To meet goals of reduced emissions, we may need to look no further than our own populace, according to a study published in PNAS. By analyzing a few different data sets and projections for population and emissions growth, a group of researchers found that a concerted effort to slow population growth could contribute as much as 29 percent of the emissions reductions needed to avert dangerous levels of climate change. They also found that a more urban population causes a significant increase in emissions, while an older one lowers emissions.

Most of the ideas being floated to address climate change involve things like renewable energy and greater efficiency, which have a direct impact on carbon emissions. From electric cars to solar power, many of these strategies face significant hurdles before they can have a serious impact on the climate.

Instead of many small changes, one group of scientists decided to take a look at how our emissions would drop if the entire world's population could be swayed into slowing its growth down. They ran population and emissions projects from the UN and the Intergovernmental Panel on Climate Change through an energy and economic growth simulation called the People-Environment-Technology (PET) model.

While investigating a lower population growth scenario sounds simple enough, it's not as easy as just cranking down the "people" knob in the PET session and watching the emissions graph plunge. The authors note that population, as well as how people are distributed and how old they are, directly and indirectly affect a lot of factors, including economic growth, labor supply, and savings and consumption behavior. Combined, all these factors can have a dramatic effect on emissions.

All of the scenarios the authors looked at involve emissions peaking at a higher level than they are at now. However, a modest emissions projection and a reduction in population growth from the UN's "medium" projection to the "low" one (just under 9 billion vs. 5.5 billion by 2100) would drop emissions by 5.1 gigatons of carbon per year.

The simulation also showed that a population that is front-loaded with old people, especially in industrialized regions, resulted in the biggest reduction in emissions. An aging population, because of the reduced labor and resulting slower economic growth, could contribute to as much as a 20 percent reduction in emissions.

An unexpected find was that an increase in urbanization could contribute to a large rise in emissions, by more than 25 percent. Again, this was a result of the relationship between labor and the economy—workers in urban areas are more productive and create more economic growth, and therefore more emissions. While urban area dwellers tend to have smaller carbon footprints in terms of lifestyle, their economic actions turn out to significantly outweigh that benefit.

The authors note that, because of its economic influence, changes in the US's population had some of the most pronounced effects on emissions. The total impact of changes in US demographics didn't outweigh the change that could result from slowed growth in China and India, but the reductions per capita were some of the largest.

While a total possible 29 percent reduction in emissions sounds great, the whole exercise is pretty firmly idealistic. Governments can't order everyone to disband their cities and start getting older faster; at least, the authors note this would not be the wisest of policy choices (though they could always secretly put something in the drinking water).

However, the authors note that a push for family planning may at least help do the trick: many countries have unmet family planning needs, and implementing them could reduce fertility by about 0.2 births per woman in in US, and by 0.6-0.7 births per woman in developing countries.

PNAS, 2010. DOI: 10.1073/pnas.1004581107 (About DOIs).

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