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The SSI Population and Environment Series

Population and Climate Change: Science and Policy Link

Between 1950 and 1998, the world's population grew from 2.5 billion to 5.9 billion human beings. United Nations demographers project world population will gain between 2 and 4 billion more people by 2040 (United Nations 1998). Just as past population increases have influenced greenhouse gas emissions and the composition of the earth's atmosphere in the late 20th century, future demographic trends will affect the earth's climate for centuries to come.

Global efforts to reduce fossil fuel use, increase energy efficiency, slow deforestation, and move to carbon-free energy sources are key components of decreasing greenhouse gas emissions. Such efforts, the objective of current negotiations on climate change, are much more likely to have enduring success if world population grows more slowly and eventually stabilizes.

The disparities in resource consumption and greenhouse gas emissions between regions, nations, and individuals complicate international efforts to address climate change, since responsibility for the problem varies so broadly. These disparities also cloud the role of population growth in changing climate, since the greatest per capita contributions to climate change are made by populations, such as those of the United States and Europe, that are growing relatively slowly or have stabilized.

Per capita emissions and populations are nonetheless increasing rapidly and in tandem in much of the world. This growth of both population and per capita emissions occurred in industrialized countries many decades ago, and their combined population of roughly 1.2 billion today helps explain the disproportionate impacts of their high consumption on the atmosphere (Engelman 1998). Efforts to stabilize the atmosphere will become even more complicated in the next century, as the population of developing nations approaches six billion or more and their economic development continues.

CLIMATE CHANGE: HUMAN INFLUENCE AND IMPACTS

The burning of fossil fuels, deforestation, and other human activities cause concentrations of greenhouse gases in the atmosphere to rise, which enhances the natural "greenhouse effect." Carbon dioxide (CO2) is the most significant among the heat-trapping greenhouse gases that human beings are adding to the atmosphere, and approximately 80-85% of anthropogenic CO2 emissions stem from the combustion of fossil fuels–coal, oil, and natural gas (Intergovernmental Panel on Climate Change 1995). Destruction of the world's forests and other changes in land use account for the remaining 15-20% of human CO2 emissions. The use of fossil fuel contributes other greenhouse gases to the atmosphere as well, including nitrous oxide (N2O) and methane (CH4).

Most climate-population analysis has focused on CO2 emissions from fossil fuel combustion, primarily due to the fact that these data are relatively easy to obtain. It is important to note, however, that greenhouse gas emissions from sources other than fossil fuels (e.g. deforestation) are significant contributors to global warming, but difficulties remain regarding measurement and accounting of these sources (Austin et al. 1998). For the developing world as a whole in 1990, fossil fuel consumption contributed an estimated 36% of greenhouse gas emissions, while deforestation and agriculture contributed 37% and 21%, respectively (Pepper et al. 1992). Therefore, analyses which use data on fossil fuel emissions alone (including most of the analyses cited in this report) understate the greenhouse gas emissions contribution of the developing world.

Anthropogenic greenhouse gas emissions have steadily increased this century. Annual fossil-fuel emissions of CO2 have increased twelve-fold since 1900 and have quadrupled since 1950 alone (Carbon Dioxide Information Analysis Center 1997). The only significant recent pauses in this upward trend have been caused by global economic downturns, such as the oil crisis of the 1970s and the break-up of the Soviet Union (Meyerson 1998). Despite the climate agreements signed in Rio in 1992 and the yet to be ratified 1997 Kyoto Protocol, global greenhouse gas emissions have continued to rise in the 1990s (Carbon Dioxide Information Analysis Center 1997).

Average global temperatures this century have risen to their highest point in 600 years, and have risen about half a degree Celsius (slightly less than one degree Fahrenheit) in the last 100 years. 1998 set a record for the highest global mean surface temperature since records have been kept (NASA Goddard Institute for Space Studies 1999).

