The effects of increasing concentrations of atmospheric carbon dioxide (C02) on agricultural yields are analyzed. It is argued that any positive effects on yields from C02 fertilization, demonstrated in controlled experiments, would be weak in farm conditions given water and nutrient limits. Furthermore, possible benefits would be more than offset by predicted consequences of climate change, ozone depletion, and additional gases created from fossil fuel combustion. The impact of including C02 fertilization on crop yield and economic welfare predictions is evaluated. The policy distorting potential of fragile claims is stressed