The effects of climate change aren’t just limited to melting glaciers and rising sea levels. Global warming is likely to affect human societies in all kinds of other ways, from higher rates of infectious disease to greater wealth disparities and even increases in civil unrest or war.
Now, new research has revealed another way climate change could affect us: increases in hot days could cause birth rates to fall.
The study, in the form of a working paper published last month by the National Bureau of Economic Research, examined the effects of “temperature shocks” — specifically, days with an average temperature above 80 degrees Fahrenheit — on birth rates in the United States.
The researchers found that each additional day above 80 degrees caused a significant decline in the birth rate approximately nine months later, suggesting that hot days can negatively affect conception. But they also found that birth rates rebound to a certain extent in the following months, suggesting that conception rates start bouncing back in cooler months.
It's unclear, though, if couples are having less sex on warmer days than on cooler days, if the weather is altering reproductive health, or both.
Lead author Alan Barreca, an associate professor of economics at Tulane University, said he first got interested in conducting the study after he started thinking about seasonal patterns in birth rates.
“I, like many people, was interested in why there are these peaks in birth rates — why most of my friends tend to be born in August or September,” Barreca said in an interview with Mashable.
“What could be driving this fact?” He said the obvious factor that occurred to him and his colleagues was temperature, which varies seasonally throughout the year.
Past research has shown that hot weather can affect fertility in a number of ways. The most obvious way is that it can make people shy away from having sex, because of changes in energy as well as the sweat factor.
But it’s also been shown that higher temperatures can affect sperm production and testosterone, as well as menstruation and ovulation. So it’s possible that temperature spikes could be affecting people’s ability to conceive on a biological level, as well.
Each hot day led to 1,165 fewer births
Barreca and his colleagues, Olivier Deschenes of the University of California, Santa Barbara, and Melanie Guldi of the University of Central Florida, examined monthly temperature data and birth rates from the years 1931 to 2010. In their statistical analysis, they controlled for certain factors like seasonal employment patterns and immigration influxes, which are not necessarily related to temperature but could have influenced conception and birth rates.
The researchers found that, overall, every additional “hot day” (above 80 degrees) reduced the birth rate nine months later by about 0.4%, which comes out to about 1,165 fewer births. Over the course of the next year, there’s a rebound in births, making up for about 50% of the warm weather-related decline.
The majority of the rebound tends to come just a few months after the decline, so there tends to be a dip in births in the spring and a peak during the summer (indicating that conception rates were higher in the fall and winter).
The mechanisms behind the birth declines and rebounds — that is, how exactly temperature is affecting them — remain unclear, according to Barreca.
“It could be coital frequency — that’s an unsexy way of saying ‘sex’ — or it could be reproductive health,” he said.
He’s leaning toward the idea that reproductive health might be the bigger factor. It looks like hot weather may be affecting people’s ability to conceive, and then as the weather cools down a few months later they start to have more success again.
One interesting thing the researchers noted, though, was that the hot days had more of an effect on birth rates earlier in the study period, between 1931 and 1969. Starting in the 1970s, the relationship between temperatures and birth rates started to diminish a little.
The researchers ran some additional tests and found that an increase in access to air conditioning starting around that time can explain some of this phenomenon.
“Air conditioning can help protect people from the [temperature] shock,” Barreca explained.
Hotter times ahead
Barreca and his colleagues were also interested in how climate change might affect birth rates in the future. While he noted that there are currently about 30 days per year, nationwide, with daily average temperatures over 80 degrees, climate models suggest that this number could triple by the end of the century.
So the researchers used models to make projections about how birth rates could be affected throughout the remainder of the century..
They found that, with the added hot days per year, birth rates could fall by 2.6% — that’s a loss of more than 100,000 births per year.
It’s important to note that the data used in this study (which was not peer-reviewed) were only taken from the United States, so it’s impossible to say for sure if the results can be extended to other parts of the world. However, if these patterns do hold true globally, the researchers believe the effects on births could be even more pronounced in other regions, particularly in developing nations where birth rates are currently higher and temperatures tend to be milder.
These effects come with certain costs, as the authors note. There are the obvious emotional costs for individuals having difficulty conceiving — but there are also certain economic considerations associated with a declining birth rate.
In countries where birth rates are already low, for instance, such declines could result in a shrinking workforce, which might not adequately be able to support its aging population through government programs such as social security.
That said, there are still many questions left to be answered in future research. There are other factors the researchers didn’t control for that are related to temperature and could be also be affecting birth rates in some way. For instance, people tend to get more sunlight in warmer months.
Barreca noted that future research should also focus on the ways seasonal changes in birth rates could affect infant health. Summers will be hotter in the future, and if more infants are born in the summer months, that could be worrisome.
Some research has suggested that hotter temperatures can have adverse effects on the health of babies, as exposure to warmer temperature in the third trimester has been associated with low birth weight, which can lead to poor infant health.
The authors said that one way to mitigate these effects would be to improve access to air conditioning. That recommendation comes with a caveat, though: Air conditioning requires large amounts of energy and can contribute to increased emissions of climate-changing greenhouse gases.
Barreca suggested that some solutions would be to improve the efficiency and eco-friendliness of air conditioners and also make corresponding emissions cuts in other places to make up for an increased use of air conditioning.
There’s also the more philosophical question of whether decreases in birth rates could actually benefit the planet’s environmental future. But Barreca’s opinion is that “reducing population growth via climate change seems unjust because it reduces people’s opportunities to have the family size that’s right for them.”
“If we did want to solve the population growth dilemma around the world, there are more efficient and just ways to do that,” he added, citing increased access for women to education, labor opportunities and birth control as positive ways of giving people the means to control their own family sizes.