What was done
Working with peer-reviewed publications that report the results of Free-Air CO2-Enrichment (FACE) studies - which they acquired via searches of the ISI Web of Science citation database (Thomson) and the ScienceDirect citation database (Elsevier BV) - the authors conducted a meta-analysis of 529 independent observations of various plant growth responses to elevated CO2 that they obtained from 53 papers that contained relevant data in graphical or numerical format pertaining to the following major crops: wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), rice (Oryza sativa L.), soybean (Glycine max L.), potato (Solanum tuberosum L.), sugar beet (Beta vulgaris L.), cotton (Gossypium hirsutum L.), maize (Zea mays L.) and sorghum (Sorghum bicolor L.), as well as the two major pasture species of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.).

What was learned
Considered en masse, Vanuytrecht et al. determined that for an approximate 200-ppm increase in the air's CO2 concentration (the mean enhancement employed in the studies they analyzed), water productivity was improved by 23% in the case of aboveground biomass production per unit of water lost to evapotranspiration, and by 27% in the case of aboveground yield produced per unit of water lost to evapotranspiration, which two productivity increases would roughly correspond to enhancements of 34% and 40% for a 300-ppm increase in the atmosphere's CO2 concentration.

It is also important to note in this regard that although "the FACE technique avoids the potential limitations of (semi-) closed systems by studying the influence of elevated CO2 on crop growth in the field without chamber enclosure," as the team of Belgian researchers write, other studies have demonstrated a significant problem caused by the rapid (sub-minute) fluctuations of CO2 concentration about a target mean that are common to most FACE experiments, as described by Bunce (2011, 2012), who found most recently that total shoot biomass of vegetative cotton plants in a typical FACE study averaged 30% less than in a constantly-elevated CO2 treatment at 27 days after planting, while wheat grain yields were 12% less in a fluctuating CO2 treatment compared with a constant elevated CO2 concentration treatment.

What it means
Getting higher crop yields per unit of water used in the process of obtaining them will be a key element of mankind's struggle to feed our ever-increasing numbers over the next four decades, when our food needs are expected to double (Parry and Hawkesford, 2010); and with both land and water shortages looming on the horizon, we are going to need all of the help we can possibly get to grow the extra needed food. Fortunately, the results of this meta-analysis coming out of Belgium point to one important avenue by which such very substantial help can come, but it will only come if the air's CO2 content is allowed to rise unimpeded.

References
Bunce, J.A. 2011. Performance characteristics of an area distributed free air carbon dioxide enrichment (FACE) system. Agricultural and Forest Meteorology 151: 1152-1157.

Bunce, J.A. 2012. Responses of cotton and wheat photosynthesis and growth to cyclic variation in carbon dioxide concentration. Photosynthetica 50: 395-400.

Parry, M.A.J. and Hawkesford, M.J. 2010. Food security: increasing yield and improving resource use efficiency. Proceedings of the Nutrition Society 69: 592-600.