Rising Temperatures, Spermatogenesis, and Fertility Decline: A 195-Country Analysis of Global and Regional Patterns

Infertility affects about 15% of couples worldwide, with male factors contributing to nearly half of all cases. Growing evidence indicates that global warming, especially the rising frequency and duration of extreme heat, may be an underestimated driver of declining male fertility. Normal sperm production requires scrotal temperatures 2–4 °C below body temperature, around 34–35 °C. When ambient temperatures exceed 33–34 °C, thermoregulation fails, leading to oxidative stress, abnormal sperm morphology, and DNA damage. Since the 1950s, the number of days above these thresholds has increased sharply, with prolonged hot seasons now common in South Asia, the Middle East, equatorial Africa, and South America. Urban heat islands add another 3–6 °C, and global person-days of heat exposure have tripled since the 1980s. An analysis using World Bank fertility data (1960–2023) and NOAA temperature anomalies (1951–1980 baseline) across 195 countries revealed a strong negative correlation between fertility and warming (r = –0.91, p < 0.001). Fertility fell from 4.69 to 2.20 births per woman as global temperatures rose by 1.19 °C, with the steepest associations in Africa (r = –0.89) and Asia (r = –0.83). These effects persisted for multiple years, indicating long-term reproductive impacts. Epidemiological and occupational studies corroborate these findings, linking chronic heat exposure to reduced sperm count, quality, and motility. While infertility is multifactorial, the temperature sensitivity of spermatogenesis suggests that climate warming is a key underlying stressor that amplifies other risks. Thus, climate change represents not only an environmental and public health crisis but also an emerging global reproductive health challenge with significant demographic consequences.