Asymmetry of thermal sensitivity and the thermal risk of climate change

Aim Understanding and predicting the biological consequences of climate change requires considering the thermal sensitivity of organisms relative to environmental temperatures. One common approach involves ‘thermal safety margins’ (TSMs), which are generally estimated as the temperature differential between the highest temperature an organism can tolerate (critical thermal maximum, CT max ) and the mean or maximum environmental temperature it experiences. Yet, organisms face thermal stress and performance loss at body temperatures below their CT max, and the steepness of that loss increases with the asymmetry of the thermal performance curve (TPC). Location Global. Time period 2015–2019. Major taxa studied Ants, fish, insects, lizards and phytoplankton. Methods We examine variability in TPC asymmetry and the implications for thermal stress for 384 populations from 289 species across taxa and for metrics including ant and lizard locomotion, fish growth, and insect and phytoplankton fitness. Results We find that the thermal optimum (T opt , beyond which performance declines) is more labile than CT max , inducing interspecific variation in asymmetry. Importantly, the degree of TPC asymmetry increases with T opt . Thus, even though populations with higher T opt s in a hot environment might experience above‐optimal body temperatures less often than do populations with lower T opt s, they nonetheless experience steeper declines in performance at high body temperatures. Estimates of the annual cumulative decline in performance for temperatures above T opt suggest that TPC asymmetry alters the onset, rate and severity of performance decrement at high body temperatures. Main conclusions Species with the same TSMs can experience different thermal risk due to differences in TPC asymmetry. Metrics that incorporate additional aspects of TPC shape better capture the thermal risk of climate change than do TSMs.