A new study from the National Center for Atmospheric Research and other bodies aims to quantify the effects of population growth on carbon dioxide emissions, and concludes that it could make a big difference.
The global population is expected to rise by more than three billion people by the middle of the century, with most growth in urban areas.
But the study showed that if the world follows one of the slower growth paths considered plausible by the United Nations, greenhouse gas emissions could be cut significantly.
Such a path could account for 16 to 29 percent of the emissions reductions thought necessary to keep global temperatures from causing serious impacts - and the effect 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," says the study's lead author, Brian O'Neill, an NCAR scientist.
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," says International Institute for Applied Systems Analysis scientist Shonali Pachauri.
The team found that growth in urban populations could lead to as much as a 25 percent rise in projected carbon dioxide emissions in some developing countries, largely due to the higher productivity and consumption preferences of an urban labor force.
In contrast, aging can reduce emission levels by up to 20 percent in some industrialized countries. This is because older populations are associated with lower labor force participation, and the resulting lower productivity leads to lower economic growth.
"Urbanization will be particularly important in many developing countries, especially China and India, and aging will be important in industrialized countries," says O'Neill.
The authors developed a set of economic growth, energy use, and emissions scenarios, using a new computer model. They also drew on data from national surveys to estimate key economic characteristics of household types.
"Households can affect emissions either directly, through their consumption patterns, or indirectly, through their effects on economic growth," O'Neill explains.
The authors suggest that developers of future emissions scenarios give greater consideration to the implications of urbanization and aging, particularly in the US, EU, China, and India.