Future humidex extremes over North America: projections and uncertainties from the CanESM5 large ensemble

Abstract Extreme heat poses growing risks to human health, ecosystems, agriculture, and infrastructure under global warming. While most studies emphasize air temperature, the humidex, which combines temperature and humidity, better captures human-perceived heat stress. This study assesses historical and projected changes in the 95th percentile of daily humidex (HX95) across North America using ERA5 reanalysis and the Canadian Earth System Model version 5 (CanESM5) single-model initial-condition large ensemble. By integrating observations with climate simulations under multiple emission scenarios, we quantify externally forced changes and uncertainties from internal variability. Results reveal a pronounced latitudinal gradient, with HX95 highest in the southern U.S. and Mexico and lowest in northern and mountainous regions. At 2 °C and 3.5 °C of global warming above the pre-industrial level, North American mean HX95 rises by about 2.6 and 5.8 units, respectively, relative to 1995–2014, with widespread expansion of areas exceeding hazardous thresholds. Responses are robust and only weakly dependent on emission scenario, as inter-scenario differences remain small at fixed warming levels. Increases stem from higher temperatures, enhanced moisture, and poleward circulation shifts. Internal variability contributes substantially to regional uncertainty, particularly in northern North America, but externally forced changes dominate, with signal-to-noise ratios above 3 at 2 °C and 6 at 3.5 °C. Overall, humidex extremes are projected to intensify and migrate poleward with continued global warming. Limiting warming to 2 °C rather than 3.5 °C would significantly reduce exposure to dangerous heat, underscoring the urgency of mitigation and adaptation.