NEW YORK • Climate change has been blamed for many things over the years. Never, until now, has it been possible to see it as a kind of contraceptive.
Hot weather leads to diminished "coital frequency", according to a paper by the private, non-profit National Bureau of Economic Research in the US.
Three economists studied 80 years of US fertility and temperature data and found that when it is hotter than 26.6 deg C, a large decline in births follows within 10 months.
Would-be parents tend not to make up for lost time in subsequent, cooler months.
What they call an extra "hot day" leads to a 0.4 per cent drop in US birth rates nine months later.
A rebound in subsequent months makes up just 32 per cent of the gap.
The researchers, from Tulane University, the University of California at Santa Barbara, and the University of Central Florida, believe that their findings give policymakers three major things to think about.
First, birth rates do not bounce back completely after heat waves. As summers heat up, developed countries may see already-low birth rates sink even lower, playing havoc with an economy.
China's leaders have acknowledged that fact by ditching the long-time one-child policy. A sub-replacement birthrate means fewer workers to support their elderly parents.
Second, more autumn conceptions means more deliveries in summer, with a higher level of poor infant health.
"Though the reasons for worse health in the summer are not well-established," the authors write. One possibility may be "third-trimester exposure to high temperatures".
However, the third key point suggests air conditioning may be an aphrodisiac. The researchers suggest that the rise of the aircon may have helped offset some of the heat-related fertility losses in the US since the 1970s.
The researchers assume that climate change will proceed according to the most severe scenarios, with no substantial efforts to reduce emissions.
Their scenario projects that from 2070 to 2099, the US may have 64 more days above 26.6 deg C than the baseline period, 1990 to 2002, which had 31.
As a result, the US may see a 2.6 per cent decline in its birth rate.