In the journal Fertility and Sterility, Japanese researchers found that in the hottest recorded summer, 2010, there was a dramatic increase in female births, whereas the coldest winter, 2011, produced more baby boys. This correlation in climate and gender fluctuations is still far too preliminary to construe as fact, but further studies have also shown that extreme outside stresses tend to favor girls: male fetuses are especially vulnerable when the mother experiences air pollution, chemical exposure, and extraordinary stresses, such as war or economic hardships.
Denmark also experienced a decline in male population from 1951 to 1995, presumably due to environmental hazards. Another study looked at the famine in China during the Great Leap Forward (1959-1961) and found that women were more likely to give birth to girls in those turbulent years. It’s worth bearing in mind, however, that these connections could be arbitrary, with scientists potentially examining narrow windows in history that conveniently encourage this evidence. Still, a Swedish study provides more support that women thrive in natural selection, saying that mothers’ bodies are evolutionarily programmed to give birth to girls, in addition to affirming the link between girls and rising temperatures.
Data from Finland and New Zealand did not show consistent connections between climate change and the death of male fetuses, although neither country experiences extreme temperature shifts like Japan. Japan has also warmed at a greater rate than the global average throughout the last century. Still, anthropologists would like to see how this data would fare in developing countries, where amenities that affect temperature changes—shelter, air conditioning, central heating—aren’t as prevalent.
National Geographic explains the basic principles of global warming in this video:
Image by Anthony J, licensed under Creative Commons