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Of course CO2 production is linked to climate change and Global Warming, Greenhouse Gases and many other complex scientific matters. What our Bordeaux wine producers need to know is whether they have to make major changes and if so, when? I have become somewhat cynical with regard to climate change having now assisted at several Global Warming and Climate Change Conferences.
The predictions and statistics presented by the most eminent, world-famous authorities on climate change have varied so widely and so hugely that one is left wondering if there is any concrete substance in them and which, if any one should believe! Some predict total disaster. A world renowned climate change authority in a presentation here in Bordeaux said, “move your vineyards 100 miles west”. A member of the audience, representing a First Growth Bordeaux, said, “that would put them in the middle of the Atlantic Ocean”. “100 miles North, then”. “That would put them in Cognac; and Cognac would not appreciate that very much”. “OK, then south”. “Now we are in the Pyrenees”. I leave it to the reader to decide whether the speaker was unfazed enough to mention the 4th cardinal point, which would have been the only remotely feasible possibility!
Recently an unemotional, deep and serious study has been published by ORACLE, “Observatoire Régional sur l’Agriculture et le Changement Climatique”, specifically on Aquitaine. It covers “situation report on climate change and its effect on agriculture in Aquitaine”. It is narrow and concentrates on the 3 main products: grain, vines and pine forests. It looks at various factors, one after the other and is the crux of this article. Here are the salient points for wine growers:
ANNUAL AVERAGE TEMPERATURE
In Aquitaine the annual average temperature has increased at the same rate as Metropolitan France, i.e., 1°C during the 20tth century or 0.1°C per decade. Since 1970 the warming has been considerably increased to the order of about 0.3°C per decade. For grain this may imply fairly rapid decisions as to when to plant, but is unlikely to affect vines for a very considerable period.
ANNUAL RAINFALL
Although a very slight decrease has been noticed in annual rainfall it is not enough to register as a significant factor over the last several decades. However, to properly qualify the evolution of hydric conditions the period of the rainfalls must be taken into account as well as any evolution of evapotranspiration. Predictions, without scientific support, suggest less rain and greater evapotranspiration, but not enough to significantly affect the vine.
NUMBER OF DAYS WITH FROST
The number of nights with frost during March and April have halved over the last 60 years. The consequences of this on both bud-break and flowering can only be accurately estimated after studying the parallel phenolic advance of the varieties in question, but is definitely significant.
NUMBER OF DAYS AT 25°C+ BETWEEN MAR-MAY
Over the last 60 years only an increase of 2.5 days during March, April and May has been recorded. For grain this is important as each day over 25°C during harvest causes the loss of 1.5 quintals per hectare of fresh grain. For vines it is insignificant.
WINKLER
This indicator, conceived by Amerine and Winkler in 1944, divides the world into different viticultural climates. It helps define the aptitude of a region in terms of production, of adaptation of grape varieties and of the speed of the phenolic cycle. It is based on temperature and properly interpreted allows a reasonably accurate estimation of temperature tendencies linked to climate change.
The findings are that the viticultural climate in Bordeaux has evolved considerably over the last decades. The Winkler reading has increased by over 400°C (base10) over the last 40 years.
NIGHT TEMPERATURES
Maturation (between colour-change and vintage) is essential for the quality of the future wine as this is the period of synthesis of the organic components influencing the typicity of the wine (sugars, acidities, polyphenols, precursors of aromas). Any change has not been sufficiently scientifically measured to make any statement. However, the negative effect of high minimal temperatures during maturation has been shown by a number of studies, although here in Bordeaux they have also not yet been measured.
VINTAGE DATES
There are double results of a rise in temperature:
- An acceleration in the accumulation of sugar resulting in an increase in alcoholic strength.
- An acceleration of the degradation of malic acid resulting in less total acidity. 1°C increase in temperature is considered to result in a diminution of acidity of the must by 1gm/litre at harvest time.
In spite of the possible diminution of annual rainfall predicted by some in France over the 21st century the yields of vines in the Bordeaux region do not seem to be in danger because of:
1. The favourable hydric context due to the proximity of the Atlantic Ocean
2. The photosynthetic metabolism of the vine in C3, which offers it a positive response to the increase of CO2 in the atmosphere
3. The excellent resistance of the vine to hydric stress
The possible adaptation of Bordeaux viticulture to climate change could concern
- Change of cultural management – direction of rows, pruning and canopy management
- Development of night harvesting and cooling of harvested grapes
- Re-localising parcels on slopes less exposed to heat if the terrain permits
- Finally, and in very long term, changing to grape varieties that better support hotter climates. This is a last resort action and should not be needed for a long time
Whatever is done, serious study is required to maintain typicity, which is vital to the reputation and continued pre-eminence of Bordeaux wine.
CHANGE IN VINTAGE DATES
The dates of the start of the vintage have advanced by 4 days every 10 years since 1970 and today the Ban des Vendanges is on average around 15 September. During the same period the average annual temperature has increased, as mentioned above, by about 0.3°C per decade.
The combined effect of these two factors amplifies the consequences of the climate change on the maturation of the grapes.
It is thought that if the increase in temperatures does not exceed +1.5°C - +2.0°C then the adaptation of the cultural management of the vine both before and after harvest should be sufficient to compensate the impact of climate change in the Bordeaux region. Should climate change go beyond these increases, growers will have to consider seriously genetic selection and production methods used by hot climate countries, BUT yields would not appear to be in danger until after the middle of the 21st century.
WATER TEMPERATURE IN THE GIRONDE
This is important as the Gironde has an important regulating role in weather conditions and meteorological factors. Since1978 the temperature in the Gironde estuary has increased by 0.8°C per decade. A significant reduction in the debit of the water in the Garonne has also been registered.
 CONCLUSION
I think it is crystal clear that although the warming is incontrovertible, none the less we do not need to cry wolf and our vines are not menaced with either extinction or smaller crops for at least 50 years or more, during which time we may well learn how to manage them to face the threat when it arrives! Serious research into genetic modification of our traditional grape varieties is now under way at Bordeaux University of Oenology.
John Salvi Master of Wine |