What was done
The crop growth and water relations model ecosys was adjusted to represent sorghum (Sorghum bicolor (L.) Moench) and run for a period of two growing seasons (1 May 1998 to 31 Oct 1999) under both wet and dry irrigation schedules at two atmospheric CO2 concentrations (approximately 368 and 561 ppm) using hourly meteorological data measured at a field south of Phoenix, Arizona, USA, after which its simulated energy balances and water relations, verified by measurements of energy flux and water potential, were used to infer the effects of free-air atmospheric CO2 enrichment on various plant parameters and processes.

What was learned
The twelve authors report that "model results, corroborated by field measurements, showed that elevated CO2 raised canopy water potential and lowered latent heat fluxes under high irrigation [both of which responses are beneficial] and delayed water stress under low irrigation [which is also beneficial]," or as they describe it elsewhere, the elevated CO2 "reduced transpiration and hence improved water status of sorghum [and] lowered the vulnerability of sorghum CO2 fixation to soil or atmospheric water deficits, even when irrigation was high."  Also, in applying their reality-tuned model to a scenario where the air's CO2 content is 50% higher and air temperature is 3°C greater, they calculate that sorghum yields would rise by about 13%, and that "current high sorghum yields could be achieved with ~120 mm or ~20% less irrigation water if these rises in temperature and CO2 were to occur."

What it means
Clearly, rising atmospheric CO2 concentrations, even in the face of rising air temperatures, should be good for both sorghum and the people who grow it, in terms of the higher yields that can be produced under these conditions and the smaller amounts of water required to produce them.