Population Wedge

When we turn to potential population wedges, we need to remember that population growth is one of the two main drivers of GCC.[i] Again according to the IPCC’s 4th Assessment Report: “The effect on global emissions of the decrease in global energy intensity (-33%) during 1970 to 2004 has been smaller than the combined effect of global per capita income growth (+77%) and global population growth (+69%); both drivers of increasing energy-related CO2 emissions.”[ii] When it comes to GCC and other environmental problems, size (of the human population) matters.[iii]

The current global population is approximately 7 billion people. Here are recent fifty-year United Nations population projections at low, medium and high rates of growth[iv]:

The medium projection is presented as the “most likely” scenario, although all three projections are considered possible depending on a variety of factors, including public policy choices. Note that all three projections, even the highest, assume fertility decreases and lower annual growth rates than in recent decades, based in part on improving efforts to provide contraception and encourage family planning. If these efforts falter, birth rates may remain high and populations fifty years from now may balloon past twelve billion.

In 2000, world per capita greenhouse gas emissions were 1.84 tons carbon equivalent. Assuming this emissions rate, each 543 million people added to Earth’s population adds another one billion tons of annual carbon emissions; conversely, preventing the existence of 543 million people fifty years from now provides a full carbon reduction wedge (543 million X 1.84 tons = 1 billion tons). If we follow the UN report and take 9.6 billion as our business as usual population scenario, then successfully holding world population growth to the lower figure of 8.0 billion would provide 2.95 global population wedges.

Conversely, allowing the world’s population to swell to the high projection of 11.8 billion (still within the realm of possibility) would create 4.05 population destabilization wedges and almost certainly doom efforts to mitigate catastrophic GCC.[v] These figures show that reducing population growth could make a huge contribution to mitigating GCC.[vi]

“Population control” tends to bring to mind coercive measures, such as forced abortions or sterilizations. In fact there are non-coercive policies that are almost as effective at reducing birth rates, and these are the ones we should pursue in constructing population wedges. First, providing free or low-cost birth control and accessible, appropriate information about how to use it has proven very effective in lowering birth rates in many poor countries.[vii] Providing cheap birth control allows those who want to have fewer children to do so, increasing reproductive freedom while decreasing population growth. Second, policies which improve the lives of women have been shown to reduce fertility rates in many developing countries.[viii] These include guaranteeing girls the same educational opportunities as boys, promoting female literacy, and improving women’s economic opportunities (and thus their value and status in society). Third, making abortion safe, legal and easily available has helped reduce birth rates in many countries. In fact, no modern nation has stabilized its population without legalizing abortion. All these measures can directly improve people’s lives at the same time that they help reduce population growth.

Given that these non-coercive methods have proven successful at reducing fertility rates in many places, and given the huge unmet need for contraception throughout the developing world, well-funded efforts to apply them globally seem capable of reducing population growth from the “most likely” scenario of 9.6 billion people to the lower projection of 8 billion people in 2060. Once again: 1.6 billion fewer people fifty years from now represents 2.95 carbon reduction wedges. That would make an immense contribution to mitigating GCC, equal to deploying all three of Pacala and Socolow’s carbon capture and sequestration wedges. Unlike carbon capture, however, the proposed population reduction measures rely on proven technologies that are available right now.

Population wedges would also provide numerous other environmental benefits, reducing human impacts across the board, in contrast to the massive environmental harms that would be caused by continued coal and uranium mining under the carbon and nuclear power wedges, or the smaller but still substantial environmental harms caused by large-scale wind or solar power generation. Smaller populations would also make an important contribution longer-term, as humanity moves closer (we hope) to creating truly sustainable societies. Ceteris paribus, smaller human populations are more likely to be sustainable, while endlessly growing populations are unsustainable by definition.[ix]

Securing women’s rights and furthering their opportunities are the right things to do, independent of their demographic effects; they can also effectively help stabilize human numbers.[x] Population wedges thus provide “win/win” scenarios with the potential to aid women and their families directly, increasing their happiness and freedom, while helping meet the grave danger of GCC.[xi] Some of the very same aims written into the UN’s Millennium Development Goals, such as improving maternal health and increasing the percentage of children receiving a full primary school education, turn out to be among the most effective means to reduce birth rates in poor countries.[xii] In addition, a recent study from the London School of Economics titled “Fewer Emitters, Lower Emissions, Less Cost," argues that reducing population growth is much cheaper than many other mitigation alternatives under consideration.[xiii] Given all this, policies to stabilize or reduce populations should be an important part of national and international climate change efforts. These are some of the best wedges we’ve got.

Still, talk of limiting or reducing human numbers makes many people uncomfortable, despite the fact that we are not proposing to kill people, but merely prevent births that would otherwise occur. Many of us have held a newborn baby and felt a sense of infinite possibility and value radiating out from that little form. How could the world possibly be better without him or her? It seems an abominable thought. Nevertheless, most of us do not have as many children as we are biologically capable of having. Resources are limited. There are the human costs of crowding, urban populations that outgrow basic services, and large numbers of unemployed young people; meanwhile, even confirmed anthropocentrists might well hesitate before accepting the total displacement of wild nature in order to maximize human numbers.

