Increasing levels of atmospheric carbon dioxide may not be as helpful to crops as previously thought, according to a new study. As global temperatures rise, soil moisture levels fall. Laboratory experiments suggested that the crop losses resulting from this loss of moisture would be offset by a boost in fertilization from rising levels of carbon dioxide. Now field studies show that carbon dioxide's beneficial effects are not nearly as strong in real crop-growing environments.

During photosynthesis, plants convert carbon dioxide into carbohydrates, fuel that allows plants to build tissue and grow. Based on the chemical conversions that take place during photosynthesis, scientists can predict how much bigger plants will become in an environment with increased carbon dioxide levels. In studies in greenhouses and enclosed chambers, this prediction is accurate. Those studies led the Intergovernmental Panel on Climate Change (IPCC) to conclude in 2001 that crop yields will increase with global warming, and that the world's food supply is not in immediate danger.

But greenhouse and chamber environments lead to higher temperatures, poor airflow and trapped humidity, and do not imitate real crop-growing conditions, according to the paper published in the current issue of Science. "A plant breeding company certainly wouldn't select varieties based on chamber studies, so we don't think our future predictions should be based on those. We think they should be based on rigorous field studies, and that's what FACE allows us to do," says co-author Elizabeth Ainsworth of the University of Illinois at Urbana Champaign.

FACE, which stands for free-air concentration enrichment, is an apparatus that enabled the researchers to control the concentration of gases in the air without affecting any other variables such as temperature or humidity. In FACE studies, carbon dioxide is released from different points surrounding a section of a crop field. A computer program constantly monitors wind direction, wind velocity and carbon dioxide concentration in the field, adjusting the positions and amount of gas released so that the carbon dioxide concentration remains constant.

The researchers studied soybeans, wheat, rice, maize and sorghum at the atmospheric carbon dioxide levels predicted to exist 50 to100 years from now. They found that in the field, only about half the expected increase in crop yield actually occurred. Although these results cast doubt on the IPCC's conclusion about the safety of the world's food supply, the researchers believe that a solution exists. According to Ainsworth, the low crop yield means there must be something limiting the plants from taking full advantage of the extra carbon dioxide. "If we can breed for plants that don't have those limitations, they can respond better to the carbon dioxide increases in the future," Ainsworth remarks. "It's a call for more research in this field."