In 1995, scientists participating in the Intergovernmental Panel on Climate Change (IPCC), an international, UN-sponsored panel of 2500 experts, concluded that "the balance of evidence suggests a discernible human influence on global climate" (IPCC 1995). (The executive summary of the IPCC document is available on the web at http://www.ipcc.ch/cc95/synt.htm.) The IPCC report represents a consensus within the scientific community.

The current warming trend is projected to accelerate in the next century and, due to the persistence of greenhouse gases in the atmosphere, warming will continue for several decades even after greenhouse gas concentrations reach a peak. The IPCC scientists estimate that if emissions are not abated, CO2 levels will double by the middle of the next century, and the global mean surface temperature will rise from 1 to 3.5 ^oC (1.8 to 6.3 ^oF) between 1990 and 2100 and will continue to rise thereafter (IPCC 1995).

Potential projected impacts of this warming are a serious concern. The IPCC "best estimate" scenario projects a sea-level rise of about half a meter by 2100, with a range of 15-95 cm (5.9-37.4 inches). Sea level rise of this magnitude would be substantially greater and faster than the increase over the last century, estimated at 10-25 cm (3.9-9.8 inches). The ecological and human impacts of rising oceans would be substantial, including increased flooding, coastal erosion, and salination of aquifers and coastal cropland (IPCC 1990, IPCC 1995).

In addition, a warming climate poses a significant public health threat. Higher average temperatures mean longer and more intense heat waves, with a corresponding potential for more cases of severe heat stress. The geographical range of temperature-sensitive tropical diseases, such as malaria and dengue fever, would also expand (Epstein et al. 1998). The frequency and intensity of hazardous weather in certain areas would likely increase. A recent study also indicates that the redistribution of precipitation patterns would markedly increase the number of people living in regions under extreme water stress (Hadley Centre 1998)–a problem that would be compounded by increasing population.

As population pressures drive the further development of environmentally sensitive areas such as hillsides, flood plains, coastal areas, and wetlands, the human and environmental impacts of a changing climate will be exacerbated. The synergistic effects of population growth and climate change may create a greater number of environmental refugees and migrants, as well as other socio-political problems.

THE ROLE OF POPULATION

Scientific literature on the greenhouse effect has noted the dual contribution of growing human numbers and rising per capita energy and materials consumption to greenhouse gas emission levels. Most analyses have focused on CO2 emissions from fossil fuel combustion, for reasons described earlier. During the quarter century from 1958 to 1983, for instance, Norman D. Newell and Leslie Marcus identified a 99.8 percent correlation between world population growth and the growing concentration of carbon dioxide in the atmosphere resulting from the combustion of fossil fuels. They called the correlation "nearly perfect." (Newell and Marcus 1987). Even nearly perfect correlations, however, do not prove causality. There is a continuing debate about the relative importance of population growth and high per capita resource consumption in "driving" the human alteration of the atmosphere.

Energy specialist John P. Holdren helped popularize a mathematical formula known as "I=PAT." The formula equates environmental impacts (I) to the product of population (P), per capita consumption levels (A, for affluence) and the per capita pollution produced by the technology used in that consumption (T). (The equation is sometimes expressed as "I=PCT," with consumption replacing affluence.)

Different interpretations of the I=PAT formula have produced a wide array of results and conclusions. In a 1991 paper, Holdren calculated that over the previous two centuries population growth was responsible for 40 percent of the increase in energy consumption (including burning of fuelwood and other traditional fuel sources). He also suggested that population growth can intensify per capita energy use in industrialized countries, citing suburban sprawl and high air conditioning in pavement-warmed cities (Holdren 1991).

In another study, population growth that occurred in industrialized countries from 1970 to 1990 was calculated to account for 18 percent of the increase in global emissions growth during that period (MacKellar et al. 1995). Several other analyses of the population-climate linkage have concluded that world population growth will contribute significantly to the growth of greenhouse emissions that occurs over the next few decades (Bongaarts et al. 1997).

