Exposure to extreme high temperatures is a major constraint on global crop productivity, yet most large-scale assessments rely on fixed temperature thresholds that overlook regional variation in genetics, environment and management. Consequently, the temperature thresholds at which heat exposure begins to cause substantial yield loss and their spatial variability remain unclear. Here we compiled subnational yield census over Northern Hemisphere (20° N–55° N) and analysed the extreme degree days (EDDs) to estimate a data-driven critical threshold (EDD_threshold). Our findings reveal EDD_threshold for maize and soybean are 34.8 ± 4.0 °C and 33.7 ± 3.9 °C, respectively. In contrast, state-of-the-art crop models significantly underestimated EDD_threshold and its spatial variations, leading to overestimated extreme heat exposure, partially explaining their underestimate in yield loss during extreme heat events. We estimate that without adaptations, growing-season extreme heat exposure could increase by 2.4%–16.1% for maize and 4.9%–16.0% for soybean by the end of the century, and sowing-date adjustment alone cannot fully offset the projected increase in extreme heat exposure.

Maize, heat, climate change