Millions of children risk experiencing stunted growth if heat-trapping emissions continue to bake the planet.
The dangers of extreme heat are already well established, with the World Health Organisation (WHO) warning that exposure to elevated temperatures exacerbates underlying illnesses such as cardiovascular disease, diabetes and asthma.
This summer, around 24,400 people died as a result of heatwaves that scorched vast parts of Europe. Without warming caused by human activities, scientists say 16,500 of these deaths could have been avoided, meaningclimate change was responsible for tripling the death toll.
Now, researchers at UC Santa Barbara have warned that the dangers of extreme weather have long been underestimated, and extend much farther than scientists and doctors realise.
The new report, published in Science Advances, found that humidity worsens the outcome of extreme heat compared to high temperatures alone. This is mainly because humans cool down by sweating, but evaporation slows when the air is humid.
“All that heat builds up in our bodies, causing heat stress,” warns lead author Katie McMahon, explaining that this can still happen when temperatures are low but humidity is high.
Pregnant women are particularly susceptible to heat stress for a number of reasons, including added weight and hormonal changes that make them more vulnerable to overheating.
Heat stress can induce premature labour late in pregnancy, resulting in developmental issues and poor health in children that can be difficult to overcome.
“Extreme weather harms many more people than it kills,” McMahon adds, arguing that focusing solely on mortality leaves out a large part of how these conditions impact our lives.
Researchers analysed the ratio of a child’s height compared to the average for their age, which is commonly used as a chronic health indicator for children under the age of five.
They found that the majority of children who experienced increases in heat and humidity during every trimester before birth would be 13 per cent shorter for their age than expected. In contrast, increases in extreme heat exposure alone translated to a one per cent reduction in height for age.
The study was conducted on pregnant women living in South Asia, which scientists argue could be hit particularly hard by extreme heat in the coming years.
If exposed to conditions expected by 2050 under a high-emissions scenario, around 3.5 million children will have experienced stunting in the study region alone.
“Even if societies succeed in limiting warming to 2°C above preindustrial levels, South Asia is expected to suffer from deadly heat events every year,” the authors write.