Demographer John Bongaarts separated CO2 emission drivers into component parts related to the I=PAT equation. Population and economic growth (affluence) were the dominant components of this mix, Bongaarts found. He calculated that projected population growth would contribute roughly 35 percent of CO2 emissions growth between 1985 and 2100 (Bongaarts 1992).

One recent study used the I=PAT formula to analyze the effects of population and affluence on carbon dioxide emissions by country (Dietz and Rosa 1997). The authors found that there is a strong positive correlation between affluence (GDP per capita) and carbon emissions among countries, until GDP per capita exceeds $10,000. At higher per capita GDP levels, additional increments in affluence continue to correlate with increases in carbon dioxide emissions, but the effect falls below strict proportionality. For most countries, however, growth to that level of affluence is not a realistic possibility in the next few decades.

Economist Nancy Birdsall analyzed costs of various options for mitigating CO2 emissions and found little basis for urging developing countries to contribute to slowing global warming by acting on their own to slow their population growth. She asserted, however, that industrialized countries would more effectively reduce CO2 emissions by spending money to reduce population growth in the developing world than by spending the same amount through carbon taxes designed to reduce their own fossil fuel consumption. She concluded that industrialized countries should include investment in family planning and girls' education in low income countries as a part of any well-designed emissions reduction strategy (Birdsall 1992).

Another recent contribution to the I=PAT debate analyzed the relative contributions of emissions drivers from 1950 – 1990 for twelve key countries and found that population was the most important factor for Mexico, the Philippines, and Ghana; affluence was the most important factor in the United States, the former Soviet Union, China, Japan, Poland, Brazil, and Indonesia; and technology was the most important factor in India (Moomaw and Tullis 1999). The authors point out that the relative importance of the factors change over time and conclude that emissions reduction strategies must address all three factors.

Finally, the authors of a recent book on population and climate change conclude that policies to slow population growth will reduce greenhouse gas emissions significantly in the long term, but that this effect will not be substantial until after the middle of the twenty-first century (O'Neill, MacKeller and Lutz, in press).

PER CAPITA EMISSIONS TRENDS

On a per capita basis, global carbon dioxide emissions from the combustion of fossil fuels have risen from 0.3 metric tons (mt) in 1900 to 1.09mt in 1970 and have remained relatively flat since then (Carbon Dioxide Information Analysis Center 1997). The result is that on a global scale, the substantial increases in total emissions over the last three decades correlate closely with population growth (Meyerson 1998).

An examination of per capita emission trends, however, presents several analytical challenges. Global per capita emission figures, for example, obscure huge disparities in per capita emissions by country. For instance, the average person in the United States contributed 5.3 tons of carbon to the atmosphere in 1995, almost five times as much as the average Mexican, and more than 16,000 times as much as the average Somali (Carbon Dioxide Information Analysis Center 1997). In 1995, the 20 percent of the world's population living in countries with the highest per capita emissions contributed 63 percent of the world's fossil-fuel CO2 emissions. This group of high emitters corresponds roughly with the population of the industrialized nations. The low emitters—the 20 percent of world population at the opposite end of the spectrum—contributed just two percent of global fossil-fuel CO2 emissions and are roughly equivalent to the world's poorest 1.2 billion people (Engelman 1998).

Likewise, approaches that compare emissions by country oversimplify a complex relationship in that they do not distinguish differences in emissions within, rather than among, nations. Inequality in distribution of wealth may mean that a small percentage of the population of a country may be responsible for a large share of greenhouse gas emissions.

Finally, a country's rapid population growth in one time period, when per capita emissions are small, can contribute to high national emissions later, when population growth may have slowed or stopped altogether and per capita emissions continue to rise. This pattern is typical of industrialized countries, and it is likely to apply in the future to the rapidly growing, low-emitting nations of today (Engelman 1998). Careful analysis that places per capita emissions into their proper context is critical to an accurate understanding of the linkage between population growth and climate change.

