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Introduction

The Earth’s population, currently 7.2 billion, is expected to rise at a rapid rate over the next 40 years. Current projections state that the Earth will need to support 9.6 billion people by the year 2050, a figure that climbs to nearly 11 billion by 2100 (United Nations, 2012). At the same time, most people envision a future Earth with a greater average standard of living than currently exists—and, as a result, greater consumption of planetary resources. How can this population growth be achieved in a manner that is sustainable from an economic, social, and environmental perspective?

The Presidents’ Committee of the National Academies, concerned that the social and natural sciences communities may not be working in concert to address these important questions, sponsored a multidisciplinary workshop to explore how the human population of the world might increase to 10 billion in a sustainable manner while simultaneously increasing the well-being and the standard of living for that population. In sponsoring this project, the presidents hoped that other sponsors would step forward for subsequent, in-depth examination of topics surfaced at this introductory workshop. To develop the workshop, three groups within the National Academies—the Board on Environmental Change and Society, Committee on Population, and Board on Life Sciences—established a planning committee to identify workshop topics and agenda items, speakers, and invited guests. The planning committee’s role was limited to planning and convening the workshop. A statement of

task, outlining the details and scope of the proposed workshop, is shown in Box 1-1.

The workshop was held at the National Academy of Sciences Building, 2101 Constitution Ave., NW, Washington, DC, September 30 and October 1, 2013.¹ Approximately 40 participants, including speakers, members of the planning committee and parent boards and committees, invited guests, and members of the public, participated in the workshop. Appendix B lists the attendees. The views contained in the report are those of individual workshop participants and do not necessarily represent the views of all workshop participants, the planning committee, or the National Research Council (NRC).

The workshop was designed as a venue to explore issues, not to reach consensus. It was organized into six sessions, focusing on the following concepts:

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¹More information, including archived webcasts of the workshop sessions and prepared slides from the presenters, can be found at http://sites.nationalacademies.org/DBASSE/BECS/CurrentProjects/DBASSE_072678 [March 2014].

• The Human-Earth System
• Challenges to the Earth System: Character and Magnitude of the Challenges in 2050
• Challenges to the Earth System: Consequences for the Earth System
• Extreme Events
• Resource Distribution and Global Inequality
• Interaction Between Earth and Societal Systems

Each section consisted of one to three presentations, followed by general discussion among all workshop participants.

During the course of the workshop, several themes recurred in presentations and discussions; all are issues that cut across the many disciplines and topics of study involved in sustainability science. These themes, which are described in the following section, were identified for this summary by the rapporteur, not by the workshop participants.

Following the section below on rapporteur-identified recurring themes, the remainder of this report summarizes the presentations and discussions for each of the workshop sessions; it was prepared by the workshop rapporteur as a factual summary of what occurred at the workshop. Appendix A shows the workshop agenda, Appendix B lists the workshop participants, and Appendix C defines acronyms used in the report. Appendix D provides short biographical information of each of the workshop presentation authors.

RECURRING THEMES

The following six theme areas were identified from the workshop and are described in more detail below:

1. Demographic variables that influence sustainability. A frequently discussed topic at the workshop related to understanding the impact of different demographic variables on future projections of population, climate, well-being, biodiversity, health, and the overall state of the Earth system.
2. Economic and policy variables that influence sustainability. Workshop participants explored financial incentives, land use, technology use, and other policies and concepts that have the potential to alter the future Earth system.
3. Suitable metrics. Some workshop participants noted the lack of suitable metrics to measure such quantities as overall human well-being, the impact of climate change on the human population, environmental impact, and inequality.

4. Carrying capacity. Originally one of the organizing principles of this workshop, some presenters did not consider the concept of carrying capacity a useful framework for evaluating the Earth system and sustainability.
5. Integrating social and natural sciences. Also one of the organizing principles of the workshop, several participants pointed out the need to enhance the interaction between the social and natural sciences.
6. Structuring a research agenda. The workshop’s charge included a request to consider future research needs. Participants suggested ways to move forward with a set of focused research ideas, spanning the relevant disciplines.

