"In general, one can associate lower populations with lower emissions, but it's not going to guarantee a low emissions outcome," explained Brian O'Neill, a scientist at the National Center for Atmospheric Research. Larger populations tend to produce higher total levels of emissions, but other factors—including urbanization, technology, and population age structure—can affect a large population's emissions output, said O'Neill. He was joined by Joseph Speidel of the University of California, San Francisco, for a February 20, 2008, discussion of the links between population and climate change sponsored by the Woodrow Wilson Center's Environmental Change and Security Program.

Population and Climate: Past and Present

O'Neill explained that it is extremely difficult to attribute environmental outcomes like climate change to specific population phenomena (e.g., urbanization, aging, and growth or decline). Nevertheless, O'Neill's examination of several recent studies revealed that historically, population size has had "a roughly proportional effect" on emissions—in other words, a large population tends to generate high levels of emissions. This relationship also tends to hold true in scenarios of possible future emissions, including those developed by the Intergovernmental Panel on Climate Change (IPCC) in its Special Report on Emissions Scenarios (SRES), published in 2000.

O'Neill and several colleagues are attempting to refine some of the SRES predictions by examining the effects of specific factors such as aging, household size, and urbanization on carbon emissions. The study found that smaller U.S. households have higher carbon-intensive expenditures (on utilities and fuel) per person than larger ones, and that older U.S. households have lower per capita carbon-intensive expenditures than younger ones. The study projected that by 2100, an aging U.S. population would result in emissions levels more than a third lower than they would be had the U.S. demographic makeup remained static. Slower population growth, a byproduct of an aging population, further decreases emissions output.

Although aging populations and larger household sizes tend to reduce overall emissions levels, urbanization tends to increase them. In a projection of aging and urbanization's effects on emissions levels in China, aging by itself led to the largest decrease in emissions output, but aging combined with urbanization led to the largest increase—more than tripling emissions output.

O'Neill emphasized that population policy cannot singlehandedly produce drastic cuts in emissions. "Population-related policies are not going to produce any immediately discernable effect on emissions…[They] can't replace reductions in per capita emissions that can happen in the shorter term and would need to happen in order to avoid some of the more extreme climate change outcomes." Technological advances, for example, can lead to much more significant and rapid emissions reductions. But O'Neill criticized the fact that population and demography "are simply not on the table at the moment" as ways to combat climate change. "Climate change is a big problem, and this would appear to be an opportunity," he said.

Policies for a Growing—and Warming—World

"It seems like the population issue has a little bit fallen off the radar screen, and it looks like global warming may be bringing it back," said Joseph Speidel. Population issues are critical for several reasons, said Spiedel: More than 200 million couples in developing countries lack access to family planning services; poor reproductive health is a leading cause of death in developing countries; rapid population growth impedes quality-of-life improvements and perpetuates poverty; and rapid population growth has serious environmental consequences. Although birth rates have declined over the past several decades, the global population continues to increase, with 95 percent of that growth occurring in developing countries.

Population growth can be addressed by both supply-side and demand-side approaches, explained Spiedel. Demand-side approaches assume that slowing population growth requires boosting the demand for small families, which can be accomplished by increasing access to education, especially for girls; improving women's economic opportunities; and improving infant and child health.

Spiedel believes the international community could slow population growth more quickly and cost-effectively with supply-side approaches—namely, by fulfilling the current unmet demand for family planning—than by trying to increase the demand for family planning services. "The history of the family planning movement shows that that demand exists," he said, noting that in developing countries between 1960 and 2005, contraceptive use increased by 50 percent and the total fertility rate declined from 6.0 to 3.1. "What happened during that period [was]…the world community and many governments made a concerted effort to provide good family planning services." This effort had rapid and significant effects on population growth in many places. For example, in 1989, the Iranian government decided to strongly support a nation-wide family planning program, and by 1996, the fertility rate had declined from 5.5 to 2.8—"one of the most rapid declines in fertility on record."

The consequences of inaction on population are dire, Speidel warned. If today's birth rates remain unchanged, world population will grow from 6.7 billion today to 11.9 billion in 2050. This growth will likely hamper socio-economic progress in the developing world, endanger past successes, and impede efforts to protect the environment.

Speidel called for a large increase in funding for family planning, especially for programs to help women avoid unintended pregnancies. He emphasized that simply meeting demand for family planning services, which are far cheaper to deliver than improvements in education and socio-economic circumstances, could significantly and quickly slow population growth. This would make improvements in education, health, and economic opportunities easier to achieve, ease the transition to a changing climate, and, in concert with other prudent policies, slow the change itself.

Drafted by Sonia Schmanski and edited by Rachel Weisshaar.