EMISSIONS EQUITY

The concept of per capita emissions must be distinguished from that of national emissions, which are the focus of the 1997 Kyoto Protocol to the Framework Convention on Climate Change and the current climate negotiating process. Under this protocol, which has not yet come into force, 38 industrialized countries would be committed to cutting their emissions of six greenhouse gases, including CO2, by an average of 5.2 percent from their 1990 emissions. Developing nations face no specific emissions limitation obligations in the Protocol, on the principle that industrialized nations have contributed the most to the problem and thus have an obligation to take the first steps to curb emissions and slow climate change (Engelman 1998).

National emissions are the total of those that occur within a nation's borders—a figure that reflects as much the size of nations as it does the use of fossil fuels by each inhabitant. Thus China gains attention as a "major contributor" to climate change, despite the fact that seven people in China contribute less on average in fossil fuel CO2 than one American—or, for that matter, one resident of Singapore. Among other populous nations, 24 Nigerians contribute the same amount as one American, as do 31 Pakistanis. In the most extreme example, hundreds of people in Burundi, Chad, and Somalia burned less fossil fuel than the average American did in 1995. To insist that the United States do nothing to limit its CO2 emissions until these countries take action to limit theirs, as some U.S. policymakers have done, is to suggest that these disparities may as well remain in place indefinitely (Engelman 1998).

As economic development has spread around the globe since 1950, per capita emissions from fossil fuel use have gradually become slightly more equitable. One-fifth of the world's population produced four-fifths of the yearly total of global CO2 emissions in 1950, but by 1995 these high emitters were responsible for a bit less than two-thirds of the global emissions total. The low emitters of 1950, by contrast, used less than one percent of the global total of fossil fuel consumed in that year, while by 1995 their share had increased to two percent. At this rate, however, it would take many centuries for the emissions gap between rich and poor to close; and in the meantime, global emissions could grow to many times their current levels (Engelman 1998).

INTEGRATING POPULATION CONSIDERATIONS INTO CLIMATE CHANGE SOLUTIONS

Population growth continues to be a significant factor driving greenhouse gas emissions, and therefore sound population polices are an important component of any effective emissions reduction strategy. At the same time, long-range strategies to address the build-up of greenhouse gases in the atmosphere must also account for the extreme differences in per capita emissions among nations.

Since the atmosphere and the biosphere have a limited capacity to absorb CO2 and other greenhouse gases without adding to the risk of climate change, greater human numbers will result in each person having a smaller piece of the atmosphere in which he or she can dispose of greenhouse gases. According to one climate model—CO2 concentration stabilizing at 450 million parts per million (Wigley et al. 1996)—the UN low population projection would allow approximately 0.45 metric tons of emissions per person in 2100, roughly equivalent to global per capita emissions in the 1920s and '30s. The high population projection, on the other hand, could sustain per capita emissions of only about one third that amount—similar to per capita emission levels of the 1870s (Engelman 1998). Decisions governments make today about population programs are instrumental in setting the course for future per capita emissions reductions.


Demographic Trends and Population Policy

Fortunately, recent world population trends represent a hopeful sign in assessing the long-term prospects for human-induced climate change. International cooperation on population has already done much to slow the growth of world population. In South Korea, for instance, the average number of children per woman fell from 6.3 in the late 1950s to 1.7 in the early 1990s. In Mexico, the average fell from 6.8 in the late '60s to 3.1 in the early '90s. Over a 15-year period from the late '70s to the early '90s, fertility in Kenya fell from 8.1 to 5.4 (Haub and Cornelius 1998). However, fertility rates in many countries still remain high.

The global movement to make family planning widely available—along with government commitments to put strong voluntary family planning programs into effect—can take much of the credit for fertility declines over the past generation. A quarter century of demographic research demonstrates the feasibility of accomplishing fertility declines by making voluntary family planning universally available and by improving educational opportunities for girls and economic opportunities for women (Engelman 1998).

