Rising alcohol levels in wines has been a trend in recent years, noted by members of the industry and consumers alike. Some blame critics’ preferences for higher-octane styles and winemakers pandering to these preferences; others blame global warming accumulating and concentrating the sugars in wines earlier in the growing seasons and forcing winemakers to harvest at higher sugar levels. 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. Within the lifetimes of today’s winemakers, this “ripeness” conundrum is a very real occurrence faced each harvest in regions throughout the wine world.

The ultimate goal for winemakers’ each vintage is to harvest grapes that are ripe, and by “ripe,” I don’t mean just the optimal sugar/acid ratio to create a semblance of balanced weight and freshness on the palate, but also fully resolved tannins (phenolics) and fully expressed flavor compounds. Without ripe tannins and flavors, what you get are wines that are, well, lean, green and mean (hard), which is precisely the effect of grapes harvested too early—something of a backlash trend in our modern era in order to assuage consumers’ desires for lower alcohol levels. Yes, yes, yes, the alcohol may arguably be more “in balance” in those scrawny, harsh, bitter wines coming from earlier harvested grapes. But at the cost of flavor and texture? Sorry, I’ll opt for the higher alcohol any day, if that is what it takes to get all the complex flavor layers and soft, fine-grained, non-bitter tannins that I want and love. 

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. 

And here’s what NASA reports is happening with CO2:

“During ice ages, CO2 levels were around 200 parts per million (ppm), and during the warmer interglacial periods, they hovered around 280 ppm. In 2013, CO2 levels surpassed 400 ppm for the first time in recorded history.”

I’ve made no secret of my fondness of Australian wines and therefore feel that I can say, without prejudice, that the higher alcohol phenomenon has been felt particularly within the Australian wine industry. I remember from a few years ago the occurrence of what I refer to as the “skinny” Chardonnay and Pinot Noir trend, whereby winemakers would early harvest these varieties in an attempt to make lower alcohol wines. The problem was they had no flavor, and the reds possessed harsh, bitter, underripe tannins, ergo, the lean, green, meanies. (As you can imagine, it was a short-lived trend, but this rather facile, Band-Aid solution has reared its ugly head in other parts of the wine world.) More recently, in efforts to hedge against what are perceived as the effects of Global Warming, there has been a scramble toward purchasing vineyard land in cooler climates such as Tasmania, looking to obtain sites that can achieve complex aroma/flavor compounds at lower alcohol levels that some styles of sought-after grapes such as Pinot Noir and Chardonnay demand. 

It is likely that Aussie winemaker frustration sparked some of the first research on the CO2 effect on vines by Rachel Kilmister, et al. in Victoria, Australia, as can be read in a 2016 article published in Wine & Viticulture Journal. This article, titled Climate change: Effect of elevated CO2 and temperature on phenology, carbohydrates, yield and grape composition - preliminary results, opens with the comment: “While there has been a great deal of interest in the effects of elevated temperature on vine phenology and a number of published scientific studies in this area (Webb et al. 2012, Petrie and Sandras 2009), there has been little study of the effects of elevated CO2 on the vine.” Among other findings, this article concludes, ominously: “The primary effect of elevated CO2 on a plant is to increase the rate of leaf photosynthesis. This generally results in increased carbohydrate levels, which may be consumed in general metabolism, used for growth of plant organs such as shoots and fruit, or stored.”

Something of a cliffhanger, for sure, but not enough yet to get the wine world up in arms. And so, imagine my surprise when, a couple of weeks ago, I arrived for a visit and tasting at Dominus Winery in the Napa Valley to have Christian Moueix greet me excitedly with a glint in his eye and some information he and winemaker Tod Mostero wanted to share. Mostero went on to explain that the team at Dominus had been looking into a possible link between rising CO2 levels in the atmosphere and rising alcohol levels. They were kind enough to offer me what they had put together thus far: a graph from NASA plotting the rising CO2 levels in Earth’s atmosphere since 1950 (something that will come as no shock to anyone who has been paying attention) and an article in The Nature Education Knowledge Project entitled: Effects of Rising Atmospheric Concentrations of Carbon Dioxide on Plants. 

In a nutshell, the report finds in the favor of our suspicions: that more CO2 availability causes plants to create more sugar. You might be thinking that this is a good thing, right? It depends. For many years it was just assumed that right along with all that extra sugar plants were creating, they were creating just as much of all the good stuff—nutrients and minerals. However, now it appears that was just wishful thinking. 

