red globe with thermometerPopulation growth obviously has an impact on climate change, but there’s been little rigorous study of exactly what that impact is. New research published today in the Proceedings of the National Academy of Sciences begins to fill that gap.

The top-level takeaway: “[S]lowing population growth could provide 16 to 29 percent of the emissions reductions suggested to be necessary by 2050 to avoid dangerous climate change,” write Brian C. O’Neill et al in “Global demographic trends and future carbon emissions.”

You could guess this part: The population of the U.S. has outsized impact on the climate. Slowing the growth in the number of Americans could have a “pronounced effect” because of the nation’s high per capita emissions.

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You might not guess this part: It’s not just about total numbers of people and the wealth of those people. Aging and urbanization are big factors. 

Aging of a populace can bring emissions down by as much as 20 percent because older people tend to leave the workforce and don’t contribute as much to economic growth.

Urbanization, on the other hand, can push emissions up by more than 25 percent, particularly in developing countries, because “urbanization tends to increase economic growth.” This trend is strong enough even to outweigh the energy-efficiency benefits of city living

So should we all move “back to the land”? No. The authors don’t advocate policies to reverse urbanization. 

You’ve heard this before: Instead, they suggest meeting the “substantial unmet need for family planning and reproductive health services” — not just in developing countries but in the U.S. (Think the U.S. doesn’t have unmet need? Think again.)  “Policies that meet [unmet] need would reduce current fertility by about 0.2 births per woman in the United States and 0.6-0.7 births per woman in the developing world.”

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The analysis of the problem is new, but the solution is not: Ensure women can get the family-planning services they want. Any questions?