If current greenhouse gas emission trends continue, average global temperatures are projected to rise by 5 to 10 degrees Fahrenheit (3 to 6 degrees Celsius) over the next century. This uncertainty is exacerbated by the unpredictable consequences of temperature changes, such as potential impacts on human fertility. Understanding the implications of climate change on fertility is a significant economic issue. World Bank estimates indicate that in the United States and many European nations, a woman typically has fewer than two children by the end of her reproductive years.
"Below-replacement" birth rates are already placing a strain on programs reliant on the working-age population, like Social Security. A continued decline in birth rates due to climate change could worsen this problem. Global warming may directly affect fertility via two main mechanisms. Firstly, hot weather might impact sexual behavior, as physically demanding activities become more challenging in higher temperatures. Secondly, temperature may adversely affect reproductive health factors, including sperm motility and menstrual cycles.
Our research focuses on the impact of "hot days," which we define as days where the average temperature is above 80°F (27°C). This average accounts for both the day's low and high temperatures, often resulting in daytime highs exceeding 90°F (32°C), typically considered extremely hot. We found that these hot days are associated with a reduction in birth rates eight to ten months later. The most pronounced effect is observed at nine months, with each hot day linked to an average 0.4% drop in birth rates, translating to roughly 1,100 fewer births. Importantly, our data also shows that the use of air conditioning can significantly alleviate the reproductive challenges brought on by hot days.
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