The idea that an increase in the air's CO2 content may be of benefit to the biosphere can be traced back in time over 200 years. As early as 1804, for example, de Saussure showed that peas exposed to high CO2 concentrations grew better than control plants in ambient air; and work conducted in the early 1900s significantly increased the number of species in which this growth-enhancing effect of atmospheric CO2 enrichment was observed to occur (Demoussy, 1902-1904; Cummings and Jones, 1918). In fact, by the time a group of scientists convened at Duke University in 1977 for a workshop on Anticipated Plant Responses to Global Carbon Dioxide Enrichment, an annotated bibliography of 590 scientific studies dealing with CO2 effects on vegetation had been prepared (Strain, 1978). This body of research demonstrated that increased levels of atmospheric CO2 generally produce increases in plant photosynthesis, decreases in plant water loss by transpiration, increases in leaf area, and increases in plant branch and fruit numbers, to name but a few of the most commonly reported benefits. And five years later, at the International Conference on Rising Atmospheric Carbon Dioxide and Plant Productivity, it was concluded that a doubling of the air's CO2 concentration would likely lead to a 50% increase in photosynthesis in C3 plants, a doubling of water use efficiency in both C3 and C4 plants, significant increases in biological nitrogen fixation in almost all biological systems, and an increase in the ability of plants to adapt to a variety of environmental stresses (Lemon, 1983).

Numerous studies conducted on hundreds of different plant species testify to the very real and measurable growth-enhancing, water-saving, and stress-alleviating advantages that elevated atmospheric CO2 concentrations bestow upon Earth's plants (Idso and Singer, 2009; Idso and Idso, 2011). In commenting on these and many other CO2-related benefits, Wittwer (1982) wrote that "the 'green revolution' has coincided with the period of recorded rapid increase in concentration of atmospheric carbon dioxide, and it seems likely that some credit for the improved [crop] yields should be laid at the door of the CO2 buildup." Similarly, Allen et al. (1987) concluded that yields of soybeans may have been rising since at least 1800 "due to global carbon dioxide increases," while more recently, Cunniff et al. (2008) hypothesized that the rise in atmospheric CO2 following deglaciation of the most recent planetary ice age, was the trigger that launched the global agricultural enterprise.

In a test of this hypothesis, Cunniff et al. designed "a controlled environment experiment using five modern-day representatives of wild C4 crop progenitors, all 'founder crops' from a variety of independent centers," which were grown individually in growth chambers maintained at atmospheric CO2 concentrations of 180, 280 and 380 ppm, characteristic of glacial, post-glacial and modern times, respectively. The results revealed that the 100-ppm increase in CO2 from glacial to postglacial levels (180 to 280 ppm) "caused a significant gain in vegetative biomass of up to 40%," together with "a reduction in the transpiration rate via decreases in stomatal conductance of ~35%," which led to "a 70% increase in water use efficiency, and a much greater productivity potential in water-limited conditions."

In discussing their results, the five researchers concluded that "these key physiological changes could have greatly enhanced the productivity of wild crop progenitors after deglaciation ... improving the productivity and survival of these wild C4 crop progenitors in early agricultural systems." And in this regard, they note that "the lowered water requirements of C4 crop progenitors under increased CO2 would have been particularly beneficial in the arid climatic regions where these plants were domesticated." For comparative purposes, they also included one C3 species in their study - Hordeum spontaneum K. Koch - and they report that it "showed a near-doubling in biomass compared with [the] 40% increase in the C4 species under growth treatments equivalent to the postglacial CO2 rise." In light of these and other similar findings (Mayeux et al., 1997), it can be appreciated that the civilizations of the past, which could not have existed without agriculture, were largely made possible by the increase in the air's CO2 content that accompanied deglaciation, and that the peoples of the Earth today are likewise indebted to this phenomenon, as well as the additional 110 ppm of CO2 the atmosphere has subsequently acquired. And as the CO2 concentration of the air continues to rise in the future, this positive externality of enhanced crop production will benefit society in the years, decades, and even centuries to come.