FPESA Research Paper Assessment: Slower Population Growth, Lower Emissions

FPESA Research Paper Assessment: Slower Population Growth, Lower Emissions

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Global Demographic Trends and Future Carbon Emissions

Brian C. O’Neill et al., “Global Demographic Trends and Future Carbon Emissions,” Proceedings of the National Academy of Sciences, vol. 107, no. 41 (12 October 2010), pp. 17521–17526. DOI: 10.1073/pnas.1004581107. URL (full paper): www.pnas.org/content/107/41/17521.full.pdf?with-ds=yes.

This paper was collaboratively assessed.

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Perhaps the most widely cited peer-reviewed paper in the past decade on population-environment connections


Perhaps the most widely cited peer-reviewed paper in the past decade on population-environment connections, this study projects that a low trajectory of world population growth could contribute between 16 and 29 percent of the carbon dioxide (CO2) emissions savings needed by 2050 to avoid a 2-degree Celsius warming of the global climate, in comparison to a projected middle path of growth. This emissions savings is broadly equivalent to what would be saved by ending all deforestation by the same year. By 2100, the low-growth trajectory would reduce global CO2 emissions from 37 to 41 percent. The estimates were based on a model that integrated population, economic, and technological trends.

The authors also considered the impacts of urbanization and population aging on changes in greenhouse gas emissions. Neither of these factors influenced emissions changes to the extent that feasible variation in population growth did. The authors cite a 1995 U.S. government estimate that fully meeting the unmet need for family planning in the United States would reduce fertility in the country by 0.2 children per woman. The authors independently estimate that meeting this need would reduce fertility by 0.6 to 0.7 children per woman in developing countries. Although the average person in developing countries contributes much less to CO2 emissions than the average person in developed ones, both regions would contribute far less emissions in this century on the lower population-growth trajectory.

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“Family planning would have a substantial environmental benefit.”


Key quote: “Family planning would have a substantial environmental benefit.”

FPESA assessments (three total) found the article to be scientifically strong and compelling, although two assessments expressed concern that the model was not fully explained or easily comprehensible, making the calculations and the paper hard to follow for the lay public. One assessment noted that the United Nations population projections that were the basis for the paper have since been superseded by later projections that would likely affect the emissions calculations—a common problem in scientific literature based on population data that are in a state of constant change.

Overall assessment: From its ambitious conception to its careful modeling and quantification of demographic impacts on emissions, this paper deserves the fame it has garnered. Its communications value could have been improved by clearer, more accessible writing and explanation, although the concepts and findings are not simple ones to convey. By virtue of its mathematical sophistication, the results obtained, and the articulation of its implications for family planning policy, this paper is in the top tier among papers assessed in the FPESA project that support the hypothesis that family planning promotes environmental sustainability. It would be especially valuable to refresh the paper’s basic model in the near future with newer data, comparing the results to the original paper.


The following annotation and assessment is excerpted from the forthcoming report of the Family Planning and Environmental Sustainability Assessment (FPESA) project. The report will contain selected annotations from more than 930 peer-reviewed articles reviewed for potential relevance to the hypothesis that family planning supports environmental sustainability. The annotations are written by project director Robert Engelman with input from members of the project team and, where indicated, members of the project’s international network of research assessors.


6 Responses

  1. The one billion people with incomes in excess of $12,000 with a TFR of 2 emit 13 times as much carbon dioxide as the one billion population with incomes less the $900 and have 4 or more children.

  2. Jenny Goldie

    In response to Bob Gillespie, that may well be true, but extend it to, say, three generations when the original couple in the richer world has 8 greatgrandchildren, the couple in the poorer world (with four children and assuming fertility stays the same), has 64. Total carbon dioxide levels from the richer couple’s descendants would still be greater but that does not take into account the land clearing for agriculture needed to feed all the extra people. And take it another generation (16 as against 256 great great grandchildren) and you’ve passed that 13 times differential anyway.

  3. Karen Pitts

    The people with the $900 income and 4 children will have 16 grandchildren and 64 great grandchildren if they don’t use birth control. And if they run out of food produced in their family farm (64+32 people on a farm originally feeding 6), we will have to feed them (if we still can), and the fossil fuels used to supply the food system will add up (25% of our energy goes to the food supply). This is assuming that they don’t improve their lifestyle as most developing countries are doing. The number of cars in the world are expected to double by 2050 – most in areas that do not have the infrastructure to support electric vehicles, so they will probably emit GHGs.

  4. Karen Pitts

    Correction: 64 + 16 people assuming two generations are alive at the same time.

  5. Jane O'Sullivan

    (in response to Bob Gillespie) … and in only four generations there would be 16 of the low income people for each of the high income people. Addressing population growth does not mean doing any less about the per capita emissions of the rich (or the poor, for that matter – most of whose emissions are not counted in the fossil-fuel-only ratio you cite). On the contrary, it makes every other approach cheaper and more effective, while greatly increasing the future prospects of the poor for development and security. How is it that this is cast as an anti-poor position by so many of the self-righteous?

  6. Peter D. Capen

    While it is true that the number of children a family has does not also consider the rate of consumption per capita, it is also true that even the poor seek a better life for their families, which translates into higher rates of consumption as they become more affluent. One need only look at China and India to see what happens to a country as more of their huge populations are lifted out of poverty to join the middle class, consuming more goods and services, as well as changing diets to more meat consumption. Moreover, economic development today is predicated on an industrial/continually trade expanding paradigm that is simply not sustainable for much longer from purely a resources standpoint, if nothing else. Modern industrial civilization has been spending with little concern the natural capital of the planet’s natural resources, rather than living off the interest. And the bills are coming due. It is if humans have become wedded to the idea of simply making the economic pie larger than facing the possibility it is not infinite and we must consider a fundamental reslicing of it to make it more equitable. Without doing so, we will see more wars and people fleeing from want and insecurity towards centers of peace and affluence. For instance, how will it be even possible to double agricultural production by 2050 in a world headed towards a human population of 9 billion people in the face of water scarcity, rampant land degradation, massive species extinction, and, course, a planet continuing to get hotter? It’s increasingly doubtful it can, no matter how creative our technology. The flip side of the coin of the increase in human numbers is the unsustainable rate of consumption inherent in the current socio-economic industrialization paradigm. Until both population and consumption rates are tackled together as one, finger pointing at one or the other will not lead to a productive solution to the conundrum that now faces Mankind. It’s not just the number of humans, it is how we, especially in the industrialized world and the rich in the developing world, consume.

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