This month The Agenda published an article called The Great Nutrient Collapse. In here, Dr. Irakli Loladze discusses new research he, as well as the USDA, are currently conducting regarding the effects of rising CO2 levels on how plants grow and, perhaps most importantly, on the amounts/types of nutrients they produce. The penny begins to drop with an experiment using zooplankton: 

I’ll cut to the chase here: it seems increasingly likely that one of the effects of rising CO2 levels on plants is an overall increase in the ratio of carbohydrates to minerals. In other words, apart from the shocking revelation that our plant-based produce is rapidly becoming more fattening and less nutritious when harvested, plants are making more sugar during the growing seasons around the world—and probably earlier in the growing seasons—and apparently apart from the effects of increased sunshine/heat, while at the expense of other grape components. And, of course, more sugar racked up before the tannins and flavors ripen equals more alcohol in wines, especially those of quality.

This very much explained what Moueix and Mostero had been experiencing in recent years, in both Napa and Bordeaux. Mostero commented on their behalf:

With many pieces of the puzzle of this association between rising CO2 levels in the atmosphere and rising alcohol levels falling into place, there was one piece for me that still didn’t fit. It was a conversation I had with Jeffrey Grosset in Australia’s Clare Valley back in 2009, in which he suspected that rising CO2 levels could be something of a silver lining for him. He had, in fact, been able to harvest earlier at lower alcohol levels since the 1990s. He recently reconfirmed his observations:

“What I can say is that we have slightly reduced alcohols compared to the 1990s and particularly since 2003... While it may seem likely that as a vineyard owner in this region I might argue against negative thoughts about the future here, I am baffled by the extent of the error in popular thought (including my own until recent years). I considered establishing a vineyard in a ‘cooler’ region as insurance or a hedge, back in the late 1980s and many times since, only for any practical support for this idea to fail to materialize. Increasing CO2 is allowing grapevines to be more efficient.”

What Grosset was saying didn’t seem to make sense to me...until I considered the grape variety he mainly works with to produce his finest wines: Riesling. 

The ripening of flavor compounds and tannins in grapes is an incredibly complex subject. This is because there are so many different compounds and the types and combinations differ with each and every grape variety used to make wine. This is precisely why wines made from different grapes taste so different. In a book that I wrote and published a few years ago, Taste Like a Wine Critic: A Guide to Understanding Wine Quality, on the topic of ripening, I noted:

The fact that Grosset works mainly with Riesling while Moueix and Mostero work mainly with Cabernet Sauvignon reminded me of an article I had read a few years ago when I was conducting research for my book. Entitled “Comparison of major volatile compounds from Riesling and Cabernet Sauvignon grapes (Vitis vinifera L.) from fruitset to harvest”, by C.M Kalua and P.K. Boss, May 26, 2010, the article concluded this:

Without dedicated research, it is difficult to make a definitive conclusion; however, based on the related research and anecdotal evidence so far, it appears that rising CO2 levels in the atmosphere may indeed be a boon for some grape varieties and styles and a curse for others.

The mounting body of evidence drawing a correlation between rising atmospheric CO2 levels and the alcoholic content in wines suggests that warmer vintages and climates may exacerbate the problem, but upping-sticks and heading for cooler ground will not get rid of the problem entirely. So, what options—besides earlier harvesting—are available? Well, there is another pretty simple winemaking solution worth considering that is going sound like heresy to most of the legal authorities in the wine industry: watering-back. Over the course of my tastings and visits over the last few years, on many occasions I’ve queried winemakers about moderate alcohol levels in wines sporting physiological ripeness—wondering how they achieved this—and I’ve had at least a handful of them look me in the eye and matter-of-fact tell me about the practical need to water-back (add water to wines—usually during fermentation and used under the excuse of preventing a stuck fermentation—to bring down the eventual alcohol level, in practice). In Australia, where this practice is illegal, they call the offending watering hose “the black snake.” California is one of the few places in the world where it is legal to add water to unfinished wine. OK, sure, in the course of my many years of tastings, I’ve tasted some god-awful wine monstrosities resulting from raisined grape juice made into wine and watered-back. Monolithic, hard, unpleasant to drink, is all I can say. BUT, I’ve also tasted some beautifully ripe, expressive wines that have had water added to the ferment to tame the alcohol, and I have no complaints. Chaptalization has been fully accepted in some of the greatest wine regions of the world (e.g. Burgundy, Champagne, Bordeaux, etc.) for centuries as a means of increasing alcohol to create balance. It may be an inconvenient truth, but given our new rising CO2 levels paradigm and the mounting body of evidence, I believe we need to consider better options for legally creating “balanced” wines other than harvesting underripe grapes.

The results of watering-back may not be as unpalatable as earlier harvesting, but it too is a Band-Aid solution to a problem that, in the long-term, requires a holistic approach to achieve the best quality results. Really, what is needed is for winemakers to be able to create balance in the vineyard and harvest grapes that do not require such adjustments. Here’s where Christian Moueix and Tod Mostero offered to share some of the measures they have been practicing in Napa and Bordeaux:

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. I do hope/implore that more scientific resources are aimed at this worthy subject in the future.