DEMOGRAPHIC VARIABLES THAT INFLUENCE SUSTAINABILITY

Much of the workshop focused on identifying demographic variables that influence sustainability, along with attempting to identify the relative importance of each variable. The variables most widely discussed were education, fertility and aging, urbanization, and migration.

Education

The importance of education was discussed by numerous speakers, and education was perhaps the most frequently recurring theme throughout the discussion. Wolfgang Lutz, in his presentation, stated that the Intergovernmental Panel on Climate Change (IPCC) has determined that education is the third most important demographic variable, following age and gender. The workshop’s discussion of education focused on identifying statistical relationships between increased education and other metrics that indicate improvement to human well-being; it did not focus on challenges associated with access to education or areas of needed research. Some of the key benefits of education discussed included the following:

• Overall impact of education on society. Brian O’Neill stated that increased education results in a substantial improvement to the Human Development Index. Lutz also pointed out that education enhances life in many ways, including enhancing cognitive skills; inclining people toward less risky behavior; extending personal planning horizons; enabling for better learning from past damage; providing better access to relevant information; providing improvements to health and physical well-being; and promoting

• higher incomes at the individual, household, and national level. Overall, Lutz linked a more educated population with decreases in disaster mortality and increases in the population’s adaptive capacity to respond to climate change.

• Relationship between maternal education and population growth. Lutz showed a strong relationship between maternal education and slowed population growth, focusing on South Korea as an example. He referred to studies that have provided evidence for causation: A more educated population leads to a lower fertility rate.
• Relationship between education and intergenerational mobility. Intergenerational mobility—the likelihood of rising out of poverty, expressed as the probability of escaping the lowest quintile of income—was discussed, along with its relationship to education. Branko Milanović stated that the correlation between parent and child income in the United States is quite high, around 0.5. In European countries, this number is closer to 0.3. Lutz pointed out that by far the most important path to upward social mobility is education in every country in the world. The Nordic countries have high-quality public education and a correspondingly high level of social mobility. In countries where the best education is private, mobility is significantly lower.
• Time lag between education and its positive effects. Lutz stated that there is a large time lag—as much as 40 years—before the benefit of the educational effect on fertility can be observed. This indicates that an age-specific education approach may be needed to increase the societal benefits of education.

Fertility and Aging

Data presented by John Bongaarts show a clear bifurcation in population: Fertility rates are high in many developing nations, while, at the same time, fertility rates are becoming low in many developed nations (leading to an aging population). The challenges are therefore different in the two populations:

1. High fertility. Parfait Eloundou-Enyegue examined many variables associated with inequality in high-fertility settings, and differential fertility was the greatest contributing factor to income. Lutz discussed in detail that increased education for women and girls leads to significant decreases in fertility rates. Bongaarts stated that fertility rates dropped significantly in nations with a comprehensive, national family planning program that included widespread access and choice, pointing to Mexico and Iran as

2. models in this area. He also suggested investing in family planning programs and in human capital (i.e., the education of girls) as the primary keys to reducing high fertility rates.

3. Aging populations. Bongaarts postulated in his presentation that fertility rates lower than 1.7 or so births per woman result in a demographic imbalance, with an aging population that puts stress on the economy and society; this imbalance is among the biggest challenges facing the United States and Europe in the next two to three decades. He suggested that solutions could include modifying the pension system; encouraging a larger labor force, including working later in life; encouraging increased childbearing; and encouraging increased immigration. Peter Marcotullio and Andrew Jorgenson noted infrastructure and energy consumption are affected by aging populations. Eloundou-Enyegue, in his presentation, examined variables that correlate to income inequality in low-fertility settings. In those settings, family structure (marriage rates and premarital fertility rates) are the largest contributing factors to inequality.

