Is elevated atmospheric CO2 to blame for rising alcohol levels in wine?

wine science

Is elevated atmospheric CO2 to blame for rising alcohol levels in wine?


Lisa Perrotti-Brown of the Wine Advocate recently published an article that caused quite a stir on the Internets. Lisa begins by saying that that the rise in alcohol levels in many wines is inexplicable. It has left her scratching her head. She’s puzzled, as are many of the winemakers that she’s been speaking with. Baffled, in fact:

Based on my experience and many conversations with winemakers, I can assure you, we critics and winemakers have been scratching our heads. Here’s our baffling modern era phenomenon: we can see that ripe phenolics (e.g. tannins) and flavors (a myriad of different compounds) are lagging further and further behind sugar accumulation in grapes with practically every vintage. And for sure, in not every case are higher than average growing season heat, sunshine days, modern vinicultural practices and/or stylistic preferences the obvious reasons. It appears that, to achieve the same physiological ripeness and qualities of wines we have known and loved in the past, we’re having to accept wines at higher and higher alcohol levels, seemingly with every vintage.

As I’m writing this I’m drinking a Pinot Noir from California (Jamie Kutch’s Signal Ridge Vineyard, Mendocino Ridge), that’s just 12% alcohol, and tastes beautifully ripe. And yesterday I met with Francisco Baettig of Errazuriz who has brought alcohol levels down in their icon wines from close to 15% to bang on 14%, and the wines are better for it. Mick and Jeanine Craven make a lovely Faure Syrah from Stellenbosch (warm climate) that’s below 12% alcohol and isn’t hard and mean! So I’d begin by questioning Lisa’s view on what constitutes ripeness, and her assertion that this is all that baffling. If you pick late in warm climates, you’ll end up with high alcohol levels. But more on that later. The real issue I want to address is the theory Lisa proposes for this problem with high alcohol.

Here’s the million dollar question: Why do the sugars in grapes appear to be rising at a much faster rate than rising temperatures (or not) around the planet alone would account for? The obvious culprit to consider is the impact of the dramatic increase in CO2 in our atmosphere since the industrial era.

She then goes on to propose a theory that rising CO2 levels increase plant photosynthesis, which results in higher production of carbohydrates. This in turn, she says, causes sugar levels in grapes to rise, and in advance of phenolic/flavour ripeness. If we want our wines to have proper flavour, she asserts, we are therefore going to have to live with higher alcohol levels. Lisa then proposes that one solution to these higher alcohol levels would be watering back wines, so that you can have ripe flavours and sensible alcohol levels.

She concludes:

This discussion clearly needs more dedicated research in order to draw definitive conclusions, but I strongly feel it is time to open this discussion. In my view, this is one of the most important issues facing our wine industry and, even more importantly, our food chain.

So, I’ll take the invitation she offers and join the discussion. The theory she proposes sounds quite interesting: but is it scientifically plausible? Does higher atmospheric CO2 result in faster accumulation of sugars in grape berries, making it hard to harvest ripe grapes without high potential alcohol? Let’s dig a bit deeper into this issue.

First of all, there is no doubt that CO2 levels have risen. From 1700-1850 they were pretty steady at around 280 ppm. Then, around 1900, things started changing, with vastly increased CO2 emissions into the atmosphere. By 1960 they had reached 320 ppm, then by 1990 they hit 350 ppm, and now they are at around 400 ppm. Projections are that by the end of this century, they’ll be around 700-800 ppm.

And there’s also no doubt that elevated CO2 levels are going to affect plant growth. But before we discuss that, let’s take a quick look at the process of photosynthesis. All life on the planet, with the exception of some weird things that live near deep sea hydrothermal vents, depends on photosynthesis. It’s the process by which plants take light, carbon dioxide and water, and make glucose and oxygen. The significance of glucose is that it’s the way the plant fixes carbon: everything in a plant is made using this process, and this carbon provides the skeleton for the organic (containing carbon) molecules that make up the structure of plants. The plant takes up mineral ions and nutrients from the soil, and uses these to combine with carbon skeleton to make stuff. Basically, plants are very sophisticated chemical factories.

Photosynthesis can be limited by three factors: light, temperature and carbon dioxide. If all three are increased, then the rate will increase, until one of the three is in short supply. Generally speaking, CO2 is the limiting factor quite a bit of the time. Part of the reason for this is that opening up the special pores on the leaf surface, known as stomata, allows precious water vapour to escape. So the plant does a calculation: I need the CO2, but I also need the water. When water is short, and I’m losing too much of it, I’ll close my stomata, and just stop photosynthesizing.

So, we have global warming. CO2 is playing two roles here: its role as a greenhouse gas in elevating global temperatures, and also its role as a frequent limiting factor in photosynthesis. And temperature also stimulates photosynthesis, up to a point: if temperatures are too high the plant will close its stomata and stop photosynthesizing. So if we think about plants adapting to climate change, we need to explore the potential dual roles of elevated CO2 concentrations.