The US and 179 other nations acknowledged the importance of these objectives at the United Nations International Conference on Population and Development (ICPD) held in Cairo in 1994. The ICPD "Programme of Action" calls for improving access to quality family planning and other reproductive health services, reducing infant and maternal mortality, eliminating the gaps between girls' and boys' education, and improving the economic, social, and political status of women. At a cost of about $17 billion annually—less than the price of a fast-food meal for every person on the planet—the Cairo conferees estimated that every woman and man in the world interested in planning their families could have access to the basic reproductive health services needed.

Such access, in combination with improved educational and economic opportunities for girls and women, would put in place the conditions needed to achieve a population growth path similar to the United Nations low projection. Under more rapid population growth scenarios, it will prove much more difficult to bring global greenhouse gas emissions to low enough levels to avoid rapid and possibly catastrophic climate change (Engelman 1998). Efforts to slow climate change will be more likely to succeed if these two long-term global trends—human population growth and the alteration of the earth's atmosphere and climate—are addressed in tandem.

Global Climate Change Policies

Long-range strategies to address the threat of climate change are unlikely to succeed without careful consideration of demographic trends. The Kyoto Protocol is an important first step towards a viable long-term climate stabilization solution. Precisely because it was recognized as a first step, the Kyoto negotiations focused on industrialized country commitments and left the matter of developing countries' participation in future emissions reductions efforts for future negotiations. That means, however, that the 97 percent of world population growth that takes place in these nations is not encompassed by the Protocol's initial emissions limitation requirements.

Although perhaps less obvious, industrialized nations' population trends are also important with regard to both Kyoto obligations and future emissions reduction agreements (Meyerson 1998). Looking first at the United States, the Department of Energy estimates that by 2012, under a business-as-usual scenario, US emissions would increase by about 30 percent from 1990 (Energy Information Administration 1999). US population is projected to increase by between 16 and 23 percent between 1990 and 2012, indicating that the strong correlation between population growth and emissions is likely to continue in this period (United Nations 1998).

Despite an expanding population, a number of different analyses show that the United States can reasonably meet its Kyoto commitments using a mix of domestic policies to overcome market barriers to the diffusion and innovation of less polluting, more secure, and more sustainable ways of producing and using energy. In this way, the Kyoto Protocol will lead to important initiatives promoting clean energy and transportation technology both in the United States and abroad. In addition to domestic action, the Protocol includes several "flexibility mechanisms"—such as emissions trading and energy or forestry projects with developing countries—that can augment domestic action to meet US commitments (Union of Concerned Scientists and Tellus Institute 1998).

That said, however, it is common sense that meeting the US emissions reductions obligations will be easier with a smaller population than with a larger one. Perhaps more important than the initial period covered by the Kyoto Protocol is the impact of US population growth on future emissions reductions requirements. Given US growth projections, the size of the post-2012 US population will continue to affect this country's ability to meet the ongoing challenge of reducing greenhouse gas emissions.

The compliance picture for the European Union, which committed to achieve similar emissions cutbacks by 2012, looks quite different than that for the United States. Because many member states of the European Union are experiencing little or no population growth, technological advances and modest shifts of fuel type (for example, from coal and oil to natural gas) are likely to enable these countries to come in under the Kyoto limit. But because per capita emissions are already lower in most European countries (driven in part by higher energy prices) and, in general, energy efficiency per unit GDP is higher, these countries will face a different set of challenges in meeting both their short-term and future emissions reductions targets.

In general, industrialized countries attempting to fulfill their Kyoto emissions reduction targets will need to bear in mind the influence of future rates of domestic population growth. The Kyoto Protocol thus makes population dynamics a silent partner in the future of greenhouse gas emissions reductions for these countries participating in the Protocol's commitments. In this context, population issues adopt a new significance in industrialized countries where population growth within their borders is often seen as benign.

