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Young children who experience extreme heat tend to know fewer words, letters and numbers, showing that global warming can harm human development from the earliest stages.

An average monthly maximum temperature of 32°C (90°F) or more reduced the likelihood that 3- and 4-year-olds would be developmentally on track by 2.8 to 12.2 per cent, compared with children who only experienced temperatures up to 26°C (79°F).

“This is the first time that is shown in the literature that excessive heat not only affects physical health, but also these developmental skills,” says at New York University.

Cuartas and his colleagues studied data from 19,600 children surveyed by UNICEF in Georgia, the Gambia, Madagascar, Malawi, Sierra Leone and the State of Palestine. Its tested children’s ability to name letters, read simple words and recognise the numbers 1 through 10, among other skills.

The researchers compared this with climate records, controlling for factors like poverty, the mothers’ education and baseline temperatures in the area. Even temperatures of 30°C (86°F) began to harm literacy and numeracy. To a lesser extent, heat also hindered children’s social, emotional and physical development.

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“Even a small impact early in life can expand,” says Cuartas. For example, a child who knows fewer numbers may have difficulty learning arithmetic and fall behind in school.

Heat stress is the leading cause of weather-related deaths, almost half a million people each year. A first-of-a-kind rapid assessment of heat mortality this year estimated that a heatwave in June and July killed 2300 people in 12 European cities, mostly aged 65 or over.

Cuartas and his colleagues found the effect extended to before birth: temperatures of 33°C (91°F) during the first trimester of pregnancy meant the child was 5.6 per cent less likely to be developmentally on track.

Heat impact was greater on children from households that were poorer, more urban and lacked water sources. “Climate change and excessive heat act as threat multipliers [on] children who are already facing disadvantages,” says Cuartas.

But the study may not have fully accounted for factors like violence or political instability that could also hamper childhood development, according to at the London School of Hygiene & Tropical Medicine.

Further research should tease out the ways in which heat is affecting development, she says. Lower-income households may lack air conditioning, or parents may be more stressed when heat hits.

Gaining a better understanding of who is being affected and how will allow us to develop adaptation strategies to help them, says Pescarini.