Why extreme weather is bad for boys

Researchers have warned climate change will lead to extreme weather events, increase human mental health problems and internally displace more than 143 million people.

A recent study from Japan, where temperatures have increased an average of 1.15 C per 100 years, suggests changing temperatures due to global warming could alter the world’s proportion of male and female newborns. The findings, published recently in the journal Fertility and Sterility, follow the scientists’ previous work on how births are affected by extreme environmental events.

At conception, scientists believe the sex ratio is equal. But during gestation, more than half of all human conceptions die, leaving a sex imbalance at birth. Due to higher female mortality, the global ratio at birth is considered to be103-106 boys to 100 girls.

In recent years, there have been nearly 90,000 newborns, and about 1,000 fetal deaths (spontaneous abortions or miscarriages after 12 weeks of pregnancy) recorded monthly in Japan. Both sex ratios of fetal deaths and temperature differences have been rising since the 1970s, whereas the sex ratios of newborn infants have been decreasing.

The conception of boys was deemed especially vulnerable to external stress, such as extreme weather events. In fact, nine months after some of Japan’s most disastrous events — the 1995 Kobe earthquake, 2011 Great East Japan earthquake and Fukushima Daichii power plant nuclear disaster — the proportion of male babies born in affected areas declined by anywhere from 6 per cent to 14 per cent compared to the previous year. Extreme weather events were also associated with fluctuations in birth weight.

These findings suggest extreme stress can influence gestation and alter the gender ratio of newborns, potentially leading to more boys born in areas with increasing temperatures and fewer boys in areas with more extreme environmental changes.