Urbanization

Urbanization was discussed in depth by Marcotullio, who argued that urban areas benefit from an economy of scale in production and consumption: In highly developed nations, urbanization can lower carbon emissions per capita through consolidation. In less developed nations, however, cities provide avenues to consumption absent elsewhere, potentially increasing per capita carbon emissions. Urbanization generally leads to increased emissions overall; as O’Neill and Marcotullio explained, urbanization leads to an increase in labor productivity, additional amenities, increased advancements in technology and innovation, and reduced transportation emissions. However, Stephen Polasky noted that urban land use is a small fraction (<1 percent) of total land use, and changes in urbanization have little to no effect on land use distributions.

Migration

Milanović informed the group that currently about 3 percent of the population migrates, but 8 percent of the population would like to do so; thus, he said, there is a pent-up demand for migration. Within a nation, there is a general migration trend from rural to urban areas. Milanović said that migration is a tool that has the potential to significantly reduce inequality. Bongaarts stated that migration currently has only a small impact in population estimates overall, but there are strong localized

effects, such as in South America. Recently there have been examples of rapid migration in Europe and, to a certain extent, in North America, and the populations that immigrate tend not to integrate quickly into their new society.

Many speakers agreed that migration is not well understood or modeled. Marcotullio and Jorgenson noted that identifying a person’s location is a difficult task, and that spatial distribution of population is a new demographic area that needs additional work, particularly because it may be important in understanding environmental change.

ECONOMIC AND POLICY VARIABLES THAT INFLUENCE SUSTAINABILITY

While the size of the human population exerts a strong and obvious influence on the Earth system, the workshop also addressed economic and policy variables that can have a strong impact on sustainability. As James Edmonds noted, it is not surprising that in a world with 10 billion people, what will matter is how much they are doing, how they are doing it, and what policies there are for regulating what they do. While a number of ideas were brought up, the workshop participants discussed the following topics most frequently.

Economic Incentives

Several workshop participants discussed a number of economic incentives that might promote behaviors that could lead toward greater sustainability. Some of these included the following:

• Demographic dividend. The demographic dividend is a “natural” (but temporary) incentive that occurs as the result of changing demographics in a society. In his presentation, Bongaarts discussed the concept of a demographic dividend, which is a 20- to 30-year financial benefit that results when a high-fertility population shifts its average age through drops in fertility rates and increases in life expectancy, leading to a larger fraction of its population in the working age group. He said that to an economist, the demographic dividend is good, as it results in a decline of poor populations and motivates governments to help decrease fertility rates. The downside is that the effect lasts for only a few decades, and then it shifts to the demographic burden of an aging population.
• Incentives for ecosystem services. Polasky suggested adding financial incentives for ecosystem services to increase output of goods

• and services without a negative impact on natural capital. He described how ecosystem services related to food production, which see a financial reward, have seen a net increase over time. The remaining ecosystem services do not deliver financial rewards to those people responsible for them, and, as a result, the future of those services is in doubt.

• Discount rate. A discount rate is the converse of an interest rate: While an interest rate expresses how something today will be valued in the future, a discount rate expresses how something in the future would be valued in today’s terms. The discount rate concept was explored in the workshop discussion as a potential avenue to incentivize responses to climate change. A participant pointed out that the IPCC has a long list of possible climate change interventions, but they are expensive to implement. No action is being taken, in part because there is no way to value making things better for the future. Several participants speculated that a discount rate, even a small one, might make some difference in actions taken today.
• Understanding return on investment. Polasky stated that the sustainability consequences of various investment possibilities are known, and he suggested the need to evaluate the return on investment of different scenarios to understand what action leads to a benefit somewhere else. The scenarios proposed would take into account different time frames and the various beneficiaries to develop an incentive structure. Another participant argued that society currently has an overemphasis on individual value systems and individual returns, and interconnections should be considered. Several participants also discussed education as an example of an area that needs additional investigation into the return on investment. William Rouse said that, when discussing the economics of investing in people, the economic valuation depends on who lives with the consequences of not investing.

