Remember, we are not talking about a reduction of greenhouse CO2 to the level of absolute zero. A reduction to any level lower than the ambient level of CO2 causes a responding reduction in plant growth. Typically, in a closed winter greenhouse the CO2 levels easily fall to 100 PPM or lower. In my greenhouse with 131 citrus trees plus a large number of other plant material, the CO2 level would quickly drop, if the structure was not open vented to the outside during the winter. Skeet, thank you very much for the math on CO2 conversion to grams of plant matter. I have no idea how many grams of plant matter growth per day occur in a greenhouse on a typical February day, but it would be considerable, as all of my trees are presently blooming, therefore the liters of air required would be a VERY large number. - Millet