Local drivers in accelerating North American heat stress.

Climate change increases heat extremes, threatening human health and economies. Using reanalysis data and climate simulations, we show that since the 1940s, population exposure to extreme heat (wet-bulb globe temperature  > 32 °C) has increased by 21% in the U.S. At 2 °C of global warming, exposure increases by 273% because heat-stress frequency increases exponentially with warming. Additionally, 2 °C warming leads to increased nighttime heat stress and decreased work capacity, indicating severe health and economic impacts. Heat stress rises fastest in high-latitude areas, while humid regions experience the greatest exposure increases. In northern regions, heatwave frequency increases with warming, whereas in southern regions, events merge into month-long heatwaves. Increasing temperatures and humidity, along with decreasing wind speed, influence regional heat stress, underscoring the need for tailored adaptation strategies. Overall, heat stress exposure is projected to escalate with additional warming, underscoring the need for mitigation and adaptation to protect vulnerable populations.