Food for thought: Lower-than-expected crop yield stimulation with rising CO2 concentrations

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 ([CO2]) will offset these losses. The CO2 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 [CO2] under fully open-air field conditions. In those trials, elevated [CO2] enhanced yield by È50 % less than in enclosure studies. This casts serious doubt on projections that rising [CO2] will fully offset losses due to climate change. M uch effort has been put into linking models of climate and crop growth to project future changes in crop yields and food supply across the globe (1–4). Pro-jections reviewed by the Intergovernmental Panel on Climate Change (IPCC) suggest that increased temperature and decreased soil mois-ture, which would otherwise reduce crop yields, will be offset by the direct fertilization effect of rising carbon dioxide concentration (ECO 2 (5–7). The IPCC projections suggest that total crop yield may rise when averaged across the globe, but this net gain will result from generally lower yields in the tropics and increased yields in temperate zones. The accuracy of these projec-tions and thus future food security depend crit-ically on the magnitude of the CO 2 fertilization effect under actual growing conditions. Atmospheric EC