Food: Costs and Consumption Habits

Several participants discussed the importance of global consumption habits and their influence on environmental sustainability. In general, they noted, as countries move toward increased affluence, they also increase their intake of refined, rather than whole grains, as well as of larger livestock. During a discussion session, Siwa Msangi said that there is still little understanding of the impact of livestock, particularly with respect to livestock intensively fed with grains. He said reducing the need for grain as livestock feed, however, would take pressure off of the

production system, but the stresses on animal productivity and animal growth (such as heat, water, stocking density, and food) need to be better understood.

Several workshop participants also considered the issue of rising food prices. Msangi provided projections of the cost increases for maize, rice, and wheat under different assumptions about climate change, income growth, and population growth. It is likely that these rising prices will increase overall levels of inequality, he said.

Land Use

Land use and land use policy were mentioned by several speakers as variables that should be modeled and included in future emissions projections. Edmonds indicated that land use policy is an important variable to consider when modeling the Earth system, but that it is not currently well studied. Land use was only discussed in detail in Polasky’s presentation. He divided land use history into five stages: (1) presettlement, (2) frontier, (3) subsistence, (4) intensifying, and (5) intensive. The intensifying stage is marked by huge changes, as ecosystem use transitions to intensive agriculture and urban areas. He stated that establishing financial incentives for preservation or enhancement of ecosystem services to increase output of goods and services without a negative impact on natural capital may be a route to increasing sustainability.

Technological Change

The impact of technology on sustainability can be complex, as noted by several presenters. Rouse stated that, on the one hand, technological advances can help alleviate environmental impact by improving efficiency and ameliorating some of the by-products or effects of consumption. On the other hand, increased technology use can lead to increased energy consumption. Both Polasky and Msangi discussed technological improvements and their influence on productivity increases in crop production and more efficient water usage, although that trend is not likely to continue domestically.

Rouse also explained that the adoption of an innovative technology is a highly selective process, and those who embrace a technology or innovation first will experience it very differently from subsequent adopters. While the adoption of a technology usually leads to an aggregate increase in human well-being, ultimately it usually also leads to increases in inequality, because early adopters tend to receive the greatest benefit. He noted that near-term technology tends to benefit those who least need the improvement.

SUITABLE METRICS

Many participants and speakers questioned how to quantify some of the topics discussed. In some cases, the ideal metric is understood although perhaps difficult to obtain; in other cases, there does not appear to be agreement on the best metric to use to understand a particular phenomenon. B.L. Turner II said metrics are critical to the discussion of sustainability, or it will be impossible to assess different options.

Some metrics that were discussed included the following:

• Overall human well-being. A participant pointed out that the community tends to use increasing gross domestic product (GDP) and life expectancy as indicators of “success,” but a different set of metrics might be able to capture the intent, rather than just measuring the throughput. One person recommended a “gross national happiness” product. Another participant proposed that it would be important to measure not just current well-being, but also the impact on future well-being. He urged an understanding of how the present may be compromising or improving the future, and GDP and measures of happiness do not account for the future. John Casti used financial markets as a measure of collective beliefs; he acknowledged that this metric was frequently questioned, but that it was an important first step toward measuring group beliefs. Lutz argued in support of a figure of merit that is static across time that could incorporate aggregate indicators of well-being, which would not decline over time or for any subpopulation. The figure of merit, he said, should also include the depletion of natural stock and how that depletion feeds back to the future well-being of humankind. He used metrics such as life expectancy at birth and the Human Development Index, acknowledging that these metrics have limitations but are a good first step. Lutz said that the community needs to identify a broad metric that can assess improvements to intergenerational inequality.
• Indicators of environmental change, including tipping points and boundaries. Turner said that it may be important to understand the structure and function of tipping points and planetary boundaries, and to have a tipping point time scale as one kind of metric. Jorgenson indicated that the ecological footprint may not be the appropriate metric when considering environmental change, although it can be a useful tool for discussion.
• The impact of climate change on humans. Lutz stressed the importance of understanding how dangerous climate change is to human well-being. Without an analytic answer to that question, he said the world cannot respond appropriately to climate change.

