Changes in population growth and composition, including aging and urbanization, a new study has suggested, could affect global emissions of carbon dioxide over the next 40 years.
Researchers said 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.
The study showed that a slowing of population growth, following one of the slower growth paths considered plausible by demographers at the United Nations, could contribute to significantly reducing greenhouse gas emissions.
The researchers found that 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.
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," the study's lead author, Brian O'Neill, an NCAR scientist, said.
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," 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.
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 labour force.
In contrast, aging can reduce emissions levels by up to 20 percent in some industrialized countries.
This is because older populations are associated with lower labour force participation, and the resulting lower productivity leads to lower economic growth.
"Demography will matter to greenhouse gas emissions over the next 40 years," O'Neill said.
"Urbanization will be particularly important in many developing countries, especially China and India, and aging will be important in industrialized countries," he stated.
The research was carried out by an international team of scientists from the National Center for Atmospheric Research (NCAR), the International Institute for Applied Systems Analysis (IIASA), and the National Oceanographic and Atmospheric Administration.
It was funded by a European Young Investigator's Award, the Hewlett Foundation, and the National Science Foundation, which is NCAR's sponsor.
The findings appear in Proceedings of the National Academy of Sciences (PNAS).