Tropical forests are facing increasing risks of exposure to critical temperature thresholds

Understanding how close tropical tree species are to critical temperature thresholds that might impede photosynthetic activity is vital in a world where heat waves have become more severe and frequent. Using remotely sensed surface temperature and species distribution maps, we studied the spatiotemporal variation in the thermal safety margins (TSM, i.e., the difference between [Formula: see text], the critical photosynthetic temperature, and the maximum canopy temperature) of [Formula: see text] tropical tree species in South America, Southeast Asia, and Central Africa during the period 2001-2020. Despite overall high-temperature tolerance with an average [Formula: see text] of [Formula: see text]C, we observed a consistent decline in the TSM of tropical forests across the globe. The average pantropical TSM decline was [Formula: see text]C per decade, with the strongest decline in South America ([Formula: see text]C per decade). Over the [Formula: see text]-y period, areas that experienced canopy temperatures surpassing the average [Formula: see text] across reported species increased from [Formula: see text] Mha to [Formula: see text] Mha in the tropics, representing [Formula: see text] of the studied area. This number increases to [Formula: see text] when computing areas where temperatures have surpassed the [Formula: see text] of the most vulnerable reported species. When considering future trends, as predicted by Earth System Models under medium-to-high emission scenarios, average [Formula: see text] may be exceeded in an area of [Formula: see text] Mha by [Formula: see text] and [Formula: see text] Mha by [Formula: see text] (over [Formula: see text] of the studied area), suggesting major feedback to the global carbon cycle and the world's biodiversity.