(100b) Effects of Elevated CO2 Concentrations on Yields of Agricultural Crops

That elevated concentrations of CO₂ can increase plant growth has been known since at least 1804, when a scientist named De Saussure reported that peas exposed to high CO₂ concentrations grew better than control plants in ambient air (about 300 ppm at that time). Since then, thousands of experiments with elevated concentrations have been conducted, mostly in greenhouses and other types of enclosures, and almost of them similarly showed increases in growth and/or yield. Indeed, it is standard horticultural practice to enrich greenhouses to about 1000 ppm CO₂ when conditions are cool enough that so that the greenhouses do not have to be ventilated for temperature control. Then, when it was realized that the atmospheric CO₂ concentrations were increasing, scientists wondered what this portended for future plant growth and agricultural productivity in open fields. Therefore, about 30 years ago, free-air CO₂ enrichment (FACE) technology was developed that enabled the air above open-field plots to be enriched with CO₂ for entire growing seasons. Since then, FACE experiments have been conducted on cotton, wheat, ryegrass, clover, potato, grape, rice, barley, sugar beet, soybean, cassava, rape, mustard, coffee (C₃ crops), and sorghum and maize (C₄ crops). Elevated CO₂ (550 ppm from an ambient concentration of about 353 ppm in 1990) decreased evapotranspiration about 10% on average and increased canopy temperatures about 0.7°C. Biomass and yield were increased by FACE in all C₃ species, but not in C₄ species except when water was limiting. Yields of C₃ grain crops were increased on average about 19%, while other C₃ crops were increased even more.