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Health Effects of CO2 -- Summary
Since atmospheric CO2 enrichment generally impacts plants in a favorable way, it obviously has a health-enhancing effect upon vegetation.  However, there are still two questions that may be asked about plants with respect to their health: (1) what do elevated levels of atmospheric CO2 do to plants that are detrimental to other plants? and (2) what do they do for plants that are suffering from various diseases?

In a study of the first of these questions, Caporn et al. (1999) grew the bracken weed (Pteridium aquilinum), which is a potential threat to human health in the United Kingdom and elsewhere, at 370 and 570 ppm CO2 for 19 months at normal and high levels of soil fertility. In both fertility treatments, elevated CO2 did not increase the biomass of any parts of the plants; but in the normal nutrient regime, it actually reduced the area of the plant fronds, suggesting, if anything, that this noxious weed may exhibit decreased growth as the CO2 content of the air continues to rise.

In a study of the second question, Malmstrom and Field (1997) grew oat plants - one third of which were infected with barley yellow dwarf virus, which plagues more that 150 species of plants, including all major cereal crops - for two months in chambers having atmospheric CO2 concentrations of 350 and 700 ppm, finding that the extra CO2 increased plant biomass by 12% in the healthy plants, but that it increased it three times more - by 36% - in the infected plants.  Hence, elevated CO2 levels clearly have a medicinal type of effect on plants affected with this potent virus.

With respect to animals, the degree of atmospheric CO2 enrichment that may reasonably be expected to occur, even in the very distant future, should not be harmful to them.  In our answer to the question Does High CO2 Harm Humans?, for example, we note that the known ill effects of atmospheric CO2 enrichment do not become evident until the CO2 concentration of the air reaches approximately 15,000 ppm, which is more than forty times greater than its current concentration.

Clearly, the potential health effects of atmospheric CO2 enrichment have not been intensively studied to date, especially in plants; but the few extant findings suggest that the ongoing rise in the air's CO2 content will probably make for a healthier biosphere.

References
Caporn, S.J.M., Brooks, A.L., Press, M.C. and Lee, J.A.  1999.  Effects of long-term exposure to elevated CO2 and increased nutrient supply on bracken (Pteridium aquilinum).  Functional Ecology 13: 107-115.

Malmstrom, C.M. and Field, C.B.  1997.  Virus-induced differences in the response of oat plants to elevated carbon dioxide.  Plant, Cell and Environment 20: 178-188.