How accurate that is beyond the last 30 years is unclear.
Thomas Labbé, a researcher at the universities of Burgundy and Leipzig and lead-author of the study, used written accounts and even proxies, such as the date on wage payments made to grape pickers, to reconstruct dates of grape harvest in Beaune - the wine capital of Burgundy - going back to 1354. He also used Beaune city council records and newspaper reports.
The authors believe grape harvest dates can be used as a proxy to study the change in climate because wine grapes are very sensitive to temperature and rainfall. So they used proxies to create dates of harvest and then use that as a proxy for changing temperature.
Extract of the expenses provided by the church of Notre-Dame of Beaune for the grape harvesting works in the region in 1385. Credit: Archives Départementales de la Côte d'Or, 2918/24, Thomas Labbé
But in the past grape picking was more art than science. If the growing season is hotter and drier, the grapes will be ready for harvest earlier than in colder years but wine is a matter of taste, and vintners across centuries varied in that, along with customers, and even the effectiveness of labor. Then there is the issue of when anything was recorded. Those are a lot of confounders.
They did their best to validate their estimates using temperature records of Paris covering the past 360 years but those are also not accurate before the 1980s. That is why controversial use of tree ring data until it stopped showing warming and then beginning to use accurate temperature measurements created confusion among the public, and claims of doubt about older climate methodology.
Vineyards in Beaune, Burgundy. Image: Olivier Duquesne via Flickr / This photograph is distributed under a Creative Commons Attribution-ShareAlike 2.0 Generic (CC BY-SA 2.0) License
"The record is clearly divided in two parts," says Labbé. Until 1987, wine grapes were typically picked from 28 September onward, while harvests have begun 13 days earlier on average since 1988. The team's analysis of the series shows very hot and dry years were uncommon in the past, but have become the norm in the last 30 years.