Most of us now accept that our climate is getting warmer, caused by human activities of one kind or another. What effect has this had on vineyards and wine and what does the future hold?
I have read several articles in the mainstream media that suggest the increase in alcohol levels in wines is down to global warming. Harvest records show that since the 1980s, grapes have been ripening earlier in most wine regions of the world.
However, going further back we see similar cycles in medieval times, before global warming. There are studies published which support and deny the effect of climate change on grape harvest dates.
While there is some connection, it is not as clear-cut as we might imagine.
Alcohol levels are determined primarily by the sugar content of the grapes at harvest. The riper the grapes, the more sugar and therefore the more potential alcohol during the fermentation.
Grape ripening is increased by higher temperatures. The main factor is the total number of degree-days during the growing season. So an increase in average temperatures will give higher sugar levels in the grapes, all things being equal.
However, all things are not equal. In particular, vignerons have the choice of when to pick. So they could pick grapes earlier to avoid higher sugar levels. The yield of the vine, measured in kilograms per vine or hectolitres per hectare, also affects ripening because if the vine has fewer bunches to ripen, it can do so more easily. Growers can prune the vines in winter to produce fewer bunches of grapes, or drop bunches in summer to speed up ripening of the remaining bunches.
Vines require water to photosynthesise and respire. If the vine lacks water, it will slow down its rate of respiration – very dry conditions can actually slow down ripening. So drought and heat can cancel each other out when it comes to ripeness.
Vignerons who irrigate can avoid drought and advance ripening but note that irrigation is prohibited in France for AOP wines.
So there are things that the vigneron can do to increase or decrease ripening, irrespective of climate.
As grapes ripen, not only do they gain sugar, they lose acidity and more importantly, the skins lose their astringent and bitter tannins and replace them with fruitier, more rounded tannins. This is most important in red wines because the skins are included in the ferment, to extract colour and tannin.
Therefore picking early or increasing yields can reduce alcohol levels but it can produce red wines which taste unripe and green, with unpleasantly astringent tannins as well as lacking colour and body. Unripe white wines would have higher acidity and less body too.
In the last 30 years, there has been a big move towards smooth, fruity red wines. In order to achieve smooth tannins, the skins of the grapes need to be very ripe or the wines need to be cellared for many years in bottle. Most consumers don’t want to wait years for their wines to mature. They want them smooth and drinkable while they are still young and fruity.
Prominent wine critics, notably Robert Parker in the USA, have lavished praise on big, smooth, rich red wines. Wines which matched that style were given high scores and could raise their prices enormously and sell more quickly.
Wine competitions will often award the best medals to wines with smooth tannins and big fruit flavours too.
There have been many techniques designed to produce such wines across all price brackets. The first is to leave the grapes hanging in the vineyard longer so the skins become soft and lose any bitterness.
New fungicides have reduced the risk of grapes rotting when left to hang into October and changes in climate which reduce harvest rains also permit this practice.
So while climate change is one factor in increasing ripening, consumer preference, partly driven by influential critics, for big, ripe wines that can be enjoyed without long cellaring, is probably the biggest reason for higher alcohol wines on the market.
Jonathan Hesford has a Postgraduate Diploma in Viticulture and Oenology from Lincoln University, New Zealand and is the owner, vigneron and winemaker of Domaine Treloar in the Roussillon - visit www.domainetreloar.com.