Abstract.The productivity of the prickly‐pear cactusOpuntia ficus‐indica, which is cultivated worldwide for its fruits and stem segments, was predicted based on the responses of its net CO2uptake to soil water status, air temperature and photosynthetic photon flux density (PPFD). Each of these environmental factors was represented by an index with a maximum value of unity when that factor was not limiting net CO2uptake over a 24‐h period. The water index, the temperature index, and the PPFD index were determined for 87 sites in the contiguous United States using data from 189 weather stations and for 148 sites worldwide using data from 1464 weather stations. The product of these three indices, the environmental productivity index (EPI), was used to predict the productivity ofO. ficus‐indicaunder current climatic conditions and under those accompanying a possible increase in the atmospheric CO2level to 650μumol mol−1. Sites with temperatures always above ‐10°C and hence suitable for prickly‐pear cultivation numbered 37 in the United States and 110 worldwide; such sites increased by 43 and 5%, respectively, for the global warming accompanying the elevated CO2. Productivity ofO. ficus‐indicawas at least 15 tonnes dry weight hectare−1year−1, comparable to that of many agronomic crops, for 20 sites with temperatures always above ‐10°C in the contiguous United States and for 12 such sites worldwide under current climatic conditions; such sites increased by 85 and 117%, respectively, under the elevated CO2condition, mainly because of direct effects of the atmospheric CO2level on net CO2uptake. In summary, simulations based on EPI indicate thatO. ficus‐indicamay presently be advantageously cultivated over a substantial fraction of the earth's surface, such regions increasing markedly with a future doublin