7. Direct Effects of Rising Atmospheric Carbon Dioxide and Ozone on Crop Yields

Rising atmospheric carbon dioxide concentration ([CO₂]) in this century will alter crop yield quantity and quality. It is important to understand the magnitude of the expected changes and the mechanisms involved in crop responses to elevated [CO₂] in order to adapt our food systems to the committed change in atmospheric [CO₂] and to accurately model future food supply. Free-Air CO₂ Enrichment (FACE) allows for crops to be grown in their production environment, under fully open air conditions, at elevated [CO₂]. Current best estimates for the response of the staple crops wheat, soybean and rice from FACE experiments are that grain yield will increase by 13% at 550 ppm CO₂. For the C₄ species, sorghum and maize, grain yield is not expected to increase at elevated [CO₂] if water supply is adequate. Grain quality is adversely affected by elevated [CO₂]. On average, protein content decreases by 10–14% in non-leguminous grain crops and concentrations of minerals, such as iron and zinc decrease by 15–30%. While these represent our best estimate of changes in crop yield quantity and quality, most studies have been done in temperate regions, and do not account for possible interactions of rising [CO₂] with other aspects of climate change, including increased temperature, drought stress and tropospheric ozone concentration.