Climate change—with its warmer global temperatures and more extreme weather events—will affect us in profound ways, including, perhaps, by tipping the balance between genders. A new study found that, at the same time that temperatures are rising in Japan, the number of boys born is falling in comparison to the number of baby girls.
This measurement is often referred to as the sex ratio, and there’s evidence that it’s sensitive to a number of factors. For example, pollution in central Scotland seems to be skewing the ratio there in favor of girls. But the science is murky. For example, war might favor the conception or birth of boys… or it might favor girls. And despite findings that tropical countries produce more girls, warmer periods in Northern Finland produce more boys.
The Japanese researchers examined records of births and spontaneous abortions from 1968 to 2012. Girls were gaining (and more male fetuses were dying) "steadily along with temperature differences," they write in the journal Fertility and Sterility. Also, an unusually hot summer followed by a cold winter first depressed the ratio (more girls than boys born nine months later) then boosted it (more boys).
Studies on other animals give more evidence that temperature and weather affects sex ratios at birth. Rainy time periods lead to more male chimpanzees. Loggerhead sea turtles lay more eggs that turn out to be females in warmer temperatures. In fact, many reptiles depend on temperature to determine the sex of incubating eggs.
But humans are not reptiles and the Japanese researchers caution that they can’t say climate change caused the skewed sex ratio they see. However, their finding does back up the idea that male fetuses are somehow more vulnerable to events that stress out expectant mothers.
So if female fetuses are hardier, why is the global sex ratio still 101 men to 100 women? At birth, there are still slightly more boys born than girls — the ratio then is close to 1.06, according to a 2002 study. Several countries, including China and India, selectively abort female fetuses and favor males.