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How does a vintner know that the grapes are ripe? By the traditional method, they taste the grapes themselves. Like this: put the grape in your mouth, bite into it, let the juice run out and in doing so determine the sugar and alcohol content. And it is precisely the latter that could become an issue in the traditional growing regions over the next few years and decades. According to a study by the University of Bern, the grape harvest in Burgundy has been starting 13 days earlier on average than it has in the past six centuries.

Grapes are sensitive to changes in temperature

This result is on the one hand interesting for winemakers and wine lovers. This is because grapes are very sensitive to temperature and rain. Higher temperatures make the grapes ripen faster. And the resulting increase in dryness also results in a higher alcohol content. Which is not what is supposed to happen,  not just because the trend is towards lighter wines. Wines with too high an alcohol content also taste slightly ‘burnt,’ In order to avoid this, the artful technical skill (for which the Germans are world-renowned, as it happens) of the master vintner is now in high demand.

According to Frank R. Schulz, head of the communications department at the German Wine Institute, German winegrowers are able to benefit from the current situation, at least for a short while. “We are now in a position to make the most of it. World vine varieties such as Merlot or Sauvignon Blanc, which were previously only native to southern Europe, are now growing here. Whereas in Spain the grape harvest is much lower than in previous years, in the main due to drought. And even in northern Germany, such as on Sylt or in Schleswig-Holstein, attempts are now being made to cultivate grape varieties. And last but not least, sparkling wine viniculture in the south of England also shows that wine regions could shift over the long term. In order to be able to react to climate changes, the southern regions are working on improving their varieties. For example, a thicker skin makes the grapes somewhat more resistant.

Grapes act as a climate proxy

Due to their sensitivity to weather, grapes are also very useful for getting a peek into past climates. This means that the start of the harvest is ideal as a kind of climate proxy. It is an indirect indicator of change, as we have seen in other natural archives. These include tree rings, ice cores and corals. Plus, historical documents like the grape harvest. The scientists from Bern used a series of data from the grape harvest of the past 664 years for their analysis.

“We did not predict that the rapid rise in temperatures since the 1980s would be so clearly visible in this time series,” explains Christian Pfister, Professor Emeritus of Climate and Environmental History at the University of Bern and a member of the Oeschger Centre for Climate Research.

He was responsible for the study together with colleagues from Switzerland, France and Germany.

Records of wages paid to harvest workers

The main author of the study, Thomas Labbé, who conducts research at the universities of Burgundy and Leipzig, meticulously reconstructed as far back as 1354 the data from grape harvests in Beaune, the wine capital of Burgundy. He used a large number of unpublished archive sources, including information on wage payments to grape pickers, records from Beaune City Council and newspaper reports. The uninterrupted recording of grape harvest data is the longest reconstructed time series of its kind and ended in 2018.

“The harvest records clearly show two phases,” says Thomas Labbé. Until 1987, the grapes were typically harvested on or after 28 September. Since 1988, however, the grape harvest has started on average 13 days earlier. Analyses of the data show that hot and dry years were unusual in the past, but have since become normal in the last 30 years. The research team consisting of historians and natural scientists has validated its time lines with the help of detailed temperature records taken over the past 360 years in Paris. This made it possible to estimate temperatures between April and July for the Beaune region throughout the 664 years covered by the dataset.

From research to action

“The transition into a period of rapid global warming after 1988 is very clear. And it is obvious to everyone that the past 30 years have been extraordinary,” Christian Pfister states.

“I hope that our recent work will help people realistically assess the current state of our planet’s climate and finally begin to take action.”

This unique reconstruction of the Burgundy grape harvest has just been published in the journal “Climate of the Past” published by the European Geosciences Union (EGU).