The article by Sherwood and Huber discusses the impact of heat stress on human and mammalian adaptation to climate change. They argue that heat stress imposes a robust upper limit to human adaptation, with peak wet-bulb temperature ($T_W$) never exceeding 31 °C in diverse climates today. Any $T_W$ exceeding 35 °C for extended periods would induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible. This threshold would be reached with a global-mean warming of about 7 °C, raising questions about the habitability of some regions. With a warming of 11–12 °C, such regions would encompass the majority of the current human population. The authors suggest that recent estimates of the costs of unmitigated climate change are too low unless the range of possible warming can be narrowed. They also propose that heat stress may help explain trends in the mammalian fossil record. The study uses observational data and climate model simulations to support these conclusions.The article by Sherwood and Huber discusses the impact of heat stress on human and mammalian adaptation to climate change. They argue that heat stress imposes a robust upper limit to human adaptation, with peak wet-bulb temperature ($T_W$) never exceeding 31 °C in diverse climates today. Any $T_W$ exceeding 35 °C for extended periods would induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible. This threshold would be reached with a global-mean warming of about 7 °C, raising questions about the habitability of some regions. With a warming of 11–12 °C, such regions would encompass the majority of the current human population. The authors suggest that recent estimates of the costs of unmitigated climate change are too low unless the range of possible warming can be narrowed. They also propose that heat stress may help explain trends in the mammalian fossil record. The study uses observational data and climate model simulations to support these conclusions.