Young children really struggle in the heat. Their bodies are too small to cool down by sweating, like adults. And they're wholly reliant on grown-ups to find any kind of relief, be it air conditioning or shade or a cool drink. Now, new research points to a potentially lifelong setback for little ones exposed to extreme heat, which is becoming more common due to climate change.
In a study of nearly 20,000 kids, young children who experienced hotter temperatures than usual were less likely to meet basic developmental milestones than those living in nearby, but slightly cooler areas, researchers report in the Journal of Child Psychology and Psychiatry. Children in lower-income households and urban areas were especially vulnerable.
"Early childhood is a sensitive period of development," says Jorge Cuartas, a researcher at New York University who led the study. "If we experience excessive stress without a buffer, that can lead to long-lasting impacts on brain and skill development."
As temperatures rise around the globe, researchers are scrambling to study the impacts on human health. They've linked extreme heat to increased risk of heart attacks, anxiety, depression and premature birth, says Cuartas. "But there was nothing about early childhood development."
In the first few years of life, we develop key physical, cognitive and social skills that can shape the rest of our lives. Meeting these basic developmental milestones at the appropriate time — including being able to grasp objects with two fingers, recognizing letters or words and playing independently — can influence later life outcomes, from mental health to economic success.
Delays in developing these skills set kids down a developmental path that can affect not just individual well-being, but society as a whole, says Cuartas.
To see if extreme heat impacted the development of these skills, Cuartas and his colleagues analyzed data for nearly 20,000 three- and four-year-olds from Gambia, Georgia, Madagascar, Malawi, Palestinian territories and Sierra Leone. These places collect data on early childhood development in similar ways, and have detailed climate records, allowing the researchers to look for links between heat and development.
Links emerged. Children living in areas where the average monthly high topped 86 degrees Fahrenheit at any time during their life were about five to seven percentage points less likely to be developmentally on track that children who lived in the same general area but experienced temps about 9 degrees cooler. Cognitive milestones, especially those related to learning to read and do math, were more likely to be affected than physical or social milestones.
That may sound like a small change, but applied across a whole region, "it's a lot of kids," says Cuartas. In their whole sample of nearly 20,000 kids, approximately 63% of children were developmentally on track. Reducing that by five to seven percentage points in the study sample alone is about 1,000 children. Even a slight decrease "can have significant consequences for our society as a whole."
Marshall Burke, an environmental economist at Stanford University who wasn't involved in the research, agrees. "The study is well done and the findings consistent with a growing body of research that shows that early life exposure to climate extremes can have really lasting impacts on health and livelihoods."
The study cannot say how extreme heat is causing these delays, but there are several theories. Lots of research suggests heat can dull thinking and slow learning. "That's consistent with their findings on literacy and numeracy," says Burke.
Extreme heat can also raise the risk of violence within a family, according to previous research by Cuartas. Extreme heat can make parents more impulsive, irritable and aggressive, he says. When that's directed towards children, it can harm their development.
Heat can also mess with children's sleep, which is essential for growth and development. Or extreme weather could close off opportunities to learn and play, an important part of growing up. "Maybe children are having more difficulties attending early childhood education and care centers, or spending time outdoors because it's too hot," says Cuartas.
Whatever the cause, the harms of heat were not evenly distributed. Children from lower-income families, those who lived in urban areas and those who had a harder time accessing clean water and sanitation were especially vulnerable, the researchers found.
Such children face more barriers to get away from the heat, says Lindsey Burghardt, a pediatrician and chief science officer at the Center on the Developing Child at Harvard University who wasn't involved in the study. Especially in lower-income countries, they may live in a house without air conditioning. Or their neighborhood may have few trees to provide shade. Those barriers add up.
"You can think of it sort of like a seesaw," she says. Extreme heat can weigh hard on one side, but that weight can be offset. The study, she says, "points to where we need to step in to protect children" — investments to ensure access to clean water is key, for example. And while air conditioning may not be realistic in many countries, there are other strategies.
"In terms of buildings and structures, shade is one of the most powerful tools," says Burghardt, so preserving greenspace, planting trees and planning development with heat mitigation in mind could help. Scaled up, such interventions could "tip things toward positive development."
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