A major new analysis of around 720,000 hospital presentations of people aged 0–24 in NSW has found that hotter than normal winter days are linked to a significant rise in acute mental health disorders, with further scenario modelling suggesting a significant rise in heat-attributable admissions by the end of this century if greenhouse gas emissions are not reduced.

Published in the Journal of the American Academy of Child & Adolescent Psychiatry, the study showed that increased risk was observed in both cold and warm seasons, but high temperatures in winter were associated with increased hospital admissions for mental health disorders among children and young adults, with rises in presentations for schizophrenia, eating disorders, and deliberate self-harm only observed in cold seasons.  

Overall, heat accounted for over eight percent of admissions in cold seasons and almost three percent in warmer seasons. The risks during cold seasons were more pronounced for female patients and young adults. 

“This is a somewhat unexpected finding. Conventional wisdom would suggest the greatest impacts would be in summer, when it’s the hottest. Instead, it was hot winter weather that was associated with the highest risks of presentations,” said lead author Dr Wen-Qiang He, an epidemiologist from the University of Sydney’s Faculty of Medicine and Health and Charles Perkins Centre.

Using hospital presentation data, the study also conducted scenario modelling, predicting that by the last decade of this century, heat-attributable admissions could rise by six percent under low emission scenarios, almost eight percent under medium emission scenarios, and almost 21 percent under high emissions.

The findings come just as Australia experiences a hotter-than-normal lead up to winter, with many parts of Australia recording early winter maximums and Sydney recording the hottest June on record since 1859. 

"Hotter-than-normal weather, which is becoming more frequent because of climate change, is already affecting the mental health of some of our most vulnerable young people," said Dr He. 

"Our findings suggest these impacts are likely to grow as the climate continues to warm, making mental health an increasingly important consideration in climate adaptation and public health planning."

While the researchers didn't examine the biological mechanisms directly, they said there could be several plausible explanations.

"Emerging research suggests heat can affect brain function and neural connectivity, with children and young people potentially more vulnerable because their brains are still developing." 

"Another possibility is that our bodies are adapted to expect cold conditions in winter. When temperatures suddenly spike, people may overheat because they're using heavier bedding and clothing, and that can disrupt sleep and place additional stress on the body's ability to regulate temperature."

The researchers’ climate modelling is based on the Shared Socioeconomic Pathways (SSPs), which map climate change scenarios by greenhouse gas emissions.  

Declaration: 

The authors declare no competing interests. The research was funded by Australian National Health and Medical Research Council (NHMRC) Investigator grant (NN-APP1197940), Financial Markets Foundation for Children (NN) and New South Wales Ministry of Health-funded Luminesce Alliance (W-QH).