Children cool off in the fountain at Grand Park in downtown Los Angeles. AP Photo/Ringo H.W. Chiu

By this century’s end, more extremely hot days could mean 107,000 fewer U.S. births per year.

Extremely hot days are becoming more and more frequent as climate change progresses. Heat is a human health hazard, to say the least: In the U.S., heat causes about 400 deaths per year. Another 1,800 die from illnesses exacerbated by hot days, such as heat exhaustion, heat stroke, and heart disease.

But there may be another, overlooked health effect of a warming planet: reduced fertility.

That’s what Alan Barreca, associate professor of economics at Tulane University and lead author of a new working paper published by the National Bureau of Economics, believes. Using federal data on birth rates and weather patterns for all U.S. states dating back to 1931, Barreca and his colleagues created a model that tested how local hot weather “shocks” impacted fertility.

The researchers found that, historically, when temperatures rose above 80 degrees, birth rates fell by 0.4 percent nine months later. Most of these temperature spikes happened in late summer.

There was some rebounding: Couples who did not successfully conceive on a hot August day, for example, may have tried again in September, October, or November. This made up for about 32 percent of the initial decline. But, “to add insult to fertility injury,” as Barreca puts it, the rebound babies wound up being born in July and August, instead of April or May. There’s some evidence that late summer births tend to pose the highest risk for infant health outcomes, compared to other times of the year.

Beyond temperature, of course, there are countless social, economic, and cultural factors that have affected U.S. fertility rates over the past century—think birth control, expanded job and educational opportunities, and a higher standard of living. Barreca says that the aim of his research is to show weather as an important added factor.

Which points to the the context of the paper: climate change. Employing a widely-used climate change model to project temperature shifts through the end of the century, the researchers estimate that, between 2070 and 2099, the median U.S. temperature will top 80 degrees about 90 days each year. Right now, we only see about 30 such days annually. According to Barreca’s model, that considerable increase will result in annual birth rates declining by 2.6 percent from what they’ve been since 1970—or about 107,000 fewer births per year.

“Our study makes the important point that temperature is affecting reproductive chances, which is something that individuals and policy makers can potentially act on to improve health and happiness,” he writes in an email to CityLab.

Can’t human fertility adapt to hotter temperatures? Barreca says that the broad adoption of air conditioning in the 1970s is correlated with greater resilience of the birth rate to heat shock, and that policymakers might encourage its use to offset some of the projected birth decline. Of course, air conditioning has its own environmental costs, so it’s certainly not a one-fit solution.

On other the hand, you might imagine that a reduced birth rate would be a good thing for our warming planet—fewer bodies means fewer emissions, right? But fewer babies also means more of an aging population, and less opportunity for people to have the families they desire. Furthermore, Barreca says it is unjust to conceive of a declining birth rate as a solution to climate change.  

“That’s like cutting off our toes because our shoes don’t fit,” he says. “There are better ways to deal with this issue. We should go buy some new shoes.” Which is to say, we need better policies that directly address the world’s rising temperatures.

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