Let me preface this piece by saying that I'm a fan of lower alcohol wines. I was first introduced to wine in 1980s Upstate New York, drinking 11.5% Rieslings and, yes, Cayuga Whites. Red wines from the area, like Marechal Foch and De Chaunac (for an overview of hybrid varieties like Cayuga White, Marechal Foch and De Chaunac, see here) were in the range of 12-12.5%. At the time, this wasn't considered low alcohol, it was considered the norm.
Perhaps because of this early exposure to lower alcohol wines, I've always preferred them, and I actively look for them when considering a purchase. New Zealand, despite being a cool climate winegrowing region, now regularly produces 13-13.5% whites and Pinot noirs are often seen with 14.5%. This goes against my, personal, ideas of what these wines should be.
I look forward to wines from the really challenging vintages where grapes struggle to get as ripe as the winemakers want them. The 2012 vintage in Hawkes Bay had a really cool and extended ripening period and grapes were brought in at much lower sugar levels (measured in degrees Brix in New Zealand and other places) than usual, but it resulted in some very good, and more elegant, in my opinion, wines as a result, with the alcohol being in balance with the other aspects of the palate. New Zealand's most recent vintage was also a challenge due to rainfall in the ripening period, resulting in grapes being brought in before target Brix were hit. I tasted my first of this vintage's wine in August, a Marlborough Sauvignon blanc, that had all the hallmarks of the region, but with only 12% alcohol: it was a much more harmonious assemblage than with the usual higher alcohol wines of the region.
Having got that out of the way, what about climbing alcohol percentages in wine?
Rising average alcohol levels in wines has been a topic of discussion for some time, and various reasons for this have been put forth over the years. A useful study to look at was published in 2011 by Alston et al. where data from California was examined to show that average harvest Brix levels between 1980 and 2010 increased. Sugars in red wine varieties increased by an average of 0.23% per year over that period, and notably, in their Figure 1, Brix for red varieties was pretty much flat from 1980 to the mid 1990s, rising from there to the 2010 average of around 23.8°. This was particularly noticeable for the North /Central Coast and Delta regions, where the rate of increase was 0.72, 0.75, and 0.96%, respectively, between 1990 and 2008, compared to 0.53% for California as a whole (Alston et al. 2011 Table 1).
So fruit sugars are going up (at least in California) and the wines made therefore have higher alcohol. But why are the sugars going up?
A different take on this has come about recently, where people are starting to look at rising carbon dioxide (CO2) concentrations in the atmosphere and linking this to increased plant productivity (e.g. here and here). This isn't a difficult link to make, as the process of photosynthesis takes CO2 and water and with the help of the enzyme Rubisco, releases oxygen and sugar (in the form of glucose). It stands to reason that if you increase the availability of a starting material, you can end up with more product. This is assuming that other starting materials (Rubisco, water, sufficiently warm temperatures and light energy in this case) aren't limiting, and that the products don't start piling up in the area where they're being produced - if glucose and oxygen keep building up in the cell, the rate of photosynthesis will slow through a process called feedback inhibition.
So increasing CO2 should mean more efficient photosynthesis and more sugars to go around? As we usually find, things aren't that simple.
There has been plenty of research into the effects of raising CO2 concentration and its positive effects on plant growth and productivity, however, much of this has been with relatively short duration experiments. When plants have a longer time and a chance to adapt to the changed conditions there is more talk of photosynthetic down-regulation, or acclimation, resulting in relatively little change.
A review by Makino and Mae notes that longer term plant adjustment is a complicated system. For example, if sugar is being produced more quickly, but the plant does not have the capability of moving the sugars out of the cell fast enough, photosynthesis will be slowed by feedback inhibition. This kind of makes sense, too, as the plant would change things so that a balance remains between production and utilisation of photosynthetic products.
There is also a suggestion that seedlings have a greater response to high CO2 compared to older plants, possibly because seedlings are generally carbohydrate supply limited, whereas older plants have a store of carbohydrates that are used when needed. Grapevines, being perennial plants, have decent carbohydrate stores even from a reasonably young age.
It's not just photosynthesis that can change, either - under climate change scenarios, increasing temperatures will also increase the respiratory activity of Rubisco. Yes, this enzyme goes both ways: it can help convert CO2 into sugar, but the same enzyme also latches onto oxygen in the process of photorespiration. This opposes photosynthesis and makes the process less efficient. Photorespiration increases faster than photosynthesis as temperatures increase, so photosynthetic efficiency suffers.
And as Jamie Goode has pointed out (here in an article where he points out a whole bunch of interesting things on the subject) with higher CO2 plants don't need to open their stomatal pores as much, because a lesser amount of air holds the same amount of CO2. This can lead to less water use, as with less air movement in and out of the leaf, there is less water vapour lost, too. A side effect of this, however, would be a rise in leaf temperature due to less evaporative cooling (you can experience this by spraying your arm with water - it feels cooler right away because the water is evaporating, and to do that your body heat is used). Higher leaf temperatures could mean more photorespiration, and more time when the leaf gets too hot to keep the enzymatic machinery going. Higher temperatures, associated with climate change, will only make this problem worse.
The multiple changes to the environment will cause plants to respond, but exactly how they respond is really too complex for us to say at the moment, especially when you start to take into account that these changes will have an influence on all the other living creatures around and on the vine (disease organisms, insect pests, and don't forget the soil ecosystem!).
So the overall effect on grapes and wine gets hazy pretty quickly, with lots of factors, and responses, involved. Having said this, I agree with Jamie in that the rise in CO2 concentration is not really what's responsible for increasing wine alcohol - that has more to do with consumer preference and technological advances.
For those that are thinking about strategies for dealing with high Brix and high alcohol wines, we have a number of tools in the viticultural toolbox, but this is a topic for another article!