Food for Thought: Lower-Than-Expected Crop Yield Stimulation with Rising CO ₂ Concentrations

Abstract

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 È50% less than in enclosure studies. This casts serious doubt on projections that rising [CO 2 ] 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). Projections reviewed by the Intergovernmental Panel on Climate Change (IPCC) suggest that increased temperature and decreased soil moisture, 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 projections and thus future food security depend critically on the magnitude of the CO 2 fertilization effect under actual growing conditions. Atmospheric ECO 2^h as risen from È260 parts per million (ppm) approximately 150 years ago to 380 ppm today (8). Yet ECO 2^i s markedly uniform across the globe; so, in contrast to temperature and soil moisture, there is no consistent spatial variation on which to estimate yield responses to increasing ECO 2^. Similarly, it is not easy to alter ECO 2^e xperimentally around a crop in the field. As a result, most information about crop responses to elevated ECO 2^i s obtained from studies in greenhouses, laboratory controlled-environment chambers, and transparent field chambers, where released CO 2 may be retained and easily controlled. These settings have provided the basis for projecting CO 2 fertilization effects on the major food crops: maize, rice, sorghum, soybeans, and wheat.Crops sense and respond directly to rising ECO 2^t hrough photosynthesis and stomatal conductance, and this is the basis for the fertilization effect on yield (9). In C 3 plants, mesophyll cells containing ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO) are in direct contact with the intercellular air space that is connected to the atmosphere via stomatal pores in the epidermis. Hence, in C 3 crops, rising CO 2 increases net photosynthetic CO 2 uptake because RuBisCO is not CO 2 -saturated in today_s atmosphere and because CO 2 inhibits the competing oxygenation reaction leading to photorespiration. RuBisCO is highly conserved across terrestrial plants, so instantaneous responses to...

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