<P> A 1993 review of scientific greenhouse studies found that a doubling of CO concentration would stimulate the growth of 156 different plant species by an average of 37% . Response varied significantly by species, with some showing much greater gains and a few showing a loss . For example, a 1979 greenhouse study found that with doubled CO concentration the dry weight of 40 - day - old cotton plants doubled, but the dry weight of 30 - day - old maize plants increased by only 20% . </P> <P> In addition to greenhouse studies, field and satellite measurements attempt to understand the effect of increased CO in more natural environments . In free - air carbon dioxide enrichment (FACE) experiments plants are grown in field plots and the CO concentration of the surrounding air is artificially elevated . These experiments generally use lower CO levels than the greenhouse studies . They show lower gains in growth than greenhouse studies, with the gains depending heavily on the species under study . A 2005 review of 12 experiments at 475 - 600 ppm showed an average gain of 17% in crop yield, with legumes typically showing a greater response than other species and C4 plants generally showing less . The review also stated that the experiments have their own limitations . The studied CO levels were lower, and most of the experiments were carried out in temperate regions . Satellite measurements found increasing leaf area index for 25% to 50% of Earth's vegetated area Earth over the past 35 years, providing evidence for a positive CO fertilization effect . </P> <P> A 2017 article states that increased CO levels have a negative impact on the nutritional quality of various human food crops, by increasing the levels of carbohydrates, such as glucose, while decreasing the levels of important nutrients such as protein, iron, and zinc . Crops experiencing a decrease in protein include rice, wheat, barley and potatoes . </P> <P> The Earth's oceans contain a large amount of CO in the form of bicarbonate and carbonate ions--much more than the amount in the atmosphere . The bicarbonate is produced in reactions between rock, water, and carbon dioxide . One example is the dissolution of calcium carbonate: </P>

What is the normal level of co2 in air