Projected Exposure of Canadian Children to Rare, High-Severity Heatwaves Under Continued Warming

Extreme heat is intensifying under climate change and poses disproportionate risks to children, yet Canadian children's lifetime exposure to rare, high-severity heatwaves remains poorly quantified at high spatial resolution. We estimate lifetime heatwave exposure across Canada under four local warming categories using bias-corrected, downscaled CMIP6 projections aggregated to 57,932 census areas and linked with age-structured population data. Heatwave severity is measured using the Heat Wave Magnitude Index daily (HWMId), which combines intensity and duration. A rare, high-severity heatwave is defined as a year in which HWMId exceeds the local 98th percentile during 1981-2010-an event historically expected less than once in an average lifetime. Projected exposure increases sharply and nonlinearly with warming. Mean lifetime exposure rises from 0.44 events under 1-3°C of local warming to 4.77 under 3-5°C, 17.67 under 5-7°C, and 39.45 under 7+°C-an 89-fold increase. The proportion of children experiencing at least one event increases from 19.8% to 84.2%, 93.7%, and 97.1% across these categories. Repeated exposure escalates even faster: no children experience five or more events under 1-3°C, compared with 39.4% under 3-5°C and 87.3% under 5-7°C. Exposure emerges earliest in western provinces, while eastern and northern regions reach similarly widespread exposure as warming increases. These findings show that historically exceptional heatwaves are projected to become recurrent over Canadian children's lifetimes and provide spatially explicit evidence for child-centered adaptation, including cooling infrastructure, heat-health early warning systems, and pediatric preparedness.