After looking at more than 400 years of harvest and climate data from France and Switzerland, researchers from Harvard University and NASA have concluded that in recent decades, warmer temperatures have pushed wine grape harvests in those countries more than 10 days earlier than in the period from 1600 to 1980—regardless of whether the growing seasons brought damp conditions or drought.
"It's evidence we have fundamentally shifted the climate system," said study co-author Elizabeth Wolkovich, assistant professor of organismic and evolutionary biology at Harvard. "It used to be that these early harvests happened in dry, hot years."
In dry soils, less moisture evaporates to cool the surface; a drought, in effect, turns up the heat to accelerate ripening in a vineyard. But average temperatures in France climbed about 2.7° F in the 20th century. "What we see happening in the 1980s is that you no longer need a dry summer," said Wolkovich.
This insight has important ramifications—good and bad—for future wine quality. In analyzing vintage ratings for Bordeaux and Burgundy from 1900 to 2001, researchers found that higher-quality wines have been typically linked to early harvests in the cooler regions of Europe. The best wines came from years with above-average rainfall early in the growing season, a warm summer and a late-season drought or dry conditions that generated a heat spike and shifted the focus of vine growth from leaf production to grape maturation.
"Wine quality also depends on factors beyond climate, including grape varieties, soils, vineyard management and winemaker practices," said lead author Benjamin Cook, a climate scientist at NASA’s Goddard Institute for Space Studies and Columbia University’s Lamont-Doherty Earth Observatory, in the announcement of the findings. "However, our research suggests the large-scale climate drivers under which these local factors operate has shifted. And that information may prove critical to wine producers as climate change intensifies during the coming decades in France, Switzerland and other winegrowing regions."
A tipping point may soon come, cautioned Wolkovich: "Climate change is the reason we've had so many great vintages of Bordeaux in the last 20 to 30 years. It's also the reason you might not get a good Bordeaux in the next 50 years. Take this forward: We've only experienced a small proportion of the warming we have created and will see in the next 50 to 80 years, and that will have radical consequences for wine regions."
As an example, she pointed to the 2003 vintage, when a record deadly heat wave across Europe resulted in the earliest harvest in their study, but mixed quality, producing some exceptional wines and some that were unbalanced.
The research, published March 21 in the journal Nature Climate Change, analyzed records in eight regions—Alsace, Bordeaux, Burgundy, Champagne, Languedoc, the lower Loire Valley, the Southern Rhône Valley and Switzerland's Leman Lake—from 1600 to 2007 to get a big-picture view covering a range of climates, soil types, slopes and grape varieties with different flowering periods and maturation rates.
Thanks to reports kept by religious orders and databases compiled by other researchers, "We had these incredible long-term harvest records," said Wolkovich. "It was a rare opportunity to see how something works before and after climate change."
As grape quality and wine character are so closely tied to climate and weather, wine is often used in climate-change modeling as an attention-getting agricultural canary in the coal mine. Over the past decade, several climate studies have predicted dramatic changes in the viability of warmer winegrowing regions—with more northern regions such as England expanding while long-established appellations see famed sites become less suitable or are forced to switch grape varieties and wine styles. But much of the research has focused on recent timeframes or future predictions.
While this isn't the first report on the longer-term shift in European harvest dates, what's unique about Cook's and Wolkovich's work is how they looked at whether the climate driving the harvest dates has changed, comparing different historical periods, with 1980 marking a dramatic turning point. They examined centuries of records of temperature, precipitation and soil moisture (an indicator of drought)—from data collected by 20th-century instruments as well as from historical documents and tree-ring analysis.
"Temperature is a similarly strong driver [of harvest] before and after ," said Wolkovich. "But what changes is drought and precipitation—they become much less coupled to harvest after 1980." The team looked at other 30-year periods, such as the one around the 19th-century phylloxera outbreak in France, when rootstocks and grape varieties were replaced, to see if climate had become decoupled from harvest at any other time, she said. "And the answer is no."
Prior research has found that each increase of 1° C (1.8° F) in average temperature bumps up the grape harvest by about six days. So when might the crucial tipping point come?
That's going to depend on the individual vineyard—which grape variety is planted, the soil type, slope, altitude and orientation and other factors—something the study's large-scale, broad analysis cannot project. (For example, in hot, stony soils, it will take less warming to tip the scales.) Along with replanting to more heat-tolerant grape varieties, winemakers can respond to climate change in they way they manage their vineyards—from pruning to canopy management to cover crops to water management.
"The silver lining, at least for me, is that the climate diversity for wine grapes as a whole is very high," said Wolkovich. "It's a question of how well the market and grower are willing to exploit that diversity."
But she added a cautionary note: "I hope people will take away that the quality of wine will be one of their lower concerns if we don't shift climate change."