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 34.07 Fahrenheit per 100 years, suggests changing temperatures due to global warming could alter the world’s proportion of male and female newborns. The findings, published in the journal Fertility and Sterility and reported on by CNN, follow the scientists’ previous work on how births are affected by extreme environmental events.

To understand the associations, researchers compared Japan’s yearly average temperature differences and extreme events to the proportion of male and female newborn births between 1968 and 2012. They also studied effects on sex ratios of spontaneous fetal deaths after 12 weeks of gestation.

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 be 103-106 boys to 100 girls. The world’s sex ratio, according to 2017 data, is 102 males to 100 females.

In recent years, there have been nearly 90,000 newborns, and about 1,000 fetal deaths recorded monthly in Japan. Fetal deaths, according to researchers, involves deaths due to spontaneous abortion or miscarriage after 12 weeks of pregnancy.

What the researchers found

Both sex ratios of fetal deaths and temperature differences have been on the rise since the 1970s, whereas the sex ratios of newborn infants have been decreasing.

The conception of boys in particular were 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 percent to 14 percent 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, such as drought or wildfire.

Lead researcher Misao Fukuda of the M&K Health Institute told CNN that stress due to “climate events caused by global warming” may also influence the sex ratio.

“Fukuda theorizes that the vulnerability of Y-bearing sperm cells, male embryos and/or male fetuses to stress is why ‘subtle significant changes in sex ratios’ occur,” the network reported.

University of California, Berkeley, professor Ray Catalano added this is because the male infant is “a relatively frail organism.”

“For every society, for every year, the human being most likely to die [prematurely] is male infants. And that’s true for every society that we have data for,” Catalano told CNN.

The National Oceanic and Atmospheric Administration labeled 2010 the hottest year on record since 1880, which rang true for Japan as well. The summer of 2010 was the country’s hottest since 1898.

Catalano explained that as climate models continue to predict warming and unpredictable temperature extremes, human stress is likely to heighten and affect both gestation and the newborn sex ratio.

“When you change the climate the way we’re changing it, you will change, profoundly, the characteristics of the population,” he said.

“The recent climate extremes, this hottest summer in 2010 together with the coldest January in 2011, may attenuate the sex ratio of births to 1.050 in 2011, the lowest since 1951, and contribute to a recent trend of decreasing sex ratios of newborn infants in Japan,” study authors wrote.

Such associations between yearly mean temperatures and sex ratios of newborn infants or fetal deaths were not significant in New Zealand and Finland, a discrepancy which may reflect the fact their neither country is exposed to the same temperature extremes as Japan. In both New Zealand and Finland, yearly mean temperatures also increased at slower paces compared to Japan.

Researchers acknowledge other confounding factors that may influence the relationship between temperature, fetal deaths and sex ratios of newborn infants. For example, air pollution has been linked to decreased sex ratios and increased preterm births. Other toxic chemicals—dioxin, Seveso and methylmercury from Japan’s tragic 1956 Minamata Bay poisoning disaster—also reduce sex ratio of births.

“However,” authors wrote, “we have demonstrated that monthly climate temperature extremes are connected to an increase in the fetal death rate and/or a decline in the sex ratio of births, in addition to the long-term effect of yearly temperature fluctuation on the sex ratios of fetal deaths and births.”

Whether climate change will affect newborn sex ratios in other parts of the world, however, depends on a variety of environmental factors. But ultimately, scientists warn extreme weather may have significant human health and evolutionary impacts.