Results/Conclusions At CO2 concentration above 400 ppm, water limitation was the primary selective factor for C4 evolution, and even at 600 ppm there is room for C4 evolution under the driest conditions. Furthermore, we find that the carbon concentration mechanism in C4 (CCM) alone leads to enough of a reduction in water use that there would have been little selection for increased hydraulic conductance within C4 grasses. When CO2 decreased below 300 ppm, high light intensity provided an enhanced advantage for C4. Low CO2, together with light intensity, were the primary drivers during the global radiation of C4 5-10 MYA, occurring hand-in-hand with grassland expansion. Subsequent to the evolution of CCM, selection could favor the reallocation of nitrogen from dark to light reactions. the selection pressure to reallocate would have been strongest when CO2 was high, i.e., during the initial evolutionary events, when the CCM alone does not give C4 a large advantage. As CO2 decreased during the C4 radiation 5-10 MYA to Pleistocene glacial periods, the CCM alone would give C4 an advantage and selection of reallocation would lessen. However, as CO2 increased again from Pleistocene, our modeling results show the N reallocation may play a major role in maintaining the high productivity of C4 species in current CO2 concentration.