People are wonderful, but it is possible to have too many people: in a family, a city, or a nation. GCC may be showing us that it is possible to have too many people on the Earth itself. Part of its message may be that with freedom to reproduce comes responsibility to limit reproduction, so as not to overwhelm global ecological services or create a world that is solely a reflection of ourselves.[xiv]


Despite thirty years of craven failure to address population issues from mainstream environmentalists, global climate change is driving home the lesson that we cannot create sustainable societies, and protect ourselves or other species, without limiting human populations. In the past few years, there has been an upsurge in efforts to address the role population stabilization or (hopefully) reduction can and must play in creating ecologically sustainable societies. Dave Foreman has just published Manswarm and the Killing of Wildlife; meanwhile, I and Eileen Crist are editing a collection of essays with the working title Apply the Brakes! Environmentalists Confront Population Growth, due out in 2012 from University of Georgia Press. Further valuable readings on climate change and population issues are attached below.

[i] Brian O’Neill, Landis Mackellar and Wolfgang Lutz, Population and Climate Change (Cambridge: Cambridge University Press, 2005); Brian O’Neill, “Climate Change and Population Growth,” in Laurie Mazur (ed.), A Pivotal Moment: Population, Justice & the Environmental Challenge (Washington, D.C.: Island Press, 2009), pp. 81-94.

[ii] IPCC, Climate Change 2007: Mitigation, Summary for Policymakers, p. 3.

[iii] Lindsey Grant, Too Many People: The Case for Reversing Growth (Santa Ana, CA: Seven Locks Press, 2001).

[iv] UN Department of Economic and Social Affairs, Population Division, “World Population to 2300,” p. 4. Original projections were to 2050; I projected out to 2060 using the annual growth rates provided. These figures might be somewhat rosy; more recently, the US Census Bureau projected that world population will grow from six billion in 1999 to nine billion by 2040. See US Census Bureau, “World Population: 1950-2050,” International Data Base (2009).

[v] One caveat: most of the population growth projected for the next fifty years is expected to be in poorer countries, which have lower-than-average greenhouse gas emissions. This might decrease the emissions benefits associated with slowing population growth. However, such lower emissions are primarily a function of poverty, which we presumably want to alleviate. Almost all attempts to come up with an equitable worldwide division of the effort to mitigate GCC allow for some increase in greenhouse gas emissions from the world’s poorest countries. So as we calculate population wedges, it seems reasonable to keep world per capita greenhouse gas emissions of 1.84 tons carbon equivalent as our “business as usual” default setting.

[vi] Frederick Meyerson, “Population, Carbon Emissions, and Global Warming: The Forgotten Relationship at Kyoto,” Population and Development Review, 24 (1998), 115-130; O’Neill et al., Population and Climate Change, chapter six.

[vii] Joseph Speidel et al., Making the Case for U.S. International Family Planning Assistance (New York: Population Connection, 2009).

[viii] Carmen Barroso, “Cairo: The Unfinished Revolution,” in Laurie Mazur (ed.), A Pivotal Moment, pp. 245-259.

[ix] Albert Bartlett, “Reflections on Sustainability, Population Growth and the Environment,” in Marco Keiner (ed.), The Future of Sustainability (Dordrecht: Springer, 2006).

[x] This happy confluence, however, should not obscure the fact that reproductive rights are not absolute. Like all rights, their proper scope and limits can only be specified in a comprehensive system of rights, whose proper end, in my view, is to further the flourishing of all members of the moral community, human and nonhuman. Philosophers have not written much of value on this topic, but see Onora O’Neill, “Begetting, Bearing, and Rearing,” in O’Neill and William Ruddick (eds.), Having Children: Philosophical and Legal Reflections on Parenthood (New York: Oxford University Press, 1979), pp. 25-38; and Carol Kates, “Reproductive Liberty and Overpopulation,” Environmental Values, 13 (2004), 51–79.

[xi] Brian O'Neill, “Cairo and Climate Change: A Win/Win Opportunity.” Global Environmental Change, 10 (2000), 93-96.

[xii] Colin Butler, “Globalisation, Population, Ecology and Conflict.” Health Promotion Journal of Australia, 18 (2007), p. 87.

[xiii] Thomas Wire, Fewer Emitters, Lower Emissions, Less Cost: Reducing Future Carbon Emissions by Investing in Family Planning: A Cost/Benefit Analysis (London: London School of Economics, 2009).

[xiv] Derek Parfit grapples with some important issues around population in part four of Reasons and Persons (Oxford: Oxford University Press, 1984). I am convinced that the full repugnance of the “repugnant conclusion” cannot be understood from within an anthropocentric ethics and that the right solution to Parfit’s Non-identity Problem cannot be specified within a narrowly individualistic ethics. But exploration of these themes lies beyond the scope of this essay.