CO2 in light fruity red wines with low levels of tannin designed to be drunk in their youth like say some Beaujolais and Beaujolais villages, even some lighter crus Beaujolais, red Vinho Verde, some light Rhône wines, perhaps made from cinsault, some entry level and even some top end pinot noirs where it contributes to the structure from the acidity, as there may be little tannin some pinots, is often beneficial.
CO2 is acidic so it increases acidity, confers a bit of zip and freshness, it protects from oxidation so preserves fruit aromas and flavours, and allows the winemaker to use less SO2. High levels of SO2 can temporarirly or permanently depress the fruit. So using more CO2 and less SO2 is good for wines which are best enjoyed early as the fruit is more apparent. It may beneficially reinforce the structure of low tannin, light red wines by contributing acidity. One would rarely want it in firm, tannic wines as it reinforces the astringency.
Its a by product of the fermentation and if the wine is lodged in closed stainlees steel tanks, glass and some ceramic vessels the CO2 produced during fermentation is retained. Winemakers can control how much is allowed into the bottle by racking before bottling. If you do not like the CO2 then decanting a wine will remove most of it.
After fermentation wines may have over 2000mg/l of CO2. White vinho verdes are often bottled about 700-800 mg/l which gives a slight prickle on the tongue. Something like 500mg/l would give a low tannin, light red wine a pleasant freshness. Less than about 300-400mg/l is below most people’s threshold of perception, but this varies between people.
Kept a long time in a porous container ie a wooden barrel, most of the CO2 escapes, especially if the wine is racked. One would not expect CO2 in an red cru Bourgeois Bordeaux and above where wines are usually racked and kept 12-24 months in barrel.
However some Burgundian producers, such as Fourrier, deliberately try and keep some CO2 in the barrel and never rack. Fourrier often advises decanting his wines to remove the CO2 before drinking. Some are experimenting with large wooden barrels and vats to retain more CO2. Others are using glass and certain ceramic containers which are inert and so the CO2 is retained. The retained CO2 preseves delicate pinot noir aromas and increases acidity which may be beneficial post global warming as the climate gets hotter.
There can be an accidental cause which results from a wine bottled with yeast, residual sugar and low SO2, perhaps because of a filtration failure, or no filtration. Closed with a cork the CO2 cannot escape. Some sediment may be produced from the yeast cells and this may lead to off flavours. If a wine has CO2 and is cloudy it is often an unwanted secondary fermentation in bottle. An excessive combination of SO2 and CO2 can lead to reduction which can produce cabbagey aromas or, in the worst cases a smell of rotten eggs, hydrogen sulphide. Light reduction is often cured in a bottled wine by allowing more oxygen into the wine by decanting.