Recent meta-analyses and compilations of past experiments show a wide range of crop productivity responses to elevated atmospheric [CO₂]. Variances, even within the same protocols, can be quite large ranging from less than 1 to greater than 30 times for [CO₂] about double the current atmospheric mean. Among the variables known to affect the CO₂ enhancement ratio (CER) are air pollutants, drought, plant competition, temperatures and soil nutrition. It is the purpose of this presentation to explore the effects and interactions of some of these factors to determine if there are practical methods of stabilizing the variability of response and /or maximizing the enhancement potential of elevated [CO₂].