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The Debt We Owe to Atmospheric CO2 Enrichment
Volume 11, Number 22: 28 May 2008

In an intriguing paper recently published in Global Change Biology, Cunniff et al. (2008) note that "early agriculture was characterized by sets of primary domesticates or 'founder crops' that were adopted in several independent centers of origin," all at about the same time; and they say and that "this synchronicity suggests the involvement of a global trigger." Further noting that Sage (1995) saw a causal link between this development and the rise in atmospheric CO2 concentration that followed deglaciation (a jump from about 180 to 270 ppm), they hypothesized that the aerial fertilization effect caused by the rise in CO2 combined with its transpiration-reducing effect led to a large increase in the water use efficiencies of the world's major C4 founder crops, and that this development was the global trigger that launched the agricultural enterprise. Consequently, as a test of this hypothesis, they designed "a controlled environment experiment using five modern day representatives of wild C4 crop progenitors, all 'founder crops' from a variety of independent centers."

The five crops employed in their study were Setaria viridis (L.) P. Beauv, Panicum miliaceum var. ruderale (Kitag.), Pennisetum violaceum (Lam.) Rich., Sorghum arundinaceum (Desv.), and Zea mays subsp. parviglumis H.H. Iltis & Doebley. They were grown individually in 6-cm x 6-cm x 6-cm pots filled with a 1:1 mix of washed sand and vermiculite for 40-50 days in growth chambers maintained at atmospheric CO2 concentrations of 180, 280 and 380 ppm, characteristic of glacial, post-glacial and modern times, respectively. This work revealed that the "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, the researchers had 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 several findings, 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 100 ppm of CO2 the atmosphere has subsequently acquired.

With an eye to the future, we have long contended that the ongoing rise in the air's CO2 content will similarly play a pivotal role in enabling us to grow the food we will need to sustain our still-expanding global population in the year 2050 without usurping all of the planet's remaining freshwater resources and much of its untapped arable land, which latter actions would likely lead to our driving most of what yet remains of "wild nature" to extinction.

Rising CO2 has served both us and the rest of the biosphere well in the past; and it will do the same in the future ... unless we turn and fight against it.

Sherwood, Keith and Craig Idso

References
Cunniff, J., Osborne, C.P., Ripley, B.S., Charles, M. and Jones, G. 2008. Response of wild C4 crop progenitors to subambient CO2 highlights a possible role in the origin of agriculture. Global Change Biology 14: 576-587.

Sage, R.F. 1995. Was low atmospheric CO2 during the Pleistocene a limiting factor for the origin of agriculture? Global Change Biology 1: 93-106.