This has been widely researched in important crop plants. Elevated CO2 levels increase the rate of photosynthesis. But they also decrease water use by plants: this is because plants don’t need to open their stomata so often to get the same dose of CO2. The production of leaf ‘non-structural’ carbohydrates increases, and the levels of nitrogen decrease. This is likely because there is a lower need for water, so fewer nutrients (dissolved in water) are taken up by the roots. This decreases the rate at which nitrogen is incorporated into organic products made by the plant: in other words, plants produce more carbohydrate and less protein.

This is where we need to start thinking about the implications for ripening of grape berries. Lisa’s assertion is that more photosynthesis means more sugar production, which in turn results in more sugar accumulation in grape berries, in advance of flavour ripeness. This ignores the fact that grape berries are themselves individual, sophisticated chemical factories.

Berry ripening consists of two distinct phases, separated by a lag phase. Berries are for the birds, and they are designed as a dispersal mechanism for seeds. In the first phase of growth, the seeds develop, and the berry chemistry is designed to make these developing berries unpalatable. They accumulate high levels of acids, tannins and methoxypyrazines, and they taste nasty. Then there’s the lag phase: they stop growing. The seeds mature. And then there’s veraison, where everything changes. This is where the berry decides it is going to put all its focus into making itself appealing to birds. It changes colour, the skin softens, it swells, the acid levels drop, the tannins begin to change and nice flavours begin to develop. And, of course, it becomes sweet.

But this ripening is a tightly regulated biochemical process. For the preceding four months or so the vine has been busy photosynthesizing, but sugars haven’t been accumulating in the berries. It’s now that the berry begins to accumulate sugars, and this is an active process. There are proteins in the cell walls that decide to let sugars through: they don’t just turn up uninvited. There are three families of proteins important in this process (acidic invertases, sucrose synthases, sugar transporters), and there’s also some biosynthesis of sucrose within the berry itself. This is not a simple matter of the vine photosynthesizing more and the sugar levels correspondingly increasing in the berries. This fact is reinforced by the observation that plant hormones such as abscisic acid (ABA) and brassinosteroids play a crucial role in berry ripening. The environmental signaling, mediated by these hormones, has a strong role in ripening. Viticultural interventions such as leaf removal, dropping crop, canopy management and deficit irrigation, for example, rely on influencing plant signaling which in turn affects ripening in complex ways.

So let’s look at attempts to study this in grape vines. One Italian group have worked on this using a technique called FACE, which stands for free-air CO2 enrichment, which is a good technique because it avoids using chambers, which can alter growth patterns. They found that when CO2 was elevated, the acids and sugars increased in the developing grapes. But by grape maturity, there were no differences in berry composition caused by higher CO2. As for global warming, their conclusion was that higher CO2 on its own had very little effect and that any yield-increasing effects were mitigated or cancelled by the higher temperatures that rises in CO2 cause. In the discussion in their paper, they point out that while for annual and perennial plants elevated CO2 increases biomass, crop yield and light/nutrient/water use efficiency. But for a crop like a vine, that sees this increase over many years, the initial increase in photosynthesis might be down-graded because of the plant’s response to excess accumulation of carbohydrates.

Another experiment was carried out on Touriga Franca grapes in the Douro region of Portugal, this time using chambers. They looked at a range of parameters, and found significant differences between the control plants and those grown in higher CO2. But when it came to berry composition (they also made wines from the different treatments) there were no significant differences. The quality of the wines produced was pretty much unaffected by the higher CO2 levels.

More recently, a project has begun in Australia, and the preliminary results have been published (although this project is ongoing). Rachel Kilminster and colleagues have collaborated in studying a Shiraz vineyard in the Murray Darling region. So far they have studied the effects of elevated CO2 and temperature on the vines over two growing seasons. They have found that elevated temperature has a stronger effect in advancing the phenology (the rate at which the vine goes through its growth stages) than elevated CO2. The higher CO2 affects starch levels (known as non-structural carbohydrates) because of a higher rate of photosynthesis. There was no difference seen in the water-soluble carbohydates (sugars), suggesting that sugar levels are actually quite tightly regulated in vines. The higher CO2 treatments increased the carbohydrate status of the plants (vines store carbohydrate in their trunks and roots as a reserve), and the result of this was increased grape yield. Interestingly, higher CO2 resulted in better acid levels in the grapes, but there were only minor differences in anthocyanin and tannin levels.

So where does this leave Lisa’s hypothesis that higher alcohol in wine is a result of runaway sugar levels in grapes caused by elevated atmospheric CO2?

The science doesn’t seem to back it up. The reality is much more complex than the simplistic picture that she paints. A faster-growing, bigger vine, doesn’t necessarily mean higher sugar levels in grapes as they develop.

Nor do anecdotal observations. Atmospheric CO2 levels have risen in an almost linear fashion, but high alcohol wine suddenly became a problem in the mid-to-late 1990s, almost out of nowhere. And it’s quite region specific: regions where American critics, with a stylistic preference for sweeter-tasting wines with very soft tannins from the outset, have been powerful. The evidence suggests that alcohol became an issue with changing stylistic preferences for bigger, sweeter red wines.

