How 1930s American scientists came to think about the impact of climate on wine


In Europe and beyond, the notion of terroir dominates ideas about the origins of the taste and quality of wine. While there’s intense debate over the term, generally it refers to the specific place where grapes are grown. The concept is largely focused on soil, but also includes the layout of the land and the elements to which it is regularly exposed – sun, rain, wind, seasons, and more. And although climate is seen as being part of the equation, the land upon which grapes are grown is its foundation. As such thinking took root over centuries, it was eventually codified into Europe’s appellation d’origine contrôlée (AOC) system, meaning “registered designation of origin”.

While European immigrants have long grown grapes and made wine around the world, the traditional regions were an ocean away, literally. So what could be done to improve wine quality in these new vineyards and wineries? The situation was particularly dire in the United States after the Prohibition forced many of its winemakers out of business.

A world away

After the Prohibition repealed in 1933, two scientists, Albert “Wink” Winkler and Maynard Amerine, launched an effort to revitalise California’s wine industry. Winkler was more of the viticulturalist and Amerine the oenologist, but both shared a passion for grapes, wine, and believed that the state could produce wines that rivalled the best of Europe. Their journey led them to collect vine samples from Fresno in the south to Ukiah in the north and westward to the coast. They planted many of these vines in test vineyards to see how they fared in different climatic regions, in order to advise growers on the best grapes for their plot of land. But vines were not the only bounty they sought.

Winkler and Amerine also collected grapes from willing viticulturalists turning them into a library of more than 500 site-specific wines over a decade. By 1943, they had observed enough seasonal variation in the hundreds of small batches of wines that Winkler and Amerine made and tasted every year to recommend specific grape varieties for specific regions. By expanding the vineyards where they collected grapes, they could both measure and taste the difference between vineyards in regions across California.

Winkler came to an epiphany from their sojourns in California’s vineyards and by analysing the wines these fields produce. The research let him to conclude that climate and regional differences were the most important factors in selecting varietals to produce high-quality wines. He came to this conclusion counter-intuitively.

By thinking about Europe and the idea of a “vintage” versus a “non-vintage” year, he realised the only thing that changed in the vineyard (not the vines, not soil type, not soil quality, not soil drainage) was the weather and, in particular, a vintage year was warmer in places like Bordeaux and Burgundy. He applied this same logic to California as he tasted the same grape in different regions and found some varieties like Zinfandel produced better wines in cooler climates in northern and coastal California while others like Alicante bouchés, which produced sweet wines, fared better in warmer, arguably hot, climates inland and in southern California. This observation had global impact.

Knowing what to grow

With Winkler’s development of a heat-based index, he and Amerine advised would-be California wine makers – from Gallo to Mondavi – not just on the varieties they should plant (or pull out) but also which ones would produce the best wines in their particular locations. The Winker Index rapidly transformed not just California vineyards but vineyards across the world as viticulturalists and oenologists paid more attention to the climate. In New World regions, it allowed them to choose varieties that produced wines best suited to the climate, thus improving the overall quality of wine.

But their research had an even deeper impact on varietal selection. Although the Winkler Index measured the temperature across the growing season, it was the taste and aroma of the wines in their wine library that was at the heart of their conclusions. In measuring the acid/sugar ratio among other compounds in their wines, Amerine and Winkler judged how climate was reflected in the wines they swirled and sipped and how their wines changed over time, especially in years when the weather deviated from the norm.

These early observations on heat and its influence on wine quality allow historians, wine makers, and climate researchers to conclude that not only is the climate warming, but how a warming climate is changing the taste of wine based not just on acid/sugar ratios – though they are – but how hotter, sunnier growing season are increasing sugar in grapes, the alcohol in wine and reducing acidity, throwing wines out of balance. A vineyard that may have consistently produced high-quality wines from the 1930s through the 1990s now produced inconsistent wine.

The opposite can also be true: A region like Bordeaux, which was historically plagued by erratic weather, sometimes losing entire vintages to hail, frost or cold summers, now had more consistent yields, smoothing the difference between a vintage and a non-vintage year. Even inexpensive wines in Bordeaux benefited from warmer growing seasons because more grapes fully ripened.

Of course, as the climate warms, that impact has other negative consequences. Hotter weather reduces the acidity of wines making them flat, flabby, or turgid. An example of mitigating low acidity is Bordeaux’s experiment allowing new varieties to be blended into their iconic – and legislated – varieties of reds and whites to increase acidity and rebalance overripe wines.

Where there’s fire there’s smoke

Could that be a hint of smoke in your wine?
Alex Maze/Unsplash

An even more difficult and frightening consequence of a warming climate are wildfires. While fires do not always destroy vineyards (grapes are just spheres of water, after all), the smoke can contaminate wine made near wildfires, resulting in smoke-tainted wine – it tastes something like burnt rubber, cigarette ash or other unpleasant flavours. Once smoke has wafted into the vineyard and engulfed ripening grapes, it is impossible to remove. Worse, winemakers cannot tell if the wine will be smoke tainted by tasting the grapes themselves, as fermentation also affects how foul a wine will taste.

Though scientists around the world are trying to find a solution, they still do not understand exactly what makes a wine taste smoke tainted or how to mitigate it. It’s become a growing concern given the rising number of fires in wine-growing regions, including California in 2020, France in 2021, and Spain in 2022. The same year two wildfires burned more than 20,000 hectares of forest in France’s Bordeaux region. Tests indicated that that year’s harvest shouldn’t be affected, but the coming years promise to be difficult for winemakers.

Adapting to a changing world

It is only because Winkler severed the link between wine and terroir that wine growers had the vision to plant and produce world-renowned wine made in places like Canberra, Australia; Mendoza, Argentina; Sussex, England; and Ningxia, China.

Given that climate change is already changing the weather in Europe’s wine-growing regions – the ones whose methods and very identity are most closely linked to traditional notions of terroir – research is also seeking to help wine makers adapt to a changing world. It’s a process that’s already taking place, not only in the Winkler Index itself, but even in the venerable AOC system. Plus ça change

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