Harvest Timing: How Climate Variability Affects Crop Yields

We must also consider Man in general, who holds the reins of destiny; understand those of the climate and its evolution in the medium and long term by controlling (or not) greenhouse gas emissions which contribute to climate change.

If the vine is therefore a human construction, it has also contributed in return to shaping the imagination of man, to giving rhythm to his life and in particular his way of segmenting the passing of time. Thus, as an example, we can recall in France that the revolutionary calendar created in 1792 named Vendémiaire (from the Latin vindemia, meaning grape harvest), the month running from September 22 to October 21. In Ancient Rome, the vine and wine were the subject of the Roman festivals Vinilia, and Vinilia Rustica in particular marked the beginning of the grape harvest.

[Un article issu de The Conversation écrit par Valérie Bonnardot, Maitre de Conférences en géographie environnementale – climatologie, Université Rennes 2]

Harvest dates: a good climate indicator

The vine, through harvest dates, has also long been a good indicator of the climate. Indeed, the ancient obligation to publish a harvest ban to begin harvesting allows us to have long series of data on harvest dates such as that published in the 19th century by the archivist of the city of Dijon Joseph Garnieret, well known to historians, such as Emmanuel Le Roy Ladurie who indicates it in his book on the History of the climate since the year 1000.

This series, about 130 years long (from 1372 to 1500 to be precise), indicates an average harvest date at the end of September with variations of one month reflecting the annual variability of the climate: the earliest harvests (early September) being associated with the hottest and/or driest years and the latest (end of October) associated with cool and/or wet years. This same series has since been used to reconstruct past variations in summer temperatures. More evocatively, this data also appears, alongside the cherry blossom dates in Japan, in the sixth IPCC report among the indicators of global changes that vegetation can reveal on a global scale.

Climate change alters harvest dates

Harvest dates have been brought forward by an average of 2 to 3 weeks over the last 40 years. But global warming is causing all stages of vine growth and grape ripening to advance, and not without consequences. The earlier emergence of the first buds generated by global warming will increase the plant's vulnerability to frost, which can always occur. This is also the climate risk that generates the most concern among winegrowers. The grape ripening period (which corresponds to the month preceding the harvest), which is shifted to a warmer period (August instead of September), will lead to a change in the composition of the grapes and therefore the wines. Thus, the potential alcohol content that increases and the acidity that decreases weaken the structure, the aromatic balance of the wines and their conservation.

Strategies for adapting to higher temperatures exist, but only up to a certain level of warming. For example, changing plant material (rootstocks and grape varieties with later maturity, more tolerant to drought and heat, etc.), changing viticultural practices to promote the resilience of vines (grassing between rows, reducing planting density, agroecology, etc.) or adjusting oenological processes (choice of fermentation yeasts, use of dealcoholization or acidification techniques, etc.). A so-called “nomadic” adaptation strategy also involves moving vine plantations to higher altitudes, on northern slopes (or southern for the southern hemisphere) or towards the sea or ocean to find coolness.

According to climate projections (varying according to greenhouse gas emissions scenarios and time horizons), some regions will be particularly affected while others could emerge. In France, some vineyards in the south of France are increasingly recording temperatures above 30°C or even 35°C, which constitute a thermal threshold of extreme stress for the vine, disrupting proper physiological functioning, in particular causing a maturity blockage as was the case in 2022. It should be noted that according to projections from an “intermediate emissions” scenario, 2022 will be an “average” climatic year by 2060. On the other hand, warming is promoting a migration of climatic areas suitable for viticulture towards the north in the northern hemisphere or towards the south in the southern hemisphere.

Vines in Brittany: regional marker of the extent of climate change

In this context, the Brittany region, better known to the general public for its cider production, is seeing the cultivation of vines develop on its territory. And this, above all, is due to a legislative change with a 2015 decree modifying the regime of planting rights on a European scale. This text in fact allows the planting of vines for commercial purposes in historically non-winegrowing regions.

But climate change also favours the thermal conditions for the production of Breton wines. Even if we cannot reduce the climates of Brittany to that of Rennes or those of the Bordeaux vineyards to that of Bordeaux, the comparison of the thermal conditions of the two cities since the middle of the last century can be provocative in this context, but it is eloquent in grasping the extent of climate change in Brittany.

The average annual temperature of Rennes over the period 1991-2020 (12.4 °C) is similar to that of Bordeaux over the historical period 1951-1980 (12.5 °C). The values ​​of Rennes in 2022 (13.8 °C) and 2023 (13.4 °C) were not reached in Bordeaux over this historical period. In the future, if the reins of destiny (climate) are not met, “the annual average in Rennes would reach 15.3°C, the same as what we currently encounter for example in Coimbra in central Portugal” thus summarized the climatologist geographer Vincent Dubreuil.

These thermal conditions in Brittany would allow the Chardonnay to be harvested before September 7 and the Chenin before September 30 with a sugar level of 190g/L necessary for the production of quality white wines.

In the meantime, the first dates for commercial harvests in Brittany have been recorded in 2022 and 2023, which were spread over the month of September depending on the location and grape varieties. Those for 2024, as is the case elsewhere in the wine-growing regions of France, are expected to be slightly later… but still within the historical norm for traditional wine-growing regions.