Plant responses to the projected future levels of CO₂ were first characterized in short-term experiments lasting days to weeks. However, longer term acclimation responses to elevated CO₂ were subsequently discovered to be very important in determining plant and ecosystem function. Free-Air CO₂ Enrichment (FACE) experiments are the culmination of efforts to assess the impact of elevated CO₂ on plants over multiple seasons and, in the case of crops, over their entire lifetime. FACE has been used to expose vegetation to elevated concentrations of atmospheric CO₂ under completely open-air conditions for nearly two decades. This review describes some of the lessons learned from the long-term investment in these experiments. First, elevated CO₂ stimulates photosynthetic carbon gain and net primary production over the long term despite down-regulation of Rubisco activity. Second, elevated CO₂ improves nitrogen use efficiency and, third, decreases water use at both the leaf and canopy scale. Fourth, elevated CO₂ stimulates dark respiration via a transcriptional reprogramming of metabolism. Fifth, elevated CO₂ does not directly stimulate C₄ photosynthesis, but can indirectly stimulate carbon gain in times and places of drought. Finally, the stimulation of yield by elevated CO₂ in crop species is much smaller than expected. While many of these lessons have been most clearly demonstrated in crop systems, all of the lessons have important implications for natural systems.

Model projections suggest that although increased temperature and decreased soil moisture will act to reduce global crop yields by 2050, the direct fertilization effect of rising carbon dioxide concentration ([CO ₂ ]) will offset these losses. The CO ₂ fertilization factors used in models to project future yields were derived from enclosure studies conducted approximately 20 years ago. Free-air concentration enrichment (FACE) technology has now facilitated large-scale trials of the major grain crops at elevated [CO ₂ ] under fully open-air field conditions. In those trials, elevated [CO ₂ ] enhanced yield by ∼50% less than in enclosure studies. This casts serious doubt on projections that rising [CO ₂ ] will fully offset losses due to climate change.

While increasing temperatures and altered soil moisture arising from climate change in the next 50 years are projected to decrease yield of food crops, elevated CO₂ concentration ([CO₂]) is predicted to enhance yield and offset these detrimental factors. However, C₄ photosynthesis is usually saturated at current [CO₂] and theoretically should not be stimulated under elevated [CO₂]. Nevertheless, some controlled environment studies have reported direct stimulation of C₄ photosynthesis and productivity, as well as physiological acclimation, under elevated [CO₂]. To test if these effects occur in the open air and within the Corn Belt, maize (Zea mays) was grown in ambient [CO₂] (376 μmol mol⁻¹) and elevated [CO₂] (550 μmol mol⁻¹) using Free-Air Concentration Enrichment technology. The 2004 season had ideal growing conditions in which the crop did not experience water stress. In the absence of water stress, growth at elevated [CO₂] did not stimulate photosynthesis, biomass, or yield. Nor was there any CO₂ effect on the activity of key photosynthetic enzymes, or metabolic markers of carbon and nitrogen status. Stomatal conductance was lower (−34%) and soil moisture was higher (up to 31%), consistent with reduced crop water use. The results provide unique field evidence that photosynthesis and production of maize may be unaffected by rising [CO₂] in the absence of drought. This suggests that rising [CO₂] may not provide the full dividend to North American maize production anticipated in projections of future global food supply.

The ratio of plant carbon gain to water use, known as water use efficiency (WUE), has long been recognized as a key constraint on crop production and an important target for crop improvement. WUE is a physiologically and genetically complex trait that can be defined at a range of scales. Many component traits directly influence WUE, including photosynthesis, stomatal and mesophyll conductances, and canopy structure. Interactions of carbon and water relations with diverse aspects of the environment and crop development also modulate WUE. As a consequence, enhancing WUE by breeding or biotechnology has proven challenging but not impossible. This review aims to synthesize new knowledge of WUE arising from advances in phenotyping, modeling, physiology, genetics, and molecular biology in the context of classical theoretical principles. In addition, we discuss how rising atmospheric CO₂ concentration has created and will continue to create opportunities for enhancing WUE by modifying the trade-off between photosynthesis and transpiration.