"Not tonight, dear. It's too damned hot." That's a phrase lovers are likely to hear more frequently as the climate warms, according to a new National Bureau of Economic Research study. The paper's authors—economists Alan Barreca of Tulane, Melanie Guldi of the University of Central Florida, and Olivier Deschenes of the University of California, Santa Barbara—looked for a causal link between heat and coital frequency using U.S seasonal and regional temperature and birth data. Their conclusion: If people are already hot and sweaty, they're less likely to want to get hot and sweaty.
The trio thus worries about global warming's effects on future fertility. If couples indulge in intercourse less frequently when it's hot, the researchers reason, then they are less likely to conceive and give birth. But before we consider whether that conclusion makes sense, let's take a look at their empirical evidence.
The study examines the seasonality of birth rates in four U.S. regions—the South, the Northeast, the Midwest, and the West. Using vital statistics data from 1931 through 2010, the researchers confirm the well-known fact that U.S. birth rates peak in August and September. From this it seems reasonable to conclude that couples enjoy more intimacy in the cooler months of November, December, and January. The authors also compile temperature and humidity data from 1931 through 2010 by county, against which they match subsequent birth rates by state.
After crunching the numbers, the researchers find that days above 80 degrees Fahrenheit do correlate with fewer subsequent births. Specifically, they report that "each additional >80°F day causes birth rates to fall by approximately 0.06% 8 months later, 0.39% 9 months later, and 0.21% 10 months later." This implies on average that from 1931 to 2010, enduring a day with temperatures greater than 80°F results in 1,165 fewer births across the whole United States than had temperatures been more moderate.
Not surprisingly, they find that the effect of >80° days is about double in "cold states" versus "hot states," suggesting those of us who dwell in balmier climes are less likely to forego romance just because it's a bit warm.
The researchers do also detect a rebound effect in which births increase in the months after an >80°F temperature shock. In other words, coital frequency apparently increases once temperatures drop. The rebound in births, however, is only half of what the average would have been if temperatures had been sustained at a lower level. Their analysis suggests that 30 additional days at 85°F and above would result in a 12 percent decline in births.
The authors acknowledge that beginning in the 1970s, the correlation between temperature and subsequent births weakens somewhat. "In the 1940s, 1950s, and 1960s, exposure to one additional >80°F day consistently causes a 0.6 percent reduction in the birth rate 9 months later," the researchers report. They find that by the 2000s that one additional >80°F causes birth rates 9 months later to decline by only 0.2 percent. What happened? Simple: Since the 1960s, the percentage of U.S. households with air conditioning rose from 21 percent to about 87 percent today. Apparently, manufactured cool air is a turn-on.
So what is the future of intimacy and fertility in a warming world? The authors cite climate models that project that the number of >80°F days per year in the U.S. will increase about 64 days from a baseline of 31 days now. Most of the additional >80°F days will occur in May and September, although June, July, and August will supposedly each experience 10 more >80°F days by 2100. Recognizing the inherent uncertainties in such long-term projections, the researchers make a back-of-the-envelope estimate using an econometric model that takes post-1970s (that is, post–air conditioning) data into account. They calculate that as a result of higher temperatures due to climate change, annual U.S. births will decline by 2.6 percent, or about 107,000 fewer births per year than there would have otherwise been. Because it will be just too hot to snuggle, they also reckon that the proportion of births will increase by 4 percent during the August peak relative to the April trough.
As amusing as it is to contemplate the effect of warmer temperatures on future sexual activity, the researchers recognize that technological innovation could make their projections moot. "Providing low-cost access to air-conditioning may be an effective tool for mitigating the fertility costs of climate change throughout the world," they note. But that concession is way too limited.
By the end of this century, technology will have radically reshaped the terrain of human sexuality and fertility. Extensions and elaborations of virtual pornography, teledildonics, sexbots, and sexual neuroenhancers will change how people experience and enjoy sex. Similarly, people will gain ever more control over their fertility, up to and including the use of artificial wombs. Whatever minor effects climate change might have on making love and making babies, they will be overwhelmed by technological enhancements in the service the ceaseless human quest for healthy children and healthy orgasms. At the end of this century, lovers will still likely croon, "Baby, it's hot outside, but not as hot as you."