Rethinking Riesling and Climate Change

Researchers say warming is only partly responsible for increase in Brix

by Peter Mitham
wine vineyard riesling climate change
Hans Schultz summarizes growing conditions for Riesling during the Riesling Rendezvous held in Washington this week.
Seattle, Wash.—Riesling has made great strides since Chateau Ste. Michelle and Dr. Ernst Loosen joined forces to organize the first Riesling Rendezvous in 2007, Chateau Ste. Michelle president Ted Baseler crowed at this week’s event in Seattle.

But greater strides are needed if producers are to keep pace with the effects of climate change, which stand to be almost as varied as the styles and classifications of Riesling being made around the world.

Germany leads the world in Riesling production, followed by the United States, where Washington state has led a resurgence of the grape. According to the United States Department of Agriculture’s 2015 harvest report, Washington growers produced 44,100 tons of Riesling that fetched an average price per ton of $760. Total Riesling production has grown 39% from 2011.

The variety is earning a reputation for quality in the Finger Lakes AVA of New York, and has champions in Oregon as a contender for one of the state’s top five varieties. (The Ontario province of Canada leads in Riesling production east of the Rockies, ensuring the grape’s prominence continent-wide.)

Climate change
Between tasting dozens of examples of Riesling from Woodinville, Wash., to Hobart, Tasmania, participants in the fifth gathering of Riesling producers and distributors were told that the effects of climate change on their favorite grape simply aren’t known.

“We have no clear idea what we’re heading for,” confessed Hans Schultz, director of the Geisenheim Research Centre and president of Geisenheim University, in summarizing growing conditions for the grape.

Shifting growing conditions promise to reveal new facets of the grape to consumers, Schultz noted, pointing out that humans have 347 genes for receiving flavor, all of which express themselves differently and in turn allow consumers to experience a grape’s expression in wine differently. A changing climate means grapes will express their terroir in different ways.

The kind of changes grapes are dealing with have been well documented: 2015 was not only the warmest growing season on record, June 2016 was the fourteenth consecutive month of record-breaking temperatures around the world—and the current month may well be the fifteenth.

But climate changes, as speakers at last week’s XI International Terroir Congress in Oregon noted, will be tailored to the regions where they occur. Congress chair Greg Jones, a professor at Southern Oregon University, reiterated the point to Riesling Rendezvous participants. “Not all regions change in the same way,” he said.

Some changes affect how vines respond during the day, others affect how vines behave at night, and others will make vines more vulnerable during specific seasons. Overall temperatures are increasing, for example, but vines may experience a greater risk of frost damage under some circumstances and also—because of how senescence takes place—be more vulnerable to any frosts that do occur.

And when it comes to Riesling, the answers aren’t at all simple.

Riesling and the unknown

Temperature shifts are more pronounced at northern latitudes, but growers shouldn’t expect them to boost sugar content, Schultz pointed out, citing research that indicated just 38% of the increase in the sugar content of Riesling grapes in recent years is attributable to rising temperatures, while evolving viticultural practices have fueled the rest of the rise.

Similarly, vine respiration is often expected to increase with warmer temperatures, but this may not be the case given that nights are shorter at the northern latitudes, which will feel the effect of rising temperatures the most. This makes for a shorter time in which vines experience the cooler temperatures night brings.

The aspect of vineyards or vines will also play a role.

The greater a location’s slope, the greater the rate of evapotranspiration. Schultz said evapotranspiration in some German vineyards with a 30% slope has increased by 207 mm per year to a current rate of 998 mm a year. How this might change in future, in other locations, demands better modeling to gauge the local impact of site conditions.

Riesling vines may also respond differently to warmer nocturnal temperatures based on their genes, affecting not just their behavior but their fruit.

Schultz referenced the work of a team led by Markus Rienth, a researcher in Montpelier, France, which has identified 1,843 genes in grapevines that are only active at night. The primary role of these is the regulation of anthocyanins and terpenes, which affect grape color and flavor, respectively.

How these genes respond to nighttime conditions will affect the composition of fruit and the characters imparted to wine, potentially delivering new kinds of Riesling to consumers.

But if the Rieslings of the future complicate consumers’ understanding of Riesling, Ernst Loosen said the challenge will be to encourage them to think more deeply about what’s going on in their glasses just as researchers think outside the glass.

While researchers pinpoint the key variables at play in shaping the Rieslings of the future, Loosen told Riesling Rendezvous participants to not be afraid of challenging drinkers.

“There is always lots of discussion about how to create the most simple message about Riesling that will get people to want to try it,” he said, but confessed that when he speaks to audiences in Asia, he hasn’t been able to reduce Riesling to less than four categories. “It’s our tradition to present different styles.…We should challenge (consumers) to be willing to learn more about what they’re drinking.”

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