A new study from researchers at the University of California, Santa Barbara, projects that climate change could cause over three million additional cases of child stunting in South Asia by 2050, primarily due to pregnant women's exposure to extreme heat combined with high humidity. The analysis focused on height-for-age ratios in children under five—a standard measure of chronic malnutrition—and found that humid heat stresses exacerbate developmental impacts far beyond dry heat alone. Pregnant individuals are particularly vulnerable because added weight, hormonal shifts, and reduced cooling efficiency make overheating more likely. Humidity impairs sweat evaporation, intensifying physiological strain during critical foetal growth periods.

The research, published in the journal Science, identified early and late pregnancy as the most sensitive windows. Lead author Katie McMahon explained that the foetus is highly susceptible at the start, while maternal health faces greater risks toward the end. Co-author Kathy Baylis noted that third-trimester exposure to combined heat and humidity produced outcomes roughly four times worse than heat alone. The team used wet-bulb globe temperature (WBGT)—which factors in humidity, radiant heat, and airflow—as a more accurate stress indicator than air temperature, with thresholds of 35°C for heat and 29°C for WBGT.

Each additional day exceeding 29°C WBGT was linked to fewer live births 6-12 months later, signalling potential broader effects on fertility and pregnancy outcomes. South Asia's large population and frequent extreme weather make it especially at risk, with projections based on high-emissions scenarios highlighting millions of preventable stunting cases. Stunting impairs cognitive and physical development, perpetuating cycles of poverty and health challenges.

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The findings emphasise that traditional heat-focused models underestimate risks by ignoring humidity, a critical oversight as climate change increases humid heat events. Researchers advocate for incorporating WBGT into public health planning, including targeted maternal protections like cooling access and nutritional support in vulnerable regions.

As global temperatures rise, addressing these intersecting climate and health threats is urgent for equitable child development. The study calls for adaptive strategies to safeguard pregnant women and mitigate long-term societal impacts in one of the world's most climate-vulnerable areas.

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