What was done
Working near Rapolano, Siena (Italy), the authors conducted a two-year (1996, 1997) FACE study of 20-year-old grapevines (Vitis vinifera L., cv Sangiovese), where they enriched the air about the plants to 550 and 700 ppm (compared to ambient CO2 levels in those two years that averaged 363 ppm, as per Mauna Loa data), measuring numerous plant parameters in the process, including - after the fermentation process was completed - "the principal chemical compounds that determine the basic red wine quality."

What was learned
In the words of the three researchers, "elevated atmospheric CO2 levels had a significant effect on biomass components (total and fruit dry weight) with increases that ranged from 40 to 45% in the 550 ppm treatment and from 45 to 50% in the 700 ppm treatment." In addition, they report that "acid and sugar contents were also stimulated by rising CO2 levels up to a maximum increase in the middle of the ripening season (8-14%)," but they note that as the grapes reached the maturity stage, the CO2 effect on these parameters gradually disappeared. In terms of the primary pigments contained in the wine itself, however, we calculate from the bar graphs of their results that in response to the ~50% increase in atmospheric CO2 concentration experienced in going from ~363 to ~550 ppm CO2, the concentrations of total polyphenols, total flavoniods, total anthocyanins and non-anthocyanin flavoniods in the wine rose by approximately 19%, 33%, 31% and 38%, respectively.

What it means
Speaking of the future, Bindi et al. conclude that "the expected rise in CO2 concentrations may strongly stimulate grapevine production without causing negative repercussions on quality of grapes and wine." In fact, the ongoing rise in the air's CO2 content might even enhance the quality of the wine.