There’s no doubting that climate change is having an effect on the world’s wine regions, and in many cases there are problems where harvest time is being pulled into summer conditions, where sugar levels rise rapidly in a short space of time, rather than the more customary autumnal conditions. But there’s no need to invoke a direct effect of higher CO2.

In many regions, there’s a move to picking earlier, and the resulting wines are often purer, taste nicer, will age better, and require less adjustment in the winery. This seems to suggest that higher alcohol levels, to a large extent, are an avoidable consequence of the ill-advised flirtation by many winemakers with sweeter, riper red wines, and a misconception of the nature of appropriate grape ripeness. The wine world had a moment of madness, and it seems that in many regions good sense is now prevailing. If you are picking late, looking for soft, ripe, smooth tannins and a sweet fruit profile in your red wines, then you may well have a problem with higher alcohol levels. Good viticulture (trying to get even ripeness) and picking at the right time is the answer to this.

Note added later: I’ve seen some of the responses to this piece, and I thought I’d emphasise that I’m not doubting that in some regions, it is getting much trickier to harvest certain varieties at good flavour ripeness but sensible potential alcohols. It’s just that we don’t need to invoke rising CO2 as a direct contributor because of its effects on photosynthesis and then berry sugar accumulation, particularly when there’s no evidence yet that this relationship exists. But CO2 could be having an indirect effect: higher levels of photosynthesis (if there is no acclimation by the vine) could increase vine carbohydrate reserves. And this could have the effect of advancing phenology, bringing harvest into a warmer part of the year. Rising temperatures (global warming) will also affect phenology. It’s this advancing of the ripening cycle that leads to the issues with high rates of berry sugar accumulation, I suspect. I’d also point readers to another article on this topic from Dr Glen Creasy, a researcher from New Zealand, here.

12 Comments on Is elevated atmospheric CO2 to blame for rising alcohol levels in wine?Tagged
wine journalist and flavour obsessive

12 thoughts on “Is elevated atmospheric CO2 to blame for rising alcohol levels in wine?

  1. Very interesting, but again looking at the kingdom under our feet, the dirt, we then start to understand sugar up, sugar down, root exudate to microbes who then source nutrients as and when required and feed these back up into the plant. The stomata is only the stop go light at the cross road. After >35 years of not using pesticides the microbes respond with positive phenolics in the plant sap, as a plant defence but also a plant strengthener. Our alcohols have decreased as well in the last 10 years as has the incidence of botrytis (yes in our climate!) and insect pests. Most importantly is the antioxidative affect of these long chained phenolics lessening the need for chemical preservation and “sweeter” skin fermented (white) wines.. So I wonder the high alcohols and Co2 condition of the atmosphere is another example of not looking where your feet are standing. What’s more…Just like blood pressure maybe it is a prerequisite for winegrowers to start doing leaf sap density testing throughout the season, and beyond.

  2. Superb article Jamie. It’s surprising that someone in such an important position as Lisa has not apparently consulted scientists such as yourself before writing her piece .

  3. a perfect example of science vs leasurly journalism. Obviously, the latter will always be more resounding than the former, because simplistic explainations are easier.

  4. Not grape-specific, but there is research substantiating that higher CO2 levels cause higher sugar accumulation in crops:

    “In C3 plants (e.g., rice, wheat), elevated concentrations of atmospheric carbon dioxide (eCO2) reduce protein and nitrogen concentrations, and can increase the total non-structural carbohydrates (TNC; mainly starch, sugars)”

    Loladze 2014, Myers et al 2013, and some more

    You make a point that grape composition and biochemistry is more complex than some of those other crops, but the body of research still being limited, perhaps it is an open question?

  5. Thanks for this Jamie. A thorough yet gin-clear explanation of the science. I’ll admit I’ve probably been guilty of imagining the CO2 up=Sugar up correlation in the past. This explains what we know (and what we don’t) brilliantly.

  6. An interesting article, and I agree that there has been a deliberate attempt to beef-up red wines over the last 10 years. This has fed into white winemaking as well, in a way that I consider non-beneficial.
    In the UK, a marginal climate, we struggle to achieve 11.5% alcohol naturally. However in the last two years we have been able to make balanced(dry white)wines without chaptalisation and without picking late, at 11.0 to 12.0% alcohol. 10 years ago we would add sugar every year.

  7. Thank you, Jamie, for this informative article.
    In the experiments you mentioned, CO2 level is the variable.
    Are there any experiments on the effects of higher temperatures on acid levels and sugar accumulation in the berries?

  8. In my innocence I thought the level of alcohol in wine was limited by the yeast. Higher alcohol levels kill yeast. Given the accepted fact that warmer climates generate a higher sugar level it seems possible that those areas, in developing wines that converted enough sugar to alcohol, migrated toward yeasts more tolerant of higher alcohol levels.
    Equally, and this is pure surmise, yeast selection by the suppliers will have promoted this aspect and possibly some research into genetic modification to allow the yeasts to work at these higher alcohol levels.
    As another reply notes even in the UK with it’s marginally increased temperature levels of alcohol have typically increased.

    So nice to see an article which is based on science rather than socially acceptable pseudo-science.

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