Rising atmospheric CO2 concentration (Ca) may alter two components (sensitivity and acclimation) of global photosynthetic carbon influx into terrestrial ecosystems (PG). Most existing global models focus on long-term acclimation. We have developed a leaf-level function (L) to quantify short-term increment of PG associated with sensitivity. The L function is the normalized response of leaf photosynthesis to a small change in Ca and has been suggested to be an invariant function for C3 plants grown in diverse environments. This paper tests the hypothesis that L is an invariant function. We calculated values of L from 9 sets of experimental data which incorporated photosynthetic responses of 12 plant species to measurement conditions of light and temperature and to growth in different light, temperature, nitrogen, phosphorus, water stress, and CO2 concentration. Absolute rates of leaf photosynthesis differed by more than tenfold due to species differences and environmental variation. However, L values derived from these data sets converged into a narrow range defined by two equations of the L function, confirming that L was insensitive to differences in photosynthetic capacity among species and between plants acclimated to different growth environments. Using the L function, we predict that a yearly increase of 1.5 parts per million (ppm) in Ca will induce an increase in PG by 0.18 to 0.34 Gt (1 Gt=1015 g) C yr-1 in 1993, provided that (1) PG=120 Gt C yr-1, (2) 85% of PG is generated by C3 plant assimilation, and (3) the 1.5-ppm increase in Ca will not induce significant photosynthetic acclimation. ¿ American Geophysical Union 1996