Climate change is among the greatest environmental challenges that we are currently facing according to reports by the Intergovernmental Panel on Climate Change. The release of greenhouse gases, especially CO2, as a result of various human activities, is seen as the main cause of climate change in general and global warming in particular.
Temperature is a key environmental factor that affects almost all aspects of plant growth and development. In terms of agricultural products, wine is among the most sensitive to the effects of climate change.
The grapevine (Vitis spp.) and its fruit are acutely sensitive to high temperatures, which have a negative impact on their natural growth cycle. One of the most immediate effects can be observed in the harvest itself. Warmer climate conditions accelerate the natural development of the grapes, leading to an earlier onset of the maturation process, which in turn brings the harvest forward significantly.
Unfortunately, temperatures are forecast to keep rising during this century, exerting an increasingly negative impact on the wine and grape sector.
The effects are manifold, but one of the most important consequences concerns phenological changes in the grapevine. Studies have shown that increases in temperature bear a significant correlation to the earlier onset of many of the grapevine’s growth stages [1,2]. Any metabolic change in the vine has consequences, in this case negative, for the composition and quality of the grapes and hence the wine.
The taste of climate change
We could describe climate change as bringing about an alteration in wine quality due to important variations in what is known as the chemistry of wine. Generally speaking, these changes can be summarized as follows:
Sugar: In oenology, this refers to simple sugars, which comprise a wide variety of constituent elements. They include fermentable sugars—glucose and fructose—which accumulate in the grapes as part of the grapevine’s metabolism and whose fermentation is essential to transforming grapes into wine.
High temperatures accelerate the maturation process, the stage at which the grapes accumulate fermentable simple sugars, thereby increasing the final concentration of said sugars.
Once the grapes have been turned into must, yeasts transform these fermentable sugars, derived from the fruit in the form of glucose and fructose, into ethanol and carbon dioxide. If the grapes have a higher concentration of fermentable simple sugars, the resulting wines will have a higher ABV.
Alcohol plays a key role in the minds of consumers and on their palates too. High-alcohol wines need high acidity to bring them into balance. Otherwise, the wines will be perceived as flabby. Furthermore, these wines tend to feel denser and often sweeter, as well as falling increasingly out of favour with end consumers who are seeking wines with lower ABV to cut down on alcohol consumption.
On the other hand, high levels of simple sugars in the must can expose the yeasts to hyperosmotic stress. During fermentation, this can lead to undesirable by-products, such as acetic acid, which have a negative impact on the end quality of the wine.
Acidity: Broadly speaking, the main acids in winemaking are tartaric and malic acid. These originate in the grapes and makeup around 80% of the total content of organic acids.
Balanced acidity— defined as the concentration of tartaric acid relative to malic acid— and potassium levels characterize the acidity of the must.
Under conditions of heat stress, especially during the ripening period, the concentration of potassium in the grapes increases, thereby raising the pH level and ultimately reducing total acidity.
High temperatures also affect the proportional balance between sugar levels and acidity. Excessive heat can boost sugar accumulation and, as a consequence, degrade the organic acids, which has a greater impact on acidity than sugar levels. For grapes grown in warmer climate conditions, this results in lower acidity for the same amount of sugar.
Organic acids play a fundamental role in the acidity that influences the stability, colour, and gustatory reception of wine, as well as its hygienic condition. A decrease in acidity leads to wines that lack brightness, aromatic intensity, wine flavour, and become a far more fragile medium from a microbiological standpoint.
Secondary compounds: other essential compounds for the sensory perception of wine, such as anthocyanins and tannins, are also affected by high temperatures.
The former give red grapes their colour and protect the plant from UV radiation. Tannins are responsible for the bitterness and astringent or dry sensation of wine and equip the vines with a line of defense against grazing animals and pests.
Anthocyanins, like most phenolic compounds, are very unstable and susceptible to heat-induced degradation. As for tannins, high temperatures can lead to insufficient ripeness, primarily as a result of picking the grapes earlier to prevent excessively high sugar levels.
In short, we can say that high temperatures and early fruit maturation bring about a decrease in the wine’s typicity, changes in its aromatic character, and an imbalance in terms of acidity and sugar in the must.
Grapes harvested earlier than expected produce wines with fewer organic acids, higher pH and ethanol levels, and alterations in their sensory characteristics, as well as a decrease in grape pigmentation due to the inhibition of anthocyanin biosynthesis.
Furthermore, these changes in temperature complicate the stabilization process in the bottle, both on a microbiological level and in terms of proteins and tartaric acid. Rising pH levels reduce the wine’s protection against oxidation, leading to a loss in aromatic compounds.
Given this scenario, it is unsurprising that the sector is working tirelessly on mitigation strategies to reduce the vines’ exposure to high temperatures by seeking out higher elevations, changing row orientation to prevent over-ripening, or looking for varieties better suited to warm climates.
For a host of reasons, climate and weather conditions are unpredictable and difficult to manage at this time. By its very nature, winegrowing is complicated, but climate change is only making it more so. Therefore, we have a shared responsibility, whether we’re big or small players, to not only control and monitor but also work together in reducing our carbon footprint and the global warming it causes.
Marta Juega, PhD
References:
1.Alikadic A., Pertot I., Eccel E., Dolcia C., Zarbo C., Caffarra A., De Filippi R., Furlanello C. The impact of climate change on grapevine phenology and the influence of altitude: A regional study. Agric. For. Meteorol. 2019;271:73–82.
2. Bock A., Sparks T., Estrella N., Menzel A. Changes in the phenology and composition of wine from Franconia, Germany. Clim. Res. 2011;50:69–81.