Meeting the long-term goal of effective climate protection will require the eventual participation of developing countries in emissions limitation agreements. Accordingly, this will require the consideration of a wide range of population growth rates and per capita emissions levels and projected trends. Clearly, world population size will determine the extent of necessary global per capita greenhouse gas emissions reductions. A world population that doubles again in the 21st century and yet again in the 22nd, as in the UN high projection, has little hope of stopping climate change without virtually eliminating the use of fossil fuels. A more feasible goal is a world population that never doubles from today's level, coupled with polices that foster the implementation of sound energy and land-use policies that lead to the long-term decarbonization of the economy. Effective climate policy clearly must address all three I=PAT factors—population, affluence (or consumption), and technology.

A FEW RECOMMENDATIONS

A number of steps could be taken to help clarify population-climate linkages and facilitate a more open discussion on world population dynamics and population policy as it relates to the climate debate.

• The IPCC could more specifically explore the interaction of population dynamics with other factors related to climate change. Several important publications are now in preparation that have the potential to help clarify these relationships. The IPCC has in the past incorporated population growth factors into its assessments of climate change and is expected to do so in its Third Assessment Report, which will be published in 2001. In addition, the IPCC is currently in the process of formulating new emissions scenarios to update the 1992 set of forecasts. The IPCC should consider aggressively the implications of the recent slowdown in population growth and the full range of possible paths of population in formulating future climate scenarios.
• In addition, the "Programme of Action" of the International Conference of Population and Development (ICPD) in Cairo should be more explicitly incorporated into both national climate change policies and the international negotiations. Specifically:
  
  
  1. The Climate Change Convention's Article IV, Section 1(f) should be interpreted as a mandate for national policymakers to explicitly connect climate change mitigation and ICPD population policies. This section of the Commitments article requires the parties to "take climate change considerations into account, to the extent feasible, in their relevant social, economic and environmental policies and actions, and employ appropriate methods …." Thus, the mandate already exists for national governments to factor climate change considerations into their population policies and to strengthen such policies as a means of meeting emission limitation commitments.
  2. The ICPD's policy recommendations should be explicitly included in any future agreements under the Climate Change Convention. A direct reference to the Cairo "Programme of Action" would signal a recognition that these two issues—climate and population—are interconnected and could clarify that the social investments called for at Cairo may also help slow human-induced climate change as well.
  3. Finally, the scientific and policy debate about greenhouse gas emissions needs a more informed contribution on differences in per capita emissions of key greenhouse gases, especially carbon dioxide. In future negotiations the foreseeable continuing disparity in per capita emissions can help inform the levels of commitments required from different countries to effectively limit the growth of greenhouse gases.

CONCLUSION

Given the increasing human influence on the earth's atmosphere and climate, population and climate change are intertwined in fact and should be more interlinked in policy. Actions taken now to encourage slower population growth, along with those aimed at shrinking per capita greenhouse gas emissions equitably, will bring closer the day when demographic and atmospheric changes are neither rapid nor threatening but mere background variables in a generally secure world.

MESSAGES FOR POLICYMAKERS AND THE MEDIA

• Future world population size will be a key determinant of the level of per capita emissions that the atmosphere and biosphere can absorb without adding to the risk of climate change.
  
  
• Efforts to slow climate change will be more likely to succeed if two long-term global trends, human population growth and per capita greenhouse gas emissions, are addressed in tandem.
  
  
• Sound population and energy policies can and should work together to preserve a global climate in which humans and nature can thrive.
  
  
• By supporting sound population policies and programs, along with new technologies of energy production and consumption, governments can increase the probability that both population and emissions growth trends will reverse course before the middle of the next century.
  
  
• Effective, voluntary family planning programs, in combination with improved educational and economic opportunities for girls and women, can be a central part of a greenhouse gas reduction strategy.
  
  
• As population pressures drive the further development of environmentally sensitive areas such as hillsides, flood plains, coastal areas, and wetlands, the human and environmental impacts of a changing climate will be exacerbated.
  
  
• Long-term strategies to reduce global greenhouse gas emissions in an equitable manner will need to account for the current extreme differences among nations in per capita emissions.

SOURCES

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February, 1999

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