• Demographic changes. Lisa Berkman’s talk highlighted the importance of examining population distributions, not just a single variable (such as a mean) to represent the distribution. A more sensitive measure would provide information about both the center of the population and the variation out to the extremes.
• Return on investment. Several participants noted that it could be useful to better understand how to use return on investment as an incentive for action. However, several workshop participants pointed out that measuring and quantifying return on investment is another challenge for the community.

CARRYING CAPACITY

Based on the statement of task (Box 1-1), the workshop was originally asked to address the concept of carrying capacity as a possible framework within which to consider the burgeoning human population of the Earth. Carrying capacity, a concept originated in animal ecology, refers to the maximum population size of a species that the environment can sustain indefinitely, to which it is able to provide an appropriate supply of food, water, habitat, and other natural resources. However, to some workshop participants, the concept of carrying capacity did not seem to be a useful framework for human populations and sustainability. In other words, although carrying capacity was expected to be a recurring theme of this workshop, it was not. Turner noted that the more built up the environment is, the more difficult it becomes to apply the concept of carrying capacity, which assumes natural limitations (such as disease or starvation through drought) on animal species that are not technologically countered by that species. Because the human species manipulates and converts its habitat and can counter the natural limits on its population (such as by vaccinating for disease or providing emergency food to drought-struck areas), the conceptual basis of carrying capacity breaks down when considering people. Turner stated that carrying capacity should be only a heuristic device, and he cautioned against calculating specific values for the human-environment system. He stated that carrying capacity has been “largely abandoned” in the social and policy sciences.

INTEGRATING SOCIAL AND NATURAL SCIENCES

In the discussion, several participants explored the motivation for the workshop, given that one of the stated workshop goals was to enhance the interaction of the social and natural sciences. Turner said that such integration is taking place, but he emphasized the need to think of the whole Earth as a system of systems. In that context, he said, social and

natural scientists need to understand that the regimes of the Earth system are changing in a fundamental way and those regime shifts have interrelated consequences. He also postulated that there is room for additional interactions among the sciences, and he singled out urbanization as one area in which this would be particularly useful. Rouse stated that no one person could leave the workshop and address the questions posed; rather, it would require a group, consensus-driven effort.

Berkman said that it was her perception that researchers in environmental issues remain mostly separated from population scientists. In addition, she said there is a need for highly skilled demographers to work on issues related to climate science. She pointed to an overall need for more nuanced thinking, rather than a black-and-white interpretation that population is ruining the world or that the world is ruining population; population and the environment are too closely intertwined to make such a distinction, she said.

STRUCTURING A RESEARCH AGENDA

The workshop discussion sessions focused in part on identifying elements of a research agenda for sustainability science moving forward. Specific research topics are addressed in the themes above. However, additional comments were made about the structure and focus of a research agenda; these were pragmatic ideas that spanned the range of topics and disciplines. Some of the salient suggestions offered by individual participants included the following:

• Catalogue the current state of knowledge, so that gaps in understanding can be addressed. Research would then target these gaps.
• Understand potential future impacts, using detailed, evidence-based scenarios to understand the possible outcome from each set of actions. This would help identify what can be put into place now for easier adoption later.
• In addition to looking at the most plausible scenarios, investigate possible, though unexpected, shocks that would disrupt these scenarios, including how to eliminate or mitigate these possible shocks.
• Consider how to implement small changes within the poorest nations, such as improvements to education and family planning programs, which would make the most impact.
• Consider a long-term framework (out to 2050), but focus on the next 5 years, because it is easier to attract attention to short-term needs, and thereby start reversing dangerous trends. An example suggested by one participant might be to help address China’s

• short-term pollution problems within a larger emissions framework. Identify multiple possible research agendas; from them, develop a broad agenda but select certain elements for deeper focus.

• Try to understand how and when a particular scenario might shift from possible to inevitable. For instance, one participant asked, how much of the reality of 2050 will be “locked in” by 2030?
• Try to understand the determinants of risk, looking across different socioeconomic and demographic variables and spatial distributions.
• Consider not just the science, but also the policy, including ideas for implementation.

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