Wine grapes are one of the world's most valuable horticultural crops; viticulture is an economic sector that counts and there are vineyards on all six continents. As with many other crops, these vines and their grapes are very sensitive to weather conditions as well as rainfall during the growing season. What are the effects of global warming on winemaking and the wine industry? Will wine be of better quality? (American) academics have analysed four centuries of French grape harvests and great growths to provide an answer.
Ahe climate and weather can affect the time of harvest, the quantity of grapes harvested, as well as the final quality of the wine. As the anthropogenic warming we are witnessing - mainly due to the increase in greenhouse gas emissions into the atmosphere - is increasing in many parts of the world, there is concern about the effects of such a phenomenon on winemaking and the wine industry over the next few decades.
My colleague Elizabeth Wolkovichfrom Harvard University, and I have studied nearly 400 years of data from France and Switzerland to show how climate change is affecting the grape harvest. This information comes from a variety of historical sources and has given us a unique opportunity to examine the changes that have taken place from centuries past to the present.
Our work indicate that harvesting is taking place earlier and earlier; while this has traditionally coincided with high quality wines, we have noted that recent climate change has made this cause-and-effect relationship less obvious; it will become less easy to 'predict' good vintages; and rising temperatures may well force winemakers to turn to new grape varieties.
Centuries of harvesting
For our study, we examined the historical registers in which the dates of harvest were recorded in many regions famous for the quality of their vineyards, such as the Gironde, Burgundy or the Loire Valley. We then reconstructed the climate - temperature, rainfall, soil moisture - based on other data, such as the growth cycles observed in the rings of trees in the regions concerned.
We have thus noted a remarkable change in the last few decades: since 1981, the harvest has taken place on average 10 days earlier. On only two occasions after 1981, there were later harvests.
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This trend is due to climate change, with warmer summers. Grapes are ripening faster and need to be harvested earlier. Another important phenomenon revealed by our study is the changes in the way the climate affects harvesting.
The temperature of the growing season is the key factor in the ripening of the fruit, thus determining the time of harvest. Prior to 1981, an episode of drought (low rainfall and low soil moisture) was required to cause early harvesting.
If the summer is rainy and the soils are wet, the evaporation process absorbs heat from the surface, keeping the soil and air temperatures relatively cool at that level. Conversely, during a drought, the soils are dry and the heat of the atmosphere causes the surface moisture to drop. Very little evaporation can occur, as the excess heat contributes to warming of the soil and air. Dry summers rhyme with hot summers, and it will be much more frequent to obtain an early ripening of the bunches and therefore an early harvest. It is such conditions that have traditionally led to superior quality wines.
However, since the 1980s, climate change has made several decades much warmer than in the past possible. This weather can cause early harvests even without a dry spell. Such a change can have serious consequences on viticulture and wine quality, as the early harvests of the last few decades are no longer just hot and dry; they can also occur in hot and humid summers.
What does this mean for viticulture?
Traditionally, in regions such as France, early harvests have produced better wines (this conclusion is based on rankings drawn up in a register in which the wines were given scores in order to control factors such as age since bottling). But because of the close link between drought and early harvests, this means that the higher quality wines were also associated with hot, dry summers. Now that early harvests can be linked to dry or wet weather, drought is no longer a good indicator of wine quality.
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However, with these new warming induced trends, there is a general trend towards greater wine quality despite changes in drought episodes. It is to be expected, with the continued rise in temperatures, that harvest dates will continue to be brought forward.
There is, however, a limit to this earliness, which is synonymous with good wines. The 2003 harvest provide a striking example: a historic heat wave that year caused the harvest to be a month earlier than usual, an absolute record over the last 400 years. But although this early harvest was exceptional, no wine from that year was particularly outstanding. Such events are likely to become frequent in the years to come.
While temperatures are on the rise, we can see that there is certainly a threshold beyond which the continued earliness of the harvest dates will not give rise to vintages. Of course, the science of viticulture and winemaking involves much more than weather and climate, and our results do not in any way predict that we will be condemned to drink inferior wines.
The production and quality of the wine depends on many factors, both human and natural, which relate to soils, topography, climate and the skill of the winemakers and other cellar masters. However, our results show that climate change is affecting harvests today, and that the patterns of yesteryear - with droughts causing early harvests and high quality wines - have already evolved.
What can viticulture do in the face of this situation? Winegrowers will undoubtedly have to consider changing their practices to adapt to a world governed by new climate laws.
Benjamin I. CookClimate Scientist, Columbia University