Scientists have long warned of the impacts that human activities are having on the Earth's climate. Our power plants and vehicles are spewing dangerous amounts of carbon dioxide, which is causing glaciers to melt, the seas to rise, and temperatures to become hotter on average around the world.

But a new study suggests there may also be a link between extreme temperatures brought about by climate change and the number of males born compared to the number of females.

That's right: Climate change may be closing the gap between male and female newborns.

Male concepti seem to be especially vulnerable to external stress factors, including climate changes.

A group of Japanese and Danish researchers evaluated two climate extremes in Japan: The exceptionally hot summer of 2010 — the hottest in Japanese history — and the country's very cold and snowy winter of 2011.

"The recent temperature fluctuations in Japan seem to be linked to a lower male:female sex ratio of newborn infants, partly via increased male fetal deaths," the authors say in the September issue of Fertility & Sterility, a journal of the American Society for Reproductive Medicine. "Male concepti seem to be especially vulnerable to external stress factors, including climate changes."

The sex ratio of newborns is fairly consistent worldwide, with a slightly higher number of males being born compared to females. The scientists, led by Misao Fukuda of M&K Health Institute in Ako, Japan, were curious about how climate change might impact this.

Average temperatures in Japan have increased by more than one degree Celsius (1.8 Fahrenheit) over the last century, a rate that is higher than the global average increase of just under one degree Celsius. Extreme weather fluctuations have also increased in the country.

Fukada and his team examined the birth records provided by Vital Statistics of Japan and temperature data obtained from the Japan Meteorological Agency.

Between 1968 and 2012, Japan recorded 90,000 newborns and about 1,000 fetal deaths each month. The mean temperature in Japan between 1968 and 2012 is 14.1 degrees Celsius.

This data provided the researchers with baselines to compare birth records following the extreme hot and cold periods of 2010 and 2011.

They found a "statistically significant" increase in the fetal death rate in September 2010 — three months after record high temperatures in June — compared to the surrounding years 2007-2011. They also found a "statistically significant" decline in the sex ratio in June 2011, nine months after the summer heat wave, compared to the surrounding years 2008-2012.

In January of 2011, Japan experienced an extremely cold period, with the most extreme snowfall since 1986. The team found a decline in the ratio between male and female births nine months later, in October 2011, compared to surrounding years 2008-2012.

The study is the first to find a correlation between the ratio of male and female births and climate change. Previous studies of temperature and birth ratios in Finland and New Zealand failed to identify a possible link.

"The discrepancy may reflect the fact that Finland and New Zealand are not exposed to the same temperature extremes as Japan, where both summers and winters can be very hot and cold, respectively," the authors say.

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