There is a strong trade-off relationship between the catalytic turnover rate (k_cat) and affinity to CO₂ (K_m(CO₂)) of Rubisco. For example, C4 Rubisco show K_m(CO₂) and k_cat much higher than C3 Rubisco. This can be understood as adaptation to the very high chloroplast CO₂ level. Even among C3 species, we can see a clear trade-off. Plants adapted to cold environments, such as wheat, have high K_m(CO₂)-k_cat Rubisco, while those adapted to warm / hot environments, such as rice, tend to have low K_m(CO₂)-k_cat Rubisco. At low temperatures where the CO₂ concentration in the stroma is high but the maximum rate of enzyme is suppressed by the low temperatures, the increase in k_cat may be essential to maintain the high photosynthetic rate. At high temperatures, the inverse would be the case. Optimal Rubisco kinetic parameters in the high CO₂ world, predicted by a simple photosynthesis model that takes stomatal conductance, mesophyll conductance, Rubisco kinetics and the temperature effects on these parameters into account, will be discussed.