Net photosynthetic CO2assimilation rate (An) decreases at leaf temperatures above a relatively mild optimum (Topt) in most higher plants. This decline is often attributed to reduced CO2conductance, increased CO2loss from photorespiration and respiration, reduced chloroplast electron transport rate (J), or deactivation of Ribulose-1,5-bisphosphate Carboxylase Oxygenase (Rubisco). However, it is unclear which of these factors can best predict species independent declines inAnat high temperature. We show that independent of species, and on a global scale, the observed decline inAnwith rising temperatures can be effectively accounted for by Rubisco deactivation and declines inJ. Our finding thatAndeclines with Rubisco deactivation andJsupports a coordinated down-regulation of Rubisco and chloroplast electron transport rates to heat stress. We provide a model that, in the absence of CO2supply limitations, can predict the response of photosynthesis to short-term increases in leaf temperature.