Plants use CO2, H2O and light to synthesize organic compounds. It then converts these compounds into elements it can use. This process is called photosynthesis. The ambient level of CO2 in the earth's atmosphere is between 300 and 600 ppm. Plants receive far more water and sunshine than they can use. Increasing CO2 lets the plant use the excess water and energy that is stored in the leaves. The result is a large increase in the growth rate. The CO2 level that is recommended is 1000 to 2000 PPM.