By mid-century it is estimated that global population could rise by more than three billion people, with most of that increase occurring in urban areas. However, the study showed that a slowing of that population growth could contribute to significantly reducing greenhouse gas emissions.

The researchers found that if population follows one of the slower growth paths foreseen as plausible by demographers at the United Nations, by 2050 it could account for 16 to 29% of the emission reductions thought necessary to keep global temperatures from causing serious impacts.

The effect of slower population growth on greenhouse gas emissions would be even larger by the end of the century.

If global population growth slows down, it is not going to solve the climate problem, but it can make a contribution, especially in the long term.

Demography will matter to greenhouse gas emissions over the next 40 years. Urbanization will be particularly important in many developing countries, especially China and India, and aging will be important in industrialized countries.

—Brian O’Neill, NCAR scientist and lead author

O’Neill’s co-author, IIASA scientist Shonali Pachauri, says that slower population growth will have different influences, depending on where it occurs.

A slowing of population growth in developing countries today will have a large impact on future global population size. However, slower population growth in developed countries will matter to emissions, too, because of higher per capita energy use.

—Shonali Pachauri

Scientists have long known that changes in population will have some effect on greenhouse gas emissions, but there has been debate on how large that effect might be. The researchers sought to quantify how demographic changes influence emissions over time, and in which regions of the world. They also went beyond changes in population size to examine the links between aging, urbanization, and emissions.

The team found that growth in urban populations could lead to as much as a 25% rise in projected carbon dioxide emissions in some developing countries. The increased economic growth associated with city-dwellers was directly correlated with increased emissions, largely due to the higher productivity and consumption preferences of an urban labor force.

In contrast, aging can reduce emissions levels by up to 20% in some industrialized countries. Older populations are associated with lower labor force participation, and the resulting lower productivity leads to lower economic growth.

The researchers worked with projections showing that population aging will occur in all regions of the world, a result of people living longer and declines in fertility. They developed a set of economic growth, energy use, and emissions scenarios, using a new computer model (the Population- Environment-Technology model, or PET). To capture the effects of future demographic change, they distinguished between household types, looking at age, size, and urban vs. rural location.

In addition, they drew on data from national surveys covering 34 countries and representative of 61% of the global population to estimate key economic characteristics of household types over time, including labor supply and demand for consumer goods.

The authors also suggest that developers of future emissions scenarios give greater consideration to the implications of urbanization and aging, particularly in the US, European Union, China and India.

The researchers caution that their findings do not imply that policies affecting aging or urbanization should be implemented as a response to climate change, but rather that better understanding of these trends would help anticipate future changes.

Resources

• Brian C. O’Neill, Michael Dalton, Regina Fuchs, Leiwen Jiang, Shonali Pachauri, and Katarina Zigova (2010) Global demographic trends and future carbon emissions. PNAS doi: 10.1073/pnas.1004581107