Specifically, climate change could alter the proportion of male and female newborns, with more boys born in places where temperatures rise and fewer boys born in places with other environmental changes, such as drought or wildfire caused by global warming.
A recent study in Japan found a link between temperature fluctuations and a lower male-to-female sex ratio at birth, with conceptions of boys especially vulnerable to external stress factors, wrote Dr. Misao Fukuda, lead study author and founder of the M&K Health Institute in Hyogo.
Last summer, Fukuda and his colleagues published a separate study looking at births in areas hit by environmental events that caused extreme stress. They included Hyogo Prefecture after the Kobe earthquake of 1995; Tohoku after the Great East Japan Earthquake of 2011 (and subsequent nuclear disaster at the Fukushima Daichii power plant); and Kumamoto Prefecture after the 2016 earthquakes.
Nine months after these disasters, the proportion of male babies born in these prefectures declined by between 6% and 14% from the previous year. This data supports the idea that major stress affects gestation, which in turn alters the newborn sex ratio, Fukuda and his co-authors wrote.
Stress stemming directly from "climate events caused by global warming" might also affect the sex ratio, Fukuda wrote in an email. Though scientists do not know how stress affects gestation, 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 newborn sex ratio
Scientists believe that the sex ratio is equal at conception, explained Steven Orzack, president and senior research scientist of the Fresh Pond Research Institute in Cambridge, Massachusetts. But more than half of all human conceptions die during gestation, and this results in a sex imbalance at birth.
"Overall, more females die during pregnancy than do males. So that's why there's an excess number of males at birth," said Orzack, who has published research on this issue.
Ray Catalano, a professor in the school of public health at the University of California, Berkeley, explained that the process of natural selection in utero is why deaths occur during gestation.
A mother's biology spontaneously aborts some conceptions in utero but not others. The factors that filter out who "gets through" from conception to birth include chromosomal or genetic abnormalities of the fetus or the mother's stress response to changes in her environment, Catalano said.
Because the ovaries of a female fetus carry all the eggs she will ever possess, the possibility of genetic defects being found in a female (and her eggs, which represent her potential children) are greater than the possibility of defects in a male fetus, which carries only his own genes.
Worldwide, the newborn sex ratio averages 103 to 106 males born for every 100 females, Catalano explained. In part, this is a result of the fact that a male infant are "a relatively frail organism," he said.
"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 said. The reasons why are not understood, but some scientists believe that boys are biologically weaker and more susceptible to diseases and premature death.
The general theory as to why the sex ratio is not equal at birth is that if you want the sex ratio to be 50:50 by reproductive age, you want a few more males than females at birth because more males than females will probably die in early childhood, Catalano said.
What Catalano found when studying populations of Danes, Finns, Norwegians and Swedes born between 1878 and 1914 is that colder years meant fewer males born. Yet years of fewer males meant hardier baby boys, who were less likely to die in infancy, he found. These boys grew into men who had a larger-than-expected number of children. This is evidence of selection in utero at work, he said.
Effects of global warming will also shape the selection process in utero, Catalano said.
The Earth is undergoing a process of rapid change. "If you start to change the environment relatively quickly -- within 100, 150 years; in evolutionary time, that's a blink of the eye -- what that means is that you're going to change the environment in which human gestations occur," Catalano said.
Add to that the fact that climate change models don't just predict that Earth will become warmer. "What they predict is that things will get less predictable," he said: We'll have greater swings of temperatures with higher highs, lower lows and faster oscillations between the two extremes.
Extreme weather and subsequent environmental effects, such as droughts, will probably lead to human stress.
That stress is likely to affect the birth sex ratio, and there will then be human adaptation -- the natural evolutionary response, Catalano said. "When you change the climate the way we're changing it, you will change, profoundly, the characteristics of the population."
'Warmer temperatures bring sons'
Samuli Helle, a senior researcher in the Section of Ecology, Department of Biology at the University of Turku in Finland, also found that "warmer temperatures bring sons."
In his study of the Sami people of Northern Finland, he was also able to quantify the effect: For every 1 degree Celsius (1.8 degrees Fahrenheit) rise in temperature, there was a 0.06% increase in the ratio of newborn boys compared with girls. For example, he said, an annual increase of 3 degrees Celsius (5.4 degrees Fahrenheit) would translate to a 0.18% higher ratio of male-to-female newborns.
"Not a dramatic influence at first sight but it should be remembered that in large populations such effect size might mean thousands of 'extra' boys annually," Helle wrote in an email.
Helle also said events caused by global warming, such as forest fires and floods, might also impact the sex ratio, though the scale would not necessarily be global.
"Such influences are more likely to be local, since the climate is warming up differently in different parts of the world. Likewise, environmental hazards are also likely to have spatially rather limited influences on human reproduction," he wrote.
Helle said there are also, potentially, several factors that influence the human birth sex ratio (and in both directions), so he doesn't expect to see global scale effects just due to climate change in the near future.
Orzack does not believe there's enough evidence to definitively state that climate change affects the newborn sex ratio. There is a trend in a number of countries towards a less male-biased sex ratio at birth. Orzack said, though he does not know whether this trend is a direct consequence of global climate change. However, this trend may be due to the effects of pollution, he said, and this "may be a secondary consequence of global climate change."
Fukuda believes that any potential effects of climate change on the newborn sex ratio "may not be uniform" around the globe. "It may depend on different environmental factors of each place," he wrote. "Extremely hot or cold weather" most markedly influenced the birth sex ratio, while more moderate shifts did not always show an effect.
Weak or strong, any effects probably would not be long-lasting, Fukuda said. His earthquake study found that the newborn sex ratio returned to normal within a few months. The "Kobe earthquake took one month, Tohoku earthquake two months, and Kumamoto one month," he said.
Ultimately, for Fukuda, the importance of the newborn sex ratio is less societal, more medical. The importance of the newborn sex ratio is as a "sensitive reproductive health indicator," he said. "Extreme temperature fluctuation affects birth weight."
Catalano's concern, though, is evolution. "Humans are incredibly adaptable, we got through the Great Ice Age," he said, so he has no fears that we will adapt to climate change.
"What will we be after that adaption? We will be different," he said. "Climate change is going to change the characteristics of the population in ways that maybe can't be anticipated."
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