The direct increase in C3 photosynthesis due to elevation of [CO2] results from two properties of Rubisco. (i) The enzyme is not saturated by present atmospheric [CO2], and so elevated [CO2] will increase the velocity of carboxylation and net photosynthesis. (ii) CO2 is a competitive inhibitor of the oxygenation reaction which leads to photorespiration. Photorespiration typically releases 20–40% of recent photosynthate as CO2. This significantly reduces net photosynthesis of C3 crops, and will be suppressed in favour of greater carbon gain by rising [CO2]. Because the kinetic properties of Rubisco are highly conserved across C3 crops, the improvement in photosynthetic gain with rising [CO2] can be calculated for all C3 crops with some confidence.