An adaptability limit to climate change due to heat stress

TLDR

Humans are often assumed able to adapt to any warming, yet fossil‑fuel scenarios could raise temperatures by up to 12 °C. The study argues that heat stress sets a hard upper bound on human adaptation to warming. Peak wet‑bulb temperatures remain below 31 °C today, yet would exceed 35 °C—causing hyperthermia—once global mean warming reaches ~7 °C, threatening most of the human population by 11–12 °C and suggesting higher climate‑change costs and fossil‑record trends.

Abstract

Despite the uncertainty in future climate-change impacts, it is often assumed that humans would be able to adapt to any possible warming. Here we argue that heat stress imposes a robust upper limit to such adaptation. Peak heat stress, quantified by the wet-bulb temperature T W , is surprisingly similar across diverse climates today. T W never exceeds 31 °C. Any exceedence of 35 °C for extended periods should induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible. While this never happens now, it would begin to occur with global-mean warming of about 7 °C, calling the habitability of some regions into question. With 11–12 °C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12 °C are possible from fossil fuel burning. One implication is that recent estimates of the costs of unmitigated climate change are too low unless the range of possible warming can somehow be narrowed. Heat stress also may help explain trends in the mammalian fossil record.