Slower population growth could contribute to significantly reducing greenhouse gas emissions, a new study suggests.

"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," said the study's lead author, Brian O'Neill, a scientist at the National Center for Atmospheric Research (NCAR).

Such slow growth paths by 2050 could account for 16 to 29 percent of the emissions reductions thought necessary to keep global temperatures from causing serious impacts, according to the study conducted by an international team of scientists from the NCAR, the International Institute for Applied Systems Analysis ( IIASA), and the National Oceanographic and Atmospheric Administration.

The effect of slower population growth on greenhouse gas emissions would be even larger by the end of the century, said the study appearing Monday in Proceedings of the National Academy of Sciences.

Slower population growth will have different influences, depending on where it occurs, said study co-author, IIASA scientist Shonali Pachauri.

"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," Pachauri said.

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.

In the latest study, 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 percent 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, the researchers said.

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

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

"Households can affect emissions either directly, through their consumption patterns, or indirectly, through their effects on economic growth," O'Neill explained.

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, according to the study.

The researchers 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 percent of the global population to estimate key economic characteristics of household types over time, including labor supply and demand for consumer goods.

The study also suggests that developers of future emissions scenarios give greater consideration to the implications of urbanization and aging, particularly in the United States, the European Union, China, and India.

"Further analysis of these trends would improve our understanding of the potential range of future energy demand and emissions," said O'Neill.

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.