The carbon dioxide concentration in the atmosphere is increasing.  In the 50-year period between 1958, when the carbon dioxide concentration in the atmosphere was first monitored, and 2008, it increased from 316 ppm to 385 ppm.  Experiments focusing on the effects of elevated carbon dioxide on plant growth began in earnest about 25 years ago, when the federal government started to support such research.  These studies have shown that elevated carbon dioxide is a benefit for agriculture.  They document that crop yields are increased a third when the carbon dioxide concentration in the atmosphere is doubled.  Under elevated carbon dioxide, evapotranspiration is reduced and water use efficiency is increased.  In the quarter century since we in the Evapotranspiration Laboratory at Kansas State University first started our studies, we can calculate that the water requirement (reciprocal of water use efficiency) of wheat and sorghum has been reduced by 6% and 4%, respectively, due to a 55 ppm increase in carbon dioxide.  Even though our knowledge about the effects of elevated levels of carbon dioxide on crop growth and yield is vast, questions still remain that require answers from experiments.  For example, why do C₄ crops increase in growth under elevated carbon dioxide, when it is predicted that they should not respond?  Evapotranspiration is less under elevated carbon dioxide, but does this reduction in water loss to the atmosphere significantly affect the amount of moisture in the air?  These and other questions will be raised in the presentation.