacity, etc. Moreover, Water, Energy, or Agriculture CC in a certain area can be imported from other regions to temporarily support a larger population and consumption [123,265,266]. Recent literature has emphasized the integrated nature of agricultural, energy, and water resources, and modeling the interactions of these subsystems is essential for studying the food–energy–water nexus [267,268,269,270,271]. In order to understand and model either Human or Earth Systems, we must model all these natural and human subsystems interactively and bidirectionally coupled.

20 A recent study focusing on the many collapses that took place in Neolithic Europe concluded that endogenous causes, i.e., overrunning CC and the associated social stresses, have been the root cause of these collapses [214,272].

21 Collapses could also happen due to rapid decline of CC as a result of environmental degradation. Droughts and climate change can decrease natural capacities and regeneration rates, which in turn lead to a decline of CC. (Motesharrei et al. [134] describe how to model these factors for a generic system, termed Nature Capacity and Regeneration Rate, and how they determine CC.) The resulting gap between total consumption and CC can lead to conflicts and the ensuing collapses. For example, see recent literature that shows the impacts of climate change on conflicts [159,160,161], a potential precursor to collapses.

22 For example, ‘Until 2008 not a single earthquake had ever been recorded by the U.S. Geological Survey from the Dallas-Fort Worth (DFW) area...Since then, close to 200 have shaken the cities and their immediate suburbs. Statewide, Texas is experiencing a sixfold increase in earthquakes over historical levels. Oklahoma has seen a 160-fold spike in quakes...In 2014 the state's earthquake rate surpassed California's’. [273].

23 The USGCRP report states that ‘Current and future climate impacts expose more people in more places to public health threats...Almost all of these threats are expected to worsen with continued climate change’.

24 Such interactions take place not just at a global scale but also at the ecosystem and local habitat scales. Ecosystems at the regional and local scales provide critical habitat for wildlife species, thus preserving biodiversity, and are also an essential source of food, fiber, and fuel for humans, and forage for livestock. Ecosystem health, composition, function, and services are strongly affected by both human activities and environmental changes. Humans have fundamentally altered land cover through diverse use of terrestrial ecosystems at the local scale, which then impact systems at the global scale. Additional changes have taken place as a result of climate change and variability [274]. Furthermore, human pressures are projected to have additional repercussions for species survival, biodiversity, and the sustainability of ecosystems, and in turn feedback on humans’ food security and economic development [141]. Thus, a key challenge to manage change and improve the resilience of terrestrial ecosystems is to understand the role that different human and environmental forces have on them, so that strategies that target the actual drivers and feedbacks of coupled components of change can be developed and implemented. Understanding how terrestrial ecosystems function, how they change, and what limits their performance is critically important to determine their Carrying Capacity for accommodating human needs as well as serving as a viable habitat for other species, especially in light of anticipated increase in global population and resource consumption for the rest of this century and beyond. Biodiversity and ecosystem services in forests, farmlands, grazing lands, and urban landscapes are dominated by complex interactions between ecological processes and human activities. In order to understand such complexity at different scales and the underlying factors affecting them, an integrated Human–Earth systems science approach that couples both societal and ecological systems is needed. Humans and their activities are as important to the changing composition and function of the Earth system as the environmental conditions and their natural variability. Thus, coupled models are needed to meet the challenges of overcoming mismatches between the social and ecological systems and to establish new pathways toward ‘development without destruction’ [242,243,244,245,246,247,248,250,251].

25 MIT: Massachusetts Institute of Technology; IGSM: Integrated Global System Modeling framework; US DOE: United States Department of Energy; GCAM: Global Change Assessment Model; IIASA: International Institute for Applied Systems Analysis; MESSAGE: Model for Energy Supply Systems And their General Environmental impact; EAA: Environmental Assessment Agency; IMAGE: Integrated Modelling of Global Environmental Change.

26 Furthermore, the use of GDP as a key measure and determinant in these future projections is itself highly problematic, because it is a very weak measure of human well-being, economic growth, or societal prosperity [275]. GDP neither accounts for the value of natural capital nor human capital, ignores income and wealth inequality, neglects both positive and negative externalities, and only captures social costs and environmental impacts to the extent that prices incorporate them. Any economic activity, whether deleterious or not, adds to GDP as long as it has a price. For example, labor and resources spent to repair or replace loss due to conflicts or environmental damages are counted as if they add to—rather than subtract from—total output. Alternative measures, such as the Genuine Progress Indicator, the Sustainable Society Indicator, the Human Development Index, and the Better Life Index of the Organization for Economic Co-operation and Development, have been developed [276,277,278]. These measures show that, especially since the 1970s, the large increases in GDP in the developed countries have not been matched by increases in human well-being. Integrated and coupled Human–Earth system models will allow for the development of much more accurate and realistic measures of the actual productivity of economic activity and its costs and benefits for human well-being. Such measures will allow for the valuation of both natural and human capital and for defining and developing sustainability metrics that are inclusive of the wealth of natural and human capital. They will also bring together the current disparate debates on environment/climate, economics, demographics, policies, and measures to put the Human–Earth System on a more sustainable path for current and future generations.

27 Input–Output analysis can account for the flows of resource inputs, intermediate and finished goods and services, and waste outputs along the production chain [279,280]. By accounting for the impacts of the full upstream supply chain, IO analysis has been used in life-cycle analysis [281] and for linking local consumption to global impacts along global supply chains [122,127,282,283]. IO models can be extended with environmental parameters to assess different environmental impacts from production and consumption activities, including water consumption [123,284,285], water pollution [285,286], carbon dioxide emissions [287,288,289], land-use and land-cover change [130,290,291], and biodiversity [283]). Such models have been developed and applied at various spatial and temporal scales. Since emissions embodied in trade have been growing rapidly, resulting in an increasing divergence between territorial-based and consumption-based emissions, territorial measures alone cannot provide a comprehensive and accurate analysis of the factors driving emissions nor the effectiveness of reduction efforts [292]. IO models can provide these consumption-based calculations and have been employed to identify and quantify key drivers for emissions and energy consumption (such as population growth, changes of consumption patterns, and technical progress [288,293,294]) as well as the environmental impacts of social factors (such as urbanization and migration) reflecting consumption patterns of different categories of households with high spatial detail [282,295]. IO analysis can also be used for simulating potential future states of the economy and the environment (e.g., [296]), through dynamically updating technological change and final demand, or employing recursive dynamics to explore explicit scenarios of change.

28 Many of the variables in such a model are affected by processes at the global scale, while decisions are often made at a local scale. Choosing variables from the subsystems depends on the specific goals of the model. Reconciling various scales spatially or temporally can be done through downscaling, aggregating, and averaging for variables defined at smaller scales. Moreover, the Human System strongly influences consumption even at these smaller local scales. For example, Srebric et al. [297] show that not only population size but also behavior of people at the community scale strongly affects local energy consumption. This example shows that coupled Human System models are needed at various scales to project consumption patterns, especially for energy and water. We thank the anonymous Reviewer No. 2 for emphasizing the importance of coupling across various scales, and for many other helpful comments.

29 There are numerous examples of policies successfully tackling many of the challenges identified in this paper. For example, the province of Misiones, Argentina, with policies for forest protection and sustaining local incomes, stands out by having very high, remotely sensed values of NDVI (vegetation index) compared to the neighboring regions [298]. The state of Kerala, India, despite a very low GDP per capita (under $300 until 1990s), through policies expanding access to education and medical care, enjoys higher life expectancy, lower birth rates, lower inequality, and superior education compared to the rest of India [299,300,301]. Formal primary, secondary, and tertiary education can also reduce societal inequalities and improve economic productivity [98,182,184,302]. Education itself can also be offered in other forms. For example, education through mass media can be influential for changing long-term cultural trends and social norms, as can be seen in the successful attempt to reduce fertility rates in Brazil using soap operas [303]. There have also been other extremely successful noncoercive family planning policies, e.g., in Thailand, Mexico, and Iran [94,99,183,304,305,306,307,308]. A recent paper in PNAS [309] showed that slowing population growth could provide 16%–29% of the emissions reductions needed by 2050 and by 37%–41% by 2100, and a study by Wire [310] shows family planning is four times as efficient as adopting low-carbon technologies for reducing carbon in the atmosphere and ocean. Successful local and regional policies on air quality include California sharply reducing NO2 in Los Angeles by 6.0 ± 0.7 % per year between 1996 and 2011 through strict policies on vehicular emissions [311]. Maryland succeeded in reducing SO2 emissions per unit energy produced from power plants by ∼90% [312]. Regulations have reduced levels of SO2 and NO2 over the eastern US by over 40% and 80%, respectively, between 2005 and 2015. Over a similar period in India, these levels grew by more than 100% (SO2) and 50% (NO2), showing the possible dangers ahead absent effective policies [313].

30 Anthropogenic climate change driven by carbon emissions, water vapor, and surface albedo was theorized as early as the 19th century, and an empirical warming trend was measured by the 1930s. Scientists came to understand the fundamental mechanisms of climate change by the 1950s and 60s (e.g., [314,315]). The first international scientific Conference on the Study of Man's Impact on Climate was held in 1971 and issued a report warning about the possibilities of melting polar ice, reduced albedo, and other unstable feedbacks that could lead to accelerated climate change ‘as a result of man's activities’ [316]. By 1979, a US National Academy of Sciences panel, chaired by Jule Charney, issued a report confirming the findings of climate change models [317], and over the course of the 1980s, the empirical evidence confirming ongoing climate change grew very rapidly. In 1988, James Hansen testified before the US Congress about the near certainty of climate change. The first IPCC Assessment Report was completed in 1990, and the Fifth in 2014, with each report successively warning of the increasingly grave consequences of our current trajectory. The latest report warns that without new and effective policies and measures, increases in global mean temperature of 3.7°C–4.8°C are projected by 2100 relative to pre-industrial levels (median values; the range is 2.5°C–7.8°C) [318]. Other national and global institutions have also warned of the disastrous consequences of such warming (e.g., [274]). As a recent World Bank assessment [261, p. xvii] states, ‘The data show that dramatic climate changes, heat and weather extremes are already impacting people, damaging crops and coastlines and putting food, water, and energy security at risk...The task of promoting human development, of ending poverty, increasing global prosperity, and reducing global inequality will be very challenging in a 2°C [increase] world, but in a 4°C [increase] world there is serious doubt whether this can be achieved at all...the time to act is now’. Scientists have also long warned about the consequences of climate change for international security (e.g., [319,320,321]).

31 With more than 180 countries, covering ∼90% of global emissions, having committed to submit INDCs, the Paris framework forms a system of country-level, nationally determined emissions reduction targets that can be regularly monitored and periodically escalated. While analysis [262] of the INDCs indicates that, if fully implemented, they can reduce the probability of reaching the highest levels of temperature change by 2100 and increase the probability of limiting global warming to 2°C, achievement of these goals still depends on the escalation of mitigation action beyond the Paris Agreement. Even if the commitments are fulfilled, they are not enough to stay below the 2°C pathway [261,262,322], but the Paris framework is an important start for an eventual transformation in policies and measures. Thus, the INDCs can only be a first step in a deeper process, and the newly created framework must form an effective foundation for further actions on emissions reductions.

32 We thank anonymous Reviewer No. 1 for guiding us to add all the important points in this paragraph and the associated citations.