Climate change isn’t just captured by thermometers—grapes can also do the trick.

By mining archival records of grape harvest dates going back to 1354, scientists have reconstructed a 664-year record of temperature traced by fruit ripening. The records, from the Burgundy region of France, represent the longest series of grape harvest dates assembled up until now and reveal strong evidence of climate change in the past few decades.

Science with Grapes

As far back as the 19th century, scientists have been using records of grape harvest dates to track climatic changes.

“Wine harvest is a really great proxy for summer warmth,” said Benjamin Cook, a climate scientist at the NASA Goddard Institute for Space Studies in New York not involved in the research. “The warmer the summer is, the faster the grapes develop, so the earlier the harvest happens.”

But there are potential pitfalls to using this method, said Thomas Labbé, a historian specializing in the Middle Ages at the Leibniz Institute for the History and Culture of Eastern Europe in Germany and the Maison des Sciences de l’Homme de Dijon in France. For instance, he said, some studies have compiled grape harvest dates from vineyards in different locations. That’s problematic because of climatic differences due to latitude—for Northern Hemisphere vineyards, grapes growing farther south tend to ripen earlier than grapes located farther north. Other investigations have relied on secondary sources of grape harvest dates riddled with transcription errors, said Labbé.

Vineyards of an Ancient City

Now, Labbé and his colleagues have assembled a 664-year record of grape harvest dates for one French city using information gleaned from original sources.

“The vineyards still surround the city, so we could extend the series to the present day.” The city of Beaune is an excellent site for long-term analysis of grape harvests, said Labbé. Rows of pinot noir, sauvignon, and gamay grapes have dotted its slopes for centuries and still do so today. (Dijon, 45 kilometers northeast and the capital of the Burgundy region, isn’t as good a site. It’s undergone pronounced urbanization since the 19th century and has accordingly lost many of its vineyards.)

“The vineyards still surround the city, so we could extend the series to the present day,” said Labbé.

The researchers mined original sources (such as medieval accounts of wage payments to vineyard laborers, city council records, and newspaper reports) to determine when Beaune’s grapes were harvested each year from 1354 to 2018. When data were missing from archival records, the researchers used harvest dates from Dijon and adjusted them to account for the capital’s more northerly location.

They were careful to analyze each date in the context of history. For instance, the harvests of 1636 and 1637 were “certainly disorganized” by warfare and an outbreak of plague, the scientists concluded. All in all, Labbé and his colleagues recovered harvest dates ranging from 16 August to 28 October.

Outliers “Become the Norm”

Labbé and his colleagues showed that the dates of Beaune’s grape harvests correlated strongly with both instrumental temperature records from Paris and tree ring–based temperature reconstructions from western Switzerland. These correlations demonstrate that grape harvest dates indeed are an accurate proxy for local temperature, Labbé and his team concluded. “It’s possible to reconstruct temperature backward,” he said.

The scientists found that the series of grape harvest dates could be clearly divided into two regimes. Grapes were, on average, picked on 28 September or later before 1988. But from 1988 onward, grapes were harvested roughly 13 days earlier. “Global warming is very, very visible.”

“Hot and dry years in the past were outliers, while they have become the norm since the transition to rapid warming in 1988,” Labbé and his team wrote in their paper, which was published in August in Climate of the Past.

It’s not surprising that the climate is warming, said Labbé. “The surprise is rather that the grape harvest date series reflects so well the temperature trend of the last 30 years. Global warming is very, very visible.”

—Katherine Kornei (@katherinekornei